Good people of the BSAOC, I am confused by some of the information I am finding in various reputable sources such as BSA Parts Books and Roy Bacon publications. I am hoping this forum can clear my head....
I have acquired an M33 engine in bits, worst way to get them I guess!
So far so good. According to the engine number (Roy Bacons book) its a 1956 engine. Number is BM33 2143.
Its been completely dismantled and missing one cam and the piston. I am trying to identify which piston I need to get for the rebuild.
Again, according to Roy Bacon, M33s and B33s had their conrod length reduced by 0.5" in 1953/54.
So I assume it should have a short conrod. But the conrod with it is a long one, it measures 7.375" on the centres. It has the casting number 66-483. I trust the guy I got it from, it is the rod from the engine for sure. I know about the stroke being the same for both short and log rod engines, and about the part numbers being one out from the casting numbers too.
To further add to my confusion the BSA Parts Book for M33 (on the BSAOC Website) shows the conrod as 65-1613. The B33 Parts Book also shows 65-1613 for the B33 with 65-482 for the B31.
Searching this forum I found the information below in another post, which contradicts the parts book for 1954/57 for both B33 and M33.
quote author=JulianS link=topic=4097.msg20069#msg20069 date=1536220503]
The stroke was the same for all the B33 engines, the later ones had a short con rod. The long rods have casting mark 66 483 and the short rods casting mark 65 1614.
So I guess the upshot is that the parts book is correct as far as the engines when they were despatched is concerned, assuming that Roy Bacon is also correct abut the changes made on 1953/54 to the short rod for the 500 engines.
The only conclusion I can come to is that at some point in the past 64 years this engine has been rebuilt using a long conrod, even is if was originally built with a short one.
I'd welcome any feedback from all you aficionados as to whether I have got this right! Were the short rods more prone to failure or are they just rarer? At least I know which piston I need.......I think!
Welcome to the forum. An excellent post. Good luck and hope you get sorted.
The rods for the B31 and early B33 are the same. If you are running a standard compression piston I don't think you will notice any difference in the running with a short or long rod. I have a B33 and an M33, the M33 runs with the long rod. Funny enough the M33 is the quicker bike. The long rod can be a problem if tuning is done. Not likely to affect you unless you try to tune it like a Gold Star
The part numbers in the books are for the rod complete with small end bush whilst the casing number is for the bare rod. Often the case with casing nubers being 1 higher than the complete part.
The parts bulletin from October 1952 shows the new parts for 1953 models.
It is quite possible that in the past 65 years the crank assembly has been changed by a previous owner due to a big end failure and that an earlier crank was availible for repair.
I think that, as long as you have the correct piston for the conrod fitted, you are unlikely to notice any difference to a short rod setup.
I would however be interested in why BSA changed the rod length to a shorter one just for the 500s. Anyone know?
What exactly the pro and cons are seems uncertain. I found the link below, which is a short attempt at explaining things, but I can't see how it would apply to an old thumper like a B/M33
http://blog.diamondracing.net/understanding-rod-length-compression-height-and-crankshaft-stroke
cheers
Pete