Oil Pressure problems on the Short Engine published September 1994
By Peter Nicholls

with input from Richard Floyd, Bob Groves,
Ray Waters, Peter Bowler, Tony Meade, and the late Andrew Macqueen.

This should perhaps be subtitled 'A problem solved and a problem gained'. Correspondence in this issue of 'Front Wheels' shows that I am not alone in worrying about low oil pressure, and some interesting points are raised.
Richard Floyd discovered that the oil pressure loss which caused big end failure in his Scout was due to wear in the rear camshaft journal.
Bob Groves has yet to find the culprit, but suspects that the rear cam shaft journal may be a prime suspect.
It is at this stage that, in this humble reader's opinion, a red herring enters the scene. Bob, no doubt in common with many other BSA owners, has a copy of the 'Book of the BSA Scout and Three Wheeler', which according to my copy is based on the Pitman publication of a similar name dated 1935, with later additions using details supplied by BSA Ltd. The section on lubrication of the four cylinder engine is reproduced below, together with the relevant diagram.
'As on the air cooled models, the engine is automatically lubricated by a rotary gear pump A, Fig.48, at the lower end of vertical shaft B which is driven through spiral gears from the camshaft at C. An extension on the pump body projects into the engine sump, the capacity of which is one gallon. In operation, the pump draws oil through filter D in the sump anddelivers it under pressure through a channel drilled in the cylinder block to the timing case, where a portion of it is diverted through a specially calibrated orifice (our highlights) to the tappet chest at E. At this point a well is formed which fills up with oil and from which the oil drains through holes in the tappet guides and then passes to the cam shaft. Any excess of oil in the well returns to the sump through two drain holes placed at the correct level, one at each end of the tappet chest.
The main oil supply, part of which is diverted to the tappets and cams as described above, passes through a passage in the timing cover to a special union F, through which it is forced to the hollow crankshaft, emerging through a hole drilled in each of the crankshaft journals and thus lubricating the big end bearings. The pistons, cylinders,and small end hearings are lubricated by splash.
An oil pressure gauge is mounted in the instrument panel. Under normal conditions when the engine is warm the pressure gauge should read about 60lb per square inch. At low speeds the pressure reading may fall somewhat,but so long as there is an adequate supply of oil in the engine sump the pressure should be maintained when the engine is running.
At G, Fig.49, a spring loaded relief valve is fitted to the pump. If the oil pressure gauge does not show a normal pressure it is either due to shortage of oil in the sump or the valve not seating properly. Pull out the dip stick on thenear side of the engine and examine the oil level. If this is in order, replace the dip stick and check the relief valve.
This valve has been designed in such away that it can be cleaned of dirt and other obstructions.
To clean the valve,leave the engine ticking over, take hold of the small projection and pull it outwards, releasing it after a second or two. In addition to this, if necessary,rotate the valve slowly in order to assist the removal of the obstruction.
There is no adjustable control in the BSA lubrication system, and the only points requiring attention are a periodic examination of the oil level in the sump and a careful check on the oil pressure reading in the gauge.
After many thousands of miles the oil pressure may fall on account of wear developing at the connecting rod big end bearings.'

The point in question is, what or where is this 'calibrated orifice' mentioned in the text? Since reading this, I have taken a close look at several of the derelict engines in my possession and cannot find a single calibrated orifice between them. In reality the lubrication of camshaft bearing and tappets agrees with the description given in my Series 4 and 5 Handbook which reads:-In operation the pump draws oil through filter D in the sump and delivers it under pressure through a channel drilled in the cylinder block to the timing case; thence through a special union Note the oil transfer device Fenabling it to pass along the drilled crankshaft to the holes in the crankshaft journals, thus lubricating the big end bearings. An oil feed under pressure is also taken to the camshaft rear bearing from which it is fed to the timing chain and the tappet gallery,ensuring adequate lubrication for the tappets and the camshaft front bearing.'
It is interesting to note that this description is accompanied by an identical diagram to the one featured in the Pitman publication, but, guess what?, no calibrated orifice marked E! It would be interesting to hear from anyone who can perhaps shed any light on this apparent discrepancy.Following my comments last month, I waded through my back issues of 'Front Wheels' and was fortunate to turn up the article by PSB to which I made reference. It contains lots of sound advice concerning our problem,so here it is again.

