Some very interesting comments turning up.
Agree with Slymo – my 800cc vee twin has modified Fiat 131 pistons. And sand casting a piston should actually be within the scope of a competent DIYer. The pattern required would be fairly easy to make. I recall one sprint bike that was the first locally to achieve 100mph – using a piston made on a TREADLE lathe.
Trev and Musk – I’d expect maybe a 2% difference, but 54 / 65 sounds a lot. If that’s to get smoothness around 5k it must be rough as hell low down. B31’s are 58% and pretty good at that.
BSA-54 – 2,000 different BSA pistons sounds a lot. Even if it’s true, our requirements are far less. As Slymo suggests, the more unusual models cannot realistically be catered for and owners will have to make do. For the rest, I’d guess about 30 models would cover most BSA classics that are actually ridden. Three sizes would be adequate – 030, 040 and 060 – which makes around a hundred. Add in a few models with alternative compression ratios, and we’re still under 200 types of piston. Now we’re not talking pressure die-casting here (something I’m well acquainted with) so I’m surprised someone hasn’t set up to sand cast small batches of, say, 50 of each of the most popular ones. Less popular ones could be cast to order, with machining carried out on an as required basis as orders are received.
Other manufacturers (eg Triumph, Velocette, Norton) made fewer bikes and models than BSA. and many of those would fall into the ‘unusual’ bracket nowadays, so to cover the spectrum of commoner British motorcycles would not be beyond economic feasibility. Or is that what BSA-54 is telling us JP do?
Sluggo – couple of answers to your queries ..
I agree, different engines need different approaches. I always checked sizes (always found within .0005) and bored / honed to fit. Weights I didn’t bother about unless a race or fast road engine. Quality pistons from the same box are very close. You don’t get paid for doing unnecessary work. A piston from an average diesel engine will weigh around five pounds and the conrod not much less, so a gram here or there is of no consequence. Add to that the need on some to machine the piston crown to get correct bump clearance, and we’re talking ounces, not grams. If a lot of crown machining was required (sometimes including deepening valve pockets) then I would weigh. Only time I would weigh a piston for a single cylinder (motorcycle) is if I was making a change – eg to a high comp piston, which would be heavier and depending on the difference I would want to check the balance factor.
Parallel twin engines were rare in my workshop, but agree that with their susceptibility to vibrate they might benefit from matching. However, if the pistons are of good quality there’s unlikely to be variations sufficient to affect a road bike. I’d be more concerned about the total weight and its effect on balance factor, as vibration is more likely from the latter than the former. If in doubt, the only way is to set the crank up on parallels and check. If going that far, and unless there is a known good balance factor for the engine / frame assembly, I usually modify the flywheel so balance can be altered without dismantling the engine again. Especially useful on newly-designed engines or when a frame has a different engine (eg Triton) when the balance factor can only be a guess.
My present nightmare is a Weslake engine in a sprint bike. By rebalancing I can reduce vibration to almost acceptable levels at low revs, only to be riding a jackhammer at 5,000. Or vice-versa. One of these days it’s going to fall to bits on the start line so I’m currently modifying the frame in an attempt to change frequencies.
Never known a Gold Star rod to fail (not to say some didn’t). The bikes I raced did hundreds of hours on methanol at 13:1, but then I always kept under 6,000 rpm. Best way to ensure rod reliability is not to ding it and cause a stress-raiser. I always protect rods right up to cylinder going on.
Just have no confidence in alloy rods. Never had, but then again if you saw the rough, spindly rods in Jaguar XK and early Aston engines, you’d wonder how they stayed inside either. Usually. I did see one Jag rod outside, but it was being raced at the tim