Classic British Bike Magazine

BSA A65 Thunderbolt

Fixing clutches is a drag, but ya gotta get a grip...


Left to right: BSA A65 clutch centre with combined shock-absorber; BSA/Triumph clutch locking tool (useful, but not essential); four-spring BSA clutch pressure plate. There's no real mystery with a BSA clutch (or any classic British bike clutch). You just have to pay attention to what the drag, slip, rumble and judder is telling you. Alternately, ignore it at your peril.

The Problem


Sumpster David Trevena wrote to us about a problem with his 650cc BSA A65 Thunderbolt. The trouble revolved, literally, around the clutch which had been "giving him a hard time for a long time".


"It was just doing everything wrong," explained David. "It was dragging and then slipping, and then making a burnt smell. It seemed to have a mind of its own. I changed the cable and adjusted the cable routing, but it just kept driving me crackers. The clutch was fairly new (less than 1000 miles) so I didn't figure there could be anything major wrong with it. That was a mistake.


"In the end, I had to get help, so I called in a friend, Paul, who listened to what I had to say, then had a good laugh, and then got out some tools and started taking down the primary side of the bike. Soon we had the entire clutch out and it was a mess. Actually, it was amazing that it ran at all.



The BSA 650cc A65 Thunderbolt. Power, style, and a perfectly decent clutchas long as you look after it and watch for the warning signs of problems.



"This is a four-spring clutch, as opposed to a more common three-spring. In fact, it seems that the bike originally had a three-spring clutch and was converted by the previous owner to four-spring. We reasoned this because I still had a box of bits that came with the bike, and in that box was a very old and rusty three spring clutch that had seen better days.


"But in any case, I think the designs are basically the same. So we decided to stay with the four-spring. The primary chain is triplex. The clutch had six steel plates, and five friction plates making eleven in total. We soon found out that the clutch bearings were shot, one of the steel plates was seriously warped, most of the friction material was blackened and at the bottom of the primary case, and the clutch pushrod was soft at the pressure plate end which was caused by massive and chronic overheating.


"The clutch bearings were so badly gone that the entire clutch was wobbling around a very eccentric circle. Then we took out the clutch shock absorber and discovered that that was ruined too. The grooves on the outside were ragged. Ditto for the grooves on the inside of the clutch drum. Basically, the whole thing was scrap.


"Also, we measured the four clutch springs, and one was distinctly shorter than the others, and when you pressed the shorter one between finger and thumb, you could actually eel it was very different. Paul was of the opinion that it wasn't even a clutch spring at all, but something else that had been put in there. Fortunately, the gearbox bearing behind the clutch was fine; there was no detectable play whatsoever.


"I learned a few things about clutches that day."


Think about movement. Spinning, sliding, and lifting. Clutches take a hell of a beating and do a fantastic job of transmitting power. During maintenance, every component needs to be examined, queried and understood. If you just bolt it all back together and close the lid on it, it might work, but what have you really learned?




The Solution


"It seemed that I had had all the signs of problems to come, but I'd ignored them. Clutch drag seemed to have started it off. I live just outside Leicester, by the way, and commute to work on the bike. One day in hot and heavy traffic I noticed that the A65 sometimes tried to creep forward a little. Then it was okay. Then it wasn't. To compensate, I was more or less constantly using the handlebar adjuster and taking slack out of the cable, and then putting it back.


"Over the course of a few weeks, the creeping got worse. I didn't think the clutch itself was the problem because, as I said, it was fairly new. So I just thought it was the cable binding. I'd bought that cable on eBay, and it was fairly cheap, and it had been hard to fit (the inner was a little too short, it seemed). So my attention was taken away from the clutch itself. I'm much more cautious about what I buy on eBay now.



▲ Aluminium alloy pressure plate. This one for a three-spring clutch. You can fit one if you want, and if you're the sensitive type you'll probably notice some difference. But the standard BSA A65 pressure plate is fine when the clutch is set up the way the factory intended.



"When I parked the bike at work and came back to it at the end of the day, it was usually okay to ride and started easily enough, and I rode it home each night without any real problems. But traffic was always lighter going out of the city than going in. That meant the bike was always running cooler on the homebound trip, and because the traffic was lighter, I wasn't using the clutch very much anyway.


"Paul suggested that the initial dragging clutch was probably the cause. That increased the overheating problem in heavy traffic. Eventually, the heat warped a steel plate. That increased the dragging. That increased the heat. And so on. Also, the weaker clutch spring wouldn't have helped and probably would have behaved differently when hot and cold.