'Every five years or so brings us a new generation of BSA enthusiasts, a few stay with us for longer, but there must be a high proportion of our members who haven't been enlightened about the various dodges and mods that us diehards have taken for granted and perhaps half forgotten over the years.The first hoary old chestnut that I can think of to pull out of the fire is that problem that used to dog us short engined trike and Scout drivers year sago, largely diminished perhaps now that better quality oils are available, but i'm sure that sagging oil pressure must be worrying someone...I used to get over the problem by adopting the following procedure. The first item for attention is the oil pump itself. when removed and dismantled it will often be found that there is end float in the gears due to wear on the end plate.
First, grind the end plate back smooth and flat using plate glass or similar and grinding paste. When the plate is finished, check both gears for end float, if this exists grind the longest back to match the other, then the pump body to match them both.Having done this you should have a pump that provides the maximum pressure within the limits of design and the wear factors of spindles and gears.
The other major culprit for the loss of oil pressure is the oil transfer device on the rear end of the crankshaft. this consists of two bronze parts that are spring loaded, forcing one end against the con cavity within the rear timing chest cover, and the other against asimilarly shaped face within the rear end of the crankshaft. Now this is a hit or miss device if ever I saw one, and it leaves itself wide open for some sort of improvement.
I had two annular rings machined in the inner part of this device and fitted rubber 'O' rings, and reassembled using a stronger spring.
The next point for attention is the oil pipe leading to the cam shaft rear hearing. This oilway provides a really low resistance path for oil pressure, and results in a flow of quite unnecessarily high rate. A good mod here, and one that had quite remarkable results whenThe pipe, from the 1934 Pitman bookI carried it out, is to remove the pipe at the lower end where it is screwed to the union on the inner rear cover, tap it 2BA and fit a brass 2BA screw shank which has been drilled to 1/16". This provides a restriction in oil flow and a consequently higher oil pressure where you want it, in the big end bearings!!Another factor worth mentioning whilst on the subject is piston blow-by.
This raises oil temperature to quite alarming levels, especially when the engine is being driven hard, and is the cause of many a big end failure. Whilst on the subject of oil temperature, various methods have been used to overcome this, including a double sump (Jack Rowe), oil coolers (myself), and aluminium deflector plates of various shapes and sizes (Bob Neal).
Of course, all the foregoing work is a complete waste of time if the condition of the big end bearings is poor. In the BSA engine the oil is unfiltered and therefore it does tend to carry with it a great deal of carbon and metal debris in suspension. This becomes embedded in the soft white metal, but there comes a time when serious wear takes place.I consider that on an unfiltered system 25000 miles and it is time to inspect and renew if the condition is bad, and it usually is at that mileage! I used to measure up one weekend and get a spare set of rods remetalled to swap over the following weekend, thus keeping the car on the road. Make sure that big end end float is not excessive when fitting, and don't run them too tight as this can cause early failure.' (Universal Torque - January1978).

What I find most interesting is that if we limit the oil flow to the camshaft rear journal then the fate of the bigends is no longer at the mercy of the degree of wear in this journal. It seems to me that this must be highly recommended and I shall endeavour to carry out this modification in my rebuild.I apologise if we appear to have flogged this topic to death, but as con-rod remetalling is such a costly enterprise these days that alone must justify giving it an airing. Next month I promise not to take up so much space with my ramblings, in the meantime I look forward to seeing as many of you as possible at the AGM.Peter Nicholls

Letters to the Editor

 

Dear Peter,
Your doubts on the lubrication of the BSA engine expressed in the Editorial of august 'Front Wheels' prompts me to write as I too am in big trouble with that very problem.

