SLIPPED DISC
Mike Kelly is rebuilding a 1953 BSA Bantam and asks about the discs on the side of the crankshaft flywheels. These pressings smooth out the profile, padding out spaces to increase primary (crankcase) compression. They have a bad reputation for coming adrift – making a ringing sound after the engine stops – and Mike wonders whether they can be left off.
I’m not sure about this; personally, I’ve had more to do with Villiers engines, which cope fine with tons of space in the crankcase and, presumably, much less primary compression. But on the other hand, they were designed that way – the Bantam wasn’t and it’s possible that the port timing of the two engines differs accordingly; after all, if the discs weren’t necessary, why did BSA fit them?
I decided to ring Swindon area Bantam enthusiast and rebuilder Rhett Fisher (07504 642590) to ask his view. Rhett said that although opinions are divided, he believes the discs are worth keeping, as given the 125’s low power you can’t afford to lose anything. As he put it: “It’s a Tesco job – ‘every little helps’!” Rhett added that he has had new discs pressed up because he finds that part of the problem is that after the old ones have been removed – as required to replace a big end bearing – or worse come loose themselves, they lose the fit that keeps them in place. So he fits new ones rather than trying to re-use them – or trying to get around it by securing them with a dab of weld.
He added that while Michael’s engine is apart, it may be worth thinking about tuning. As it came, Rhett’s 125 did 35-40mph, but with a few mild tuning mods it now rolls happily along at 55mph, which sounds much safer on modern roads.