I've read a few times (but I can't exactly remember where) that on the unit twins, not only are the crankcases machined as a pair, and therefore supplied as a matched pair (as everyone well knows), but the inner timing cover is machined in situ on the TS crankcase half, therefore it is also matched to the crankcases.
I noticed this three digit number stamped onto the side of the bottom mount lug on the TS case.
There is also a three digit number stamped on each inner timing cover that I have.
Stamped on the bulge that clears the end of the crank.
I think that maybe only once in the last decade have I seen a set of A65 cases advertised with a matched timing cover.
So does it matter in the slightest ?
Nobody seems to be bothered about this.
But then why did BSA bother stamping the matching parts ?
This might seem to be an excessively pedantic question, but I've always wondered if I was doing something wrong just fitting any old timing cover ?
With the milling machines they had available at the time, it would have been necessary I believe to line bore each pair of bearing mounts or bushes with the crankcases bolted together (eg for the camshaft bushes). The same would have applied to the timing idler pinion bushes requiring the inner timing cover to be bolted in place. Once the machining was done, the crankcases and inner timing cover had to be kept together as matched components. In principle, you can mix the crankcases and inner timing cover in a subsequent engine rebuild as long as you line ream the bushes afterwards to give the required alignment accuracy.
With more modern engines machined using CNC machine, I think the accuracy is so great that you can mix crankcase halves even if they use ball bearings or roller bearings throughout. When I rebuilt my non-BSA 1980's bike, this is exactly what I did without any problems.
Hi,
There was 6 different inner covers over the years, some thought will be needed if changing to a different part number. If using the same part number an unmatched pair should be okay (testing if the idler turns freely of course), although in my experience the bushes have been so worn there's been a need to replace them making a matched set anyway.
The kickstart bush shouldn't be a problem, its always well worn.
Best wishes...