However, firstly a little background. In January 1991 I purchased a dismantled trike, four cylinder 1935 Special Sports BKT175, engine number 1822, chassis number 7018, previously the property of a Mr King, who I believe was a Club member. Mr King dismantled the vehicle with the intention of a full and thorough rebuild, but unfortunately was never to achieve this. His son Jason kept the trike for some time with the intention of carrying on the rebuild, but change ofcircumstances led him to sell it to me.
I collected BKT from somewhere near Ipswich and the enclosed photos indicate the state of play once the items were unloaded and sorted. It was well and truly dismantled as can be seen, the chassis was down to a bare 'A' frame, the engine block, gear box and differential were empty castings; but as it turned out hardly a a nut and bolt were missing, and the real big advantage was that the cylinder block had been re bored, new pistons, gudgeons and rings still boxed. New valves,springs, guides were there, the crankshaft reground, big ends remetalled, and all new ball and roller bearings for both engine mains, gear box and differential were included, also a reconditioned radiator.
As I have rebuilt a number of both cars and motorcycles over the years, the work was approached with great confidence, and it took around two years before the vehicle was complete. An enclosed photo shows the final result, though I must admit the modern film in my Box Brownie does rather enhance the paint work,which is brush finish.
The engine started without trouble and sounded good. With an oil pressure at 60 psi all seemed well, so May '93 saw it with MOT and tax, ready for use. A few odd problems were found and sorted in local 'round the houses' trips before a more serious run was tackled. Here the real trouble showed itself, the car went well enough but once fully warmed up the oil pressure dropped, and continued to drop. Stop and let the oil cool, then oil pressure would be fully restored, only to drop away again as the thing warmed up.
On the good advice of many people a number of attempts were made to overcome the problem. The oil pressure relief valve was dismantled, cleaned and reseated more than once, the crankshaft feed device inspected, reseated and new spring fitted, oil pump dismantled and inspected by professional engineers, three oil changes trying different makes and grades, even the oil gauge itself was changed,none of which gave any improvement. On the last run earlier this year, with the pressure down to 15 psi and still falling, I tried to coax it home the last few mile sand finished with a 'rattle' that said it all.
With hindsight, I now feel the root of the cause is the flow restrictor, as mentioned by yourself, (also see enclosed photocopy from the 'Book of the BSA Scout and three Wheeler) not preventing the hot thin oil from bypassing the big ends. It would seem reasonable that some method is a must, and I suspect that two different ways were tried; the calibrated orifice, or the fit of the camshaft rear bearing.
Questions I feel need answers are :-

1) Was in fact more than one method of oil control tried.
2) If an orifice, exactly what size should it be.
3) If bush controls flow, what is the maximum bush clearance permitted.
4) What should be the correct oil pressure when hot.
5) Who can be recommended for the remetalling of the con rods.
6) Is it possible to convert the rods to accept modern type shell bearings.
Over the coming winter it is my intention to dismantle and rebuild the unit, and needless to say any advice from yourself or any other member would be greatly appreciated.
Yours sincerely,Bob Groves

 

And another---

Dear Peter,
I read with interest your problem with low oil pressure and big end failure happening to your engine (August Editorial).
My Series 2 engine suffered the same fate many years ago, this was after I had rebuilt the engine with new pistons, big ends, etc. The big end failure that occurred demolished the end cap and let the piston and rod flail about which damaged the bottom of the bore and punched a hole through the side of the block.So that welding could be done, the engine had to be completely stripped (never been done before) and the reason for the lack of oil pressure became apparent.The cam shaft bearing was about 30 thou. oval, caused, I am sure, by the pressure of the valve springs being downwards and the timing chain pull being upwards.This makes for a hammering effect on the bearing, and in time the oval wear will take place. This wear cannot be found with the engine in the car, and even with the timing cover removed I doubt whether, with the pressure of the valve springs,any vertical movement could be detected.
I had a new phosphor bronze bush machined from solid for £40 and I now run theengine with 20/40 oil and a respectable 40psi. when hot. It was lOpsi. with 50 grade gear oil and an oil cooler! If anyone has an engine which, when going through various rev ranges, (mine was about 30 and 55 mph) makes a sound similar to a machine gun, then I can guarantee that the source is this bush.

Yours sincerely,
Richard Floyd.

Footnote

A close examination of the BSA parts list and diagram reveals the fact that the item E on the drawing points to a timing cover fixing stud!!(35-1617)
whilst the item referred to as "Timing case oil feed pipe adaptor"(35-1652) cannot be found on either drawing!In my opinion the "calibrated orifice" only existed on the very early four cylinder engines, to be replaced by that pipe that we all like to put a restrictor in!which, according to the 1935 three-wheeler parts list is "cam-shaft rear bearing, complete with oil-feed pipe"(part number 35-1297)
Nowadays of course in-line oil filters are available, which must go a long way to improve matters
It's worth noting that the RWD version of these engines has white metalled main bearings, and oil is fed in the conventional way, to both ends of the crank, via (I think) an external oil pipe from one end to the other.
psb

