I have just stripped a B Series engine that was making alarming noises and found a problem relating to the use of an incorrect timing side main bearing....
These are supplied by at least one shop (who I won't name to avoid embarrassment) as a replacement for the timing side lipped roller. Dimensionally the bearing is OK but it does not have the 'lip' of the standard bearing.
The problem in this case was that the inner bearing was not a tight fit on the mainshaft.
Over a period of time it moved along the shaft, through the unlipped bearing outer and started to eat its way through the crankcase wall, which isn't very thick at that point. Needless to say there was also nothing like full engagement of the rollers with the outer race so load capacity would have been seriously reduced as well...
Loose bearings are a fairly common occurrence as the engines get older and the problem should be rectified if they are too bad.
However, the point of the posting is to highlight a possible problem resulting from the use of a non standard bearing type which is being sold to people...Ian

thanks for that ian i am rebuilding my motor this winter can i place my trust with russels for bearings or whom would you rekamend thanks for your time martin

Am I correct that this can't happen with M20? Ball bearing should keep inner bearing on its place.

Thats right Kalle...the problem can only occur in B Series motors or the early '8 head bolt' M20/21s which don't have the extra bearing fitted..Ian