My current cases are not a matched pair, but now I have a clean, good set. Can I - with all due precautions - move the three bearings over to the new cases? Or do I have to go to the considerable expense of new bearings? Please advise

email (option): viaconsu # planet dot nl

Hi Hans,

If the bearings are good, I don’t see why it cannot be moved.
Of course you have to press only the outer crowns when taking them out and in.

I have seen BSA “mechanics” doing much wores things, like finding a used un pitted crown
For the magneto or engine main bearing and match it with inner part of totally another (also used) bearing and install it.

I personally think it’s a crime against nature to do so, but it’s cheap, people don’t understand and pay,
And it works…


Noam.

email (option): noam10@gmail.com

Hans, as long as the bearings are not worn or pitted then I see no reason at all why you shouldn't re-use them. M20 main bearings are pretty bullet proof and known for their long life. Ian has seen them last for over 100,000 miles.
Wait till Mrs Hans is at M&S before putting the cases in the oven though.Ron

email (option): ronpier@talk21.com

Hi Hans..No problems using the main bearings again if they are good with undamaged/uncorroded races etc. Likewise with the crankcases...you can mix odd halves on these engines without any problems...
I would fit four, not three main bearings though... ...Ian

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

Single lipped rollers are easy to inspect so yes, but double lipped ball type are not easy to inspect but are extremely cheap so i would always replace the ball type. I have the 6305 bearing 25x62x17 here, they are only £10 new i will post you one if you want.

Hi PJ...Your right about the difficulty of thoroughly inspecting the ball bearings...but the drive side ball bearing is a 6205 C3...and is still quite easy to get in the correct C3 fit...Ian

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

That's what it is a C3.
Having said that i never simply fit the recommend clearance, i check what it fits like, on an old crank, c3 are often not tight on the journals & i rather use the bearing that is going to give me the correct running clearance for the engine im building,

The drive side ball bearing shouldn't be a tight fit on the mainshaft...A push fit by hand is specified by BSA, though in practice the difference between that and a looser fit is minimal and that is usually the case, as wear on the shafts is common.
The crank has to be pulled through the bearing by the tightening of the cush drive to position it within the cases.
C3 (and other fits) indicate the degree of internal clearance in the bearing, inner and outer diameters being identical to a standard fit bearing. The C3 specification allows for the 'squash' of being shrunk into the crankcase, vibration when running and the higher temperature environment. So if anything the fit of the bearing into the crankcases is the most important factor.....Ian

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

Journal fit affects internal clearance aswell as casing fit.
Hoffman's 00 spec was for new engines.
If you are going to fit mains its best to fit it all up & see how it feels & that's better than reading the original bearing spec & just fitting that.
Im not talking about an interferance fit when i say tight, i mean "not loose' :-D

Never had problems & the first m20 i put bearings in was in 1979 :-D