Further Thoughts on Oil Pressure, by Ray Waters

Last time I tried to offer a few constructive comments about the material which appeared in the September 1994 edition of 'Front Wheels' concerning oil pressure on the four cylinder three wheeler and Scout Series 1 to 5 engines. The following are a few additional thoughts on the matter.
Further study of the Pitman publication (figure 31) and of BSA's threewheeler manual (page 17) shows a picture of the specially calibrated orifice nut I mentioned last time. The Orifice!!It can be seen about an inch north-east of the figure '8'. This picture does not appear in the Scout manuals, it is obviously of a very early engine as I ain't never seen a fuel pump like that. Certainly the engine oil filler is also early. Can anyone tell me if it is early Scout or T9? All the books do, however,show a view of the timing chain driving the dynamo and camshaft. Here the oilfeed to the tappet chamber, rather than the rear camshaft bearing, can clearly be seen.
With reference to the oil pressure relief valve, (Figure 1), my own personal 'bodge' was to unsolder the end so that I could remove the ball bearing and spring to clean it (remember to note the position of the setscrew which compresses the spring by counting the turns of the setscrew before it falls out). I did replace the ball bearing on one occasion when I simply could not get it to seat properly. In this case I seated the new ball bearing by dropping it into place and using a drift to give it a smart tap with a hammer. I have also heard the suggestion that it could ground in using grinding paste. All I can say is that you would have to be very careful to get all the paste out!
Having rebuilt the pressure relief valve by reassembling the ball bearing, spring and setscrew, it is necessary to reposition the setscrew to give the required cold oil pressure. Be careful, if you screw the setscrew up too far to start with you can bend the oil gauge needle! Obviously a good starting point is to put the setscrew back in the same position as it came from. Personally I would set for maximum cold pressure (say 60 to 80psi), this should produce, I believe, the maximum hot pressure, which with this type of engine is what life's all about. I always found the original ignition key to be the ideal tool to adjust said setscrew. In my experience very little oil leaks past the setscrew so I did not re solder the whole thing up again.If you are concerned about it, try locking and sealing the setscrew threads with a drop of Red Hermetite.
The description of big end failure in Richard Floyd's letter in the September magazine which caused 'a demolished end cap, and let the piston and rod flail about' etc., is a bit extreme. Usually big end failure gives you this awful death rattle and lost oil pressure. In days gone by I struggled home on three cylinders with the appropriate spark plug removed to minimize damage to the crankshaft journal of the failed bearing, but without other major catastrophe. Pete Bowler always described Richard's machine gun noise as like 'a stick being drawn along a wooden slatted fence'.
I often wondered why the occurrence of this noise could be altered (but not got rid of) by changing the timing chain tension. Now I know from where the noise emanated I understand.
While I accept that Richard has cured his problem by replacing the rear camshaft bearing, 50 grade gear oil would, I should think, give low oil pressure even if the entire engine was perfect. Gear oil simply is not designed to run at the temperatures engine oil reaches, and would be much thinner than 50 grade engine oil at the normal working temperature found in an engine. Incidentally, does anyone agree with me that 20/50 (as opposed to 20/40) grade engine oil would provide better lubrication for an engine which runs as hot as ours do?With reference to Bob Groves' letter in the same magazine, I believe that Peter Nicholls, Pete Bowler (PSB), and Richard Floyd in the September magazine, plus my own effort 'Oil Pressure - The Next Generation' have answered the first two questions he listed at the end of his letter. I would agree with Richard Floyd that 40psi is very respectable and personally found that 20psi hot at 30mph in top gear) was THE ABSOLUTE MINIMUM. Regarding Bob's other questions, could Richard tell us what dimension he had the bore of his new rear cam shaft bearing made to, or can Tony Meade come up with a drawing for publication in the magazine of said rear camshaft bearing? Finally, can other members recommend conrod remetallers and comment on the use of modern shells?
One last piece of advice to anyone who does not already know, and is having their conrods remetalled. If you look carefully at where the straight section of the conrod changes to the semi-circular section which forms the big end, you will see a small hole. This is supposed to continue through the white metal. It provides an oil jet which is fired at the bore/little end every time this little hole lines up with the hole in the crankshaft journal which brings the oil supply to the big end bearing. So remember to drill your new white metal bearings to give this supply to the little end and bore.
Finally one 'last last' question. Is the scene now set for an ardent V-twin enthusiast to write an article entitled: 'Oil Pressure...Who Cares? A Treatise on the Dangers of High Oil Pressure in the BSA Three wheeler V-twin Engine'.

