A65 ENGINE ADVICE
I have several complete BSA A65s plus engines. I am piecing one good one together for a Tribeezaha project; a 500 Triumph frame, BSA engine with RZ Yamaha wheels and forks, and I’m most impressed by the BSA unit 650 engine. However, I can’t get the logic behind the commonly-discussed engine mod, that changing the timing side main bearing is good idea. Why is it that putting in a weaker bearing is considered an improvement which will stop the left connecting rod from seizing?
Consider some basic laws of physics here. If the oil supply to the crank fails then both rods will seize. A plain bearing is several times the strength and load bearing capacity of a roller or ball type; all car engines use them. I have heard all the stories about how the end float needs to be dead-nuts blueprinted, and how BSA’s design of the plain bearing is all wrong. The suggestion is that you need to spend an amount of money equal to the value of the bike on a conversion to make it reliable enough to ride any distance. And don’t even start the engine if you don’t have the special steel oil pump…
One of my engines has about .065” end play and the main bearing clearance is .009”. It must have run for quite a while to wear that much! I was looking at the engine with the head off and could not help but notice that the tops of the pistons did not match up...there was a .009” step between them – but I’m more concerned with how much more I can cut off them to lighten them.
I got the engine cheap enough because it had thrown a chain and broken the casting, which had then been cut away. So I got out my power grinder and cut, cut, cut and then clean, clean, cleaned. The motor looked like it might have had its oil changed sometime around 1970, so I made sure to blow out the gorp and grot to get the crankcase as clean as I can. Looking down in the crankcase, there are funny gouges. Hmm. The guys who busted these castings out of the sand were a little long at the pub that day, eh wot!
Next I groove on the design of the mounts (like, peace out man) because I need to make a mount for the bench so I can jig the remains of the 500 Triumph frame around the BSA engine. This will use the front and top downtube and centrestand mount, so I can use a bolt-on hardtail section. (No it’s not a bobber. It’s a super lightweight street racer, geared for redline at 80mph.) Looking at the bottom and rear mounts and the mount-mounts, I have another cold one while I admire the engine design and the challenge it will offer to my engineering skills. It’ll either be a testament to my abilities or perhaps I will prove Darwin right…
Measuring the bottom mounting bosses, I see numbers stamped on each side of the cases and they are different. I check my other Beezers and the numbers on those cases are all the same. The trouble light (all mechanics know why it’s called that) comes on over my head just like in a comic strip. I think about the step between the two cases, the funny gouges inside which, upon closer inspection, reveal small cracks outside. This motor has been grenaded and they replaced only the left crankcase half. And even with a step like that it
still ran long enough to wear the main bearing. To me this sounds like a fantastic engine design!
Getting further into the A65 engine, the transmission appears about 50% stronger than the ones in my Commandos... and on this one there wasn’t even a spring fitted on the kickstart. Somebody had put a lot of hard miles on this BSA and it stood up to the abuse.
The crucial point for anyone rebuilding an old A65 is that you should seriously consider pulling the engine apart and cleaning the crankshaft sludge trap. To my way of thinking this is the cause of the bad reputation of the A-series engines. They were so good that nobody did the maintenance, so the trap filled and the left rod seized and somebody came up with the idea that they could make a living off of offering a bearing conversion they said would cure it. I challenge anyone to show me a BSA where only the right side seized!
This is not peculiar to BSA engines either. I recall a recent article in RC about a maroon 500 Triumph that took out the left side rod. All twins are subject to this if people don’t do the maintenance. ‘Oh we won’t be riding far... I change the oil often... It’s so original… I wouldn’t want to spoil the patina... Factory original, never been opened up...’
Well I hate to tell you: they need to be clean internally. I am going to take a close look at the drillings in my crankshafts to see if they can be modified such that the detritus does not accumulate in the sludge trap. Installing a modern filter should do what the laws of physics in the form of centrifugal force have done in the past, with the ultimate result being that the bike reminds the negligent user that they didn’t clean the filter.
It is through bitter experience that I relate this wisdom, having gleaned it by the catastrophic failure of the left side rod in my first supercharged Commando, a failure I attributed to excessive wrist syndrome at the time. In retrospect and with retro-inspection of the crank this was not a failing of the engineering of the bike. It was a failing of the previous owners who never cleaned it out.
These old British engines were designed by people who were steeped in the tradition of pulling off the oil pan (sump) of their car engine, and pulling a few shims out of the bearings to tighten them up every couple of years. By the 1970s that maintenance had dwindled to getting a tune-up every six months when they changed the points. Remember getting a tune-up? We don’t even change oil now! This is a safety thing: I know what it’s like to be rolling on the throttle and grooving on the sensation at 80mph – and then having the rear wheel lock. It is a pathway to a great spiritual experience…
Please consider this level of maintenance. No one wants to buy a wrecked bike from an estate sale.