Quote Ian: Interesting about the oil groove...I posted about potential main shaft differences when I spotted different part numbers but have yet to get all mine out and do some comparisons...I was hoping someone might know the answer...Personally, I wasn't aware of any potential differences until recently and I've handled scores of them over the years!!...

Lubrication design issues aside, I find the quickest way to destroy the sleeve gear bushes is not to maintain the gearbox oil at a sufficiently high level...With the standard set up and no sealed bearing, particularly if the field stand is used, the oil level can drop to critical levels in just a few hundred miles...I recommend a check every 300 miles...I'd be interested in anyone else's view on this....Of course with the sealed bearing fitted the situation is much improved and that should be considered a 'must have' modification at rebuild time...Ian


Hi Ian, Could not find the old thread about differences in the mainshaft but here is a picture of the difference in oil grooves:



It looks like the oil is picked up by the short, longitudinal, deeper groove and forced into the sleeve bush by the spiral.

Some will collect in the middle of the bush where the relief is. The groove at the end of the RHS mainshaft clearly tries to keep the oil inside.



Think the 3 holes in the gear act as a sort of exit/return, oil inside the holes will be forced outwards when spinning, it creates a circulation of oil through the sleeve bush.

When the oil level drops the short deep groove in the mainshaft will pick up less oil and the circulation gets worse or even halts and the wear starts..

Yes, today we are luckily blessed with sealed bearings to keep the level up! . BR Michiel

HMMM! I thought the three holes were for the oil to go in. Never thought about it like that:thinking_face: Ron

email (option): ronpier@talk21.com

I guess it's safe to assume the left hand shaft is the later one...I'll have to go through my parts lists to determine when the change was made...Could it be the tapered felt is not there primarily to keep oil out of the clutch but to keep it in the bush?....I've never had oil contamination in the clutch even though I don't fit the felt....Ian

email (option): ian@wright52.plus.com

How full is full? I fill mine up to the bottom end of the thread for the filler cap.

That's about right...If you can see the oil in the 'spout' when you remove the cap it's OK....Ian

email (option): ian@wright52.plus.com

Not sure Ron but just my analysis. There are 3 holes in the 3rd gear on the mainshaft also, one hole can be seen in the top picture. When they act as an exit probably some oil will be drawn into the bush along the mainshaft also...

Think they tried to get some circulation going with all the grooves and holes..... when you look at the kickstart end of the layshaft, the spiral forces some oil into the blind layshaftbush, the surplus oil can flow back through the long hole in the center of the shaft and exits in the heart of the 1st and 2nd gear bushes.

I've checked out some parts lists and as is often the case the result still does not definitively answer the question...

The 1940 WD list and the 1940 'combined' WD and Civilian parts list shows the gearbox main shaft part number as 15-4260...

The WD lists for contract number S1048 (09-1942 to 11-1943)and S7218 (08-1944-1946) shows the number 66-3283...I don't have the list that covers the contract before S1048...

However, the post war parts list for side valve models (1949-55) and the parts list for OHV models (1949-52) both show the 'earlier' number 15-4260...

So, possibilities are...Both post war lists have an error showing the earlier number...Not all that likely I'd have thought...
The post war boxes reverted to the early pattern shaft...That seems illogical at first sight but maybe there was no gain in bush life from the modification that was made but the later shaft was more complicated/more costly to manufacture...Possible...In that case a reversion to the earlier pattern would be advantageous...Any other views on the subject..?...Ian

email (option): ian@wright52.plus.com

Yes, think the feltring is there to keep the oil inside and the corkbush is there to keep the feltring in place.

I have just found a prewar shaft which has the 'cross-over' groove pattern like the LHS shaft, so guess that is the early one and the RHS with the separate "return" scroll at the end is the later one.

Probably too much oil leaking out from the early ones ??

So it appears 66-3283 was the later pattern shaft (without the crossover grooves)...

The reason for the modification is unclear. Though probably an attempt to improve lubrication to the bush it might have been an attempt to reduce oil losses from the gearbox by reducing the supply to the bush....Finally, and perhaps least likely, it might have been a production simplification!!.. Whatever it was, BSA seem to have concluded eventually the first pattern was a better option and reverted to that post war....Ian

email (option): ian@wright52.plus.com

Strangely the postwar 47-52 partslist all show the separate 'return' oilgroove in the drawing, so maybe the change in the partnumber stands for another difference besides the grooves..... would be interesting to see what groove is actually in a postwar box.....


Just thinking that the small feltring and the 'return' groove might, besides preventing oil spilling, also slightly increase the oil-pressure in the bearing ??...... less pressure will be build up if the oil just runs out...

Maybe the feltring has a bit more benefits than we generally think?

['Maybe the felt ring has a bit more benefits than we generally think?...']

I'm beginning to suspect that is the case...Regarding pressure in the bush I'm not so sure...Whilst the volume of oil could be controlled by dimensional differences as a liquid will follow the easiest path, something would be required to generate a pressure...Perhaps the rotation of the shaft against the bush and the grooves has a 'pumping effect'...Or maybe centrifugal force comes into play...Or am I just overthinking it?...:laughing: ...Ian

email (option): ian@wright52.plus.com

I have some nos main shafts here I can check this evening
Br
Jo’b

email (option): jonnyob1@googlemail.com

I hope anyone who thinks it's OK to use grease (Semi fluid or not) in an M20 box is reading this. Ron

email (option): ronpier@talk21.com

Yes, think so... the spiral grooves create a difference in pressure, just like a screwpump/augerpomp....the centrifigal force a suction, oil will circulate and refresh the bushes....

I can imagine the viscosity of the oil is also of great influence here... if there turns out to be a special 'WD-mainshaft" the groove might be suited to the oil used by the War Department ??

That could explain the postwar change back to the early version...

But just throwing out idea's, no expert at all and overthinking could indeed be the case :relaxed:

The RAF illustrated parts list quotes the 66-3283 shaft and the illustration seems to show the left hand shaft in Michiel's original post. Ron

email (option): ronpier@talk21.com