Ray Waters

 

Read oil about it!, Andrew Macqueen

In the September editorial, Peter apologises for flogging the oil pressure topic to death. I'm not so sure that we should put the big stick away yet since this problem keeps cropping up sixty years on.
So, for what it is worth, the following observations on the oil supply to the rear camshaft bearing on the 'short' engine may be relevant still. It so happens that I have one engine (block cast in 1932) on the 1932 T9 which has the elusive 'E' orifice, and one engine on the 1933 trike which has not. I know not whether the trike block is original (and we all know that the engine number is stamped on the alloy clutch flange and could be transferred from block to block).
The fact that the rear camshaft bearing wear can be a problem is clearly stated by Richard Floyd, and his 'machine gun hammer' theory is intriguing (I hadn't thought of that), and the fact that BSA changed from the comparatively simple 'E' squirt to a more sophisticated direct supply to the rear bearing would seem to indicate that this was known to the makers sixty-odd years ago.
As mentioned in the editorial, that doyen of sound hands-on practical advice, Peter bowler, wrote way back in January '78 of using a 1/8" restrictor in the oil line to thecamshaft bearing, so let's look at how much oil this orifice can supply. The trouble with oil is that the viscosity can vary so much (even with multigrades) depending on temperature, and this affects the flow rate, velocity and friction loss through pipework and fittings, and the rate at which flow (and pressure) can escape through worn bearings etc.
Going back to PSB again, he sagely stated that other work was a waste of time if some bearing etc. condition is poor. Nevertheless, I think that the 1/8" restrictor recommended is too large. How can we add some theoretical bones to such a statement? We calculate it first with WATER, that's what. Water does not have much of a viscosity problem - unless you let it freeze!Then you look for a conversion factor to make your calculation roughly appropriate to oil. This cannot be a highly sophisticated exercise unless you have access to much more detailed monitoring than any of us are likely to have, but we do have a way which I reckon is generally acceptable enough to make an educated assessment - good enough for our purposes.
So where do I go from here? I go back to PSB again! This time way, way back to August 1964 (yes, thirty years ago) where 'Universal Torque' reports on a by-pass oil filter system Peter was fitting to his trike. He had fitted a 1/16" restrictor in the oil supply line to the filter, and found that the return line (to the sump) was passing the equivalent of one gallon in four minutes. Again we can only guess at the pressure, temperature, etc., but I have taken 30psi for this purpose. Now comes the really theoretical bit. The most appropriate water flow formula for orifice throughput is probably the following simplified one.

25d² x §P = UK gallons per minute

(note § should be sq root sign, I don't have it on my editor!!!,psb)

I'm showing my age here as this is strictly Imperial stuff, ie.d = diameter in inches
P = pressure in pounds per square inch
From this you can calculate for yourself that at 30psi:-

a 1/32" restrictor will pass 0.13igpm water
a 1/16" restrictor will pass 0.53igpm water
a 1/8" restrictor will pass 2.13igpm water

Now Peter's 1/16" restrictor was passing 0.25igpm OIL, so oil gives about 50 of the water throughput. I have included a 1/32" reading for interest, but even that would pass a good half pint (a fair sized oil can's worth) every minute of oil. But, would you be happy with such a small orifice in an unfiltered oil system? I would have reservations. Having said that, I've just checked on a spare set of con rods in the garage to ascertain the size of the drilling which squirts the oil up into the bores at each revolution. It's 1/32"!But what if the pressure drops to lOpsi, which we all know is rather more than possible? The following would be the theoretical throughput:-

1/32" = 0.076 water / 0.038 oil igpm
1/16" = 0.308 water / 0.154 oil igpm
1/8" = 1.23 water / 0.615 oil igpm

Remember that by the time the oil pressure is down at l0psi the oil will be fairly hot and viscosity low, and consequently the flow will be somewhere between theoil and water figures given above.What we really need is someone out there to make up a jury rig to measure theactual flow of oil at different pressures and temperatures through various sizes ofrestrictor orifice. That would put an end to all this theorising nonsense! Liquidpressure is a bit like that new car you drive out of the showroom - as soon as youmove it it depreciates.