Check the grooves on this new clutch drum. That's ideal. but they'll soon wear ragged. Clean 'em up during maintenance, if you can. And take out any obvious burrs. But the clutch doesn't so much wear in as wear out. Fortunately, it's very forgivving.




"Next, clutch pushrods need a little clearance to prevent it rubbing constantly against the pressure plate. I kinda knew that, but I didn't really pay attention to that fact. Because I'd kept fiddling the handlebar adjuster, I'd regularly taken out the free play on the clutch push rod. That caused more friction, and that friction had taken the hardening off the pushrod end. That added to the vicious circle. If I'd just recognised the fault in the first plate and made some simple adjustments, a lot of trouble could have been avoided.


"We ended up by fitting a new clutch and new bearings. I suggested fitting a seven plate clutch set-up and maybe an aluminium alloy pressure plate. I heard of other people saying that this made a real difference and gave them with a lighter clutch that was less prone to slippage. But Paul said that if we just set it up the way the factory had set it up, and made sure the parts we used were good, and fitted right, that would be good enough.


"Apparently, many years ago, Paul had despatched in Central London on a BSA A65. At that time, there were a few guys despatching on old British bikes, and Paul used to see them around sometimes. He used his A65 day-in, day-out, for about a year and a half, and that bike was box-standard.


"He said that occasionally little things went wrong, but it never let him down. And even on scorching days in Central London traffic, the clutch worked fine. So did the gearbox, which I've had some trouble with.



▲ No matter how many springs you've got in your clutch, you need to check spring length as dictated by your manual. If only one spring is weak, replace them all as a set, and measure them before you fit. There are plenty of useless spares around.



"Yes, his A65 occasionally complained and got a little fretful in hot weather, so he was constantly trying to keep the bike moving in the breeze. The point is, if you can despatch on a 650cc BSA A65 in Central London, you can ride it pretty much anywhere.


"Anyway, we fitted a new clutch shock absorber too. There was no point trying to repair the old one. If the body of the shock absorber is okay, you can open it up and check the spider and the rubbers. The spider rubs up against the back plate, and that can't be allowed to happen or you'll get a rough transmission. So we junked it and fitted a completely new shock absorber. We fitted noew clutch springs and a new pushrod. And a new cable.




This BSA A65 had a four-spring clutch and eleven plates (five-friction, and six steel). But all kinds of variations are likely. And possible. Our advice is to stay as close to the way the factory set it up unless there are compelling reasons to change it, or unless you know exactly what you're doing, and why.



"The whole thing cost me about three hundred and fifty quid and two afternoons of work. Since then, I've had no clutch trouble and no complaints from the gearbox—and the bike has got pretty hot sometimes.


"I asked Paul why the clutch had started slipping suddenly at one point, and he asked what oil I'd been using in the primary case. Then I remembered that I'd put a straight 40-weight in there. Then I drained it and tried a 10W40 simply because another friend had suggested it, and that must have flushed out the 40-weight. Also, it seems that I had overfilled the primary. That's the theory, anyway.


"BSA specified Castrolite for the primary case, and no more than one-quarter of a pint. Castrolite is a 10W30. So when it's cold, it's a 10-weight.


When it's hot, however, it think thinks it's a 50-weight. Something like that anyway. But a straight 30, 40 or 50-weight is too heavy. Too thick. It will cause the clutch plates to slip in a wet clutch. No mistake. And too much oil will exacerbate the problem.


Usually,  when you contaminate clutch plates with the wrong oil, you need to strip the clutch and clean the plates with petrol or something. But I was too thick to understand what was going on, and eventually the clutch plates cleared themselves.


The trick, which is no trick at all really, is to use exactly what the manufacturer suggested, or at least something very close to it. The A65 wants a much lighter oil in the primary case than engine oil. A low detergent 10W30 is fine. You might get away with a 10W40. Or even a 20W50. Or you might be better advised to try a lightweight machine oil or transmission oil.




"Nowadays, I've got a very different attitude about the clutch on my BSA A65. Actually, I've changed my entire view of bike maintenance. I tend to examine the components one by one. I try to understand how the mechanism is supposed to work rather than assemble it like a jigsaw and wait to see the big picture. I carefully watch what I buy on eBay and try to do business only with established bike shops which know their stuff (and they're getting rare).


"The BSA is running perfectly now, and I still commute on it. But as soon as I hear or see or feel anything different, I check it out properly and take nothing for granted. Trouble is, these things sometimes creep up on you, don't they?"




Think you know better? Good, drop us a line at Sump and we'll include your thoughts/experience.






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