Andrew Macqueen

 

Once More into the Breech Tony Meade, June 1995

Having noted with interest the various points that have been raised on the subject of the four cylinder engine lubrication system, I thought that a delve into the drawings archive might provide a few answers.
In the February '95 magazine Ray Waters mentioned the Pitman Book of the Three wheeler which does in fact show the V9 van set-up.
The parts lists clearly indicate that the early type of cam shaft oil feed was used up to engine No.F1048, with the oil pipe attached to the cam shaft bush from engine No. F1049.
The drawing for the oil feed nozzle shows the 'calibrated orifice' to be 1/32" on the drawing dated 9.5.31, changed to .040" on 13.11.31.
What is definitely worth checking is the condition of the oil pump gears themselves. Up to engine No. F209 the gears had 11 teeth, length 1.000" - 0.999",diameter 0.929" - 0.928". On and after engine F210 there were 9 teeth, with the same length and diameter (June 1932). The tolerances were changed on 12.3.35 to 1.0005" - 0.9995" and 0.930" - 0.9295" respectively, which would have increased the oil pressure. The oil pump used on Series 2 Scouts had gears of the same diameter, but length 1.3755" - 1.3745", at which time the oil pressure release valve was changed from 55 psi to 45psi blow-off pressure. The bore of the oil pump body is shown as 0.931" - 0.930", with depth 1.001" - l.000".
In one letter there is mention of the oil feed to the cam shaft being of 1/4" diameter on the later engines, but this is incorrect. Certainly the bore from the camshaft bush to the tappet chest is that size, but it must be remembered that the oil has squeeze around the cam shaft bush oil grooves before it reaches that hole. The cam shaft itself also had an oil groove machined on the rear journal which is shaped so that it lines up with the oil pipe hole and the hole in the tappet chest only once per revolution of the cam shaft to meter a squirt of oil to the tappet chest. I would be very surprised, looking at the size of this groove, if the amount of oil was as great as in the earlier engines, and its position on the journal is strictly controlled in relation to the cam lobes. The clearance in the rear cam shaft bush should only be 0.0015" - 0.0035", so the amount of wear is important. I have still got the original shaft and bush in APH 775 which, although well worn, still gives 45psi running pressure when hot.
With regard to the oil transfer device feeding the crankshaft, PSB noted the importance of the clearance between its components, which should only be 0.0001"- 0.0011" on the diameter. When I checked this on my original car, MJ 2033, I could get a 10 thou feeler in the gap, and only 20psi running pressure when hot!In APH 775, the clearance is about 2½ thou.
The final item which other scribes have mentioned is the pressure release valve.I have, on several occasions, had almost zero oil pressure on the gauge, which has been miraculously restored when I've cleaned the valve, even though i couldn't see any dirt. There were at least four different models according to the parts lists in my possession, with the part number and pressure stamped on the solder sealing plug in the end. Trikes should have 36-431, set at 55psi; RWD and Series 1 should have 36-432, set at 45psi; Series 2 to Series 5 should have 38-155, set at 55psi(22.7.35), but changed to 45psi (20.6.36). The components of the latter two types are identical, 38-155 being marked with RED paint, 36-432 with GREEN. Having looked at the drawings of the valve bodies and the housing in the timing cover, it is obvious that for the valve to work it must be screwed down tight on to the timing cover before the locknut is tightened, otherwise oil can leak round the end of the body into the timing cover, thus reducing the pressure. It is worth checking the seat in the cover casting for blow holes or corrosion because this will have the same effect.
In Jeff Calver's letter in the January '95 magazine, he suggests cutting off the oil supply to the rear cam shaft bearing altogether (if I interpreted his article correctly). The crankshaft does NOT splash oil into the tappet chest - it can only enter via the oil feed pipe. The oil in the tappet chest lubricates the tappets, the front cam shaft bush, and the oil pump drive skew gears via the various drillings and the cam shaft worm oil feed nozzle, which is the hexagon headed device with a 9/^'diameter bore in the floor of the tappet chest. It is this device which sets the level of oil in the tappet chest - any surplus flows through its bore on to the camshaft worm below and thence back into the sump. There are also three angled holes 3/8"diameter at the back of the tappet chest to drain surplus oil into the sump. I discovered while researching this article that in August 1936 there was a modification made to increase the oil level in the tappet chest by blocking off the rear two angled holes and using a longer headed oil feed nozzle. The purpose of this was to increase the oil supply to the front camshaft bush (which must have suffered with the Scout engine tilting backwards). It would, therefore, seem worth using this modification on any Scout if it isn't already fitted. I am sending copies of the relevant drawings to Peter Nicholls with this article but if he is unable to print them the numbers are: D.M.693 modification to cylinder block 'to increase oil supply to camshaft bush 38-57 Oil hole plug'.38-58 Camshaft worm oil feed nozzle.
This would not, of course, be necessary for the trike engine.

Tony Meade

 

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