Mind the grind
Nik Mace emailed my website (rick. parkington.co.uk) to ask if I could recommend anyone to regrind/refresh the big-end of his 1937 Ariel Red Hunter and balance the crank.
Well, I’m not sure I’d go that way, Nik. It used to be common to address big-end wear by fitting slightly oversize rollers, but uneven loading at the big-end means the pin is unlikely still to be round so simply fitting larger rollers will result in tight spots. The proper method is to regrind the pin and rod eye as necessary and take up the slack with the larger rollers.
Fine... except that crankpins are not through-hard – otherwise they’d snap like
a carrot. Typically, they’re surface hardened to a depth of around 40 thou (1mm) before finish grinding, the rest remaining ‘soft’ to retain flexibility. The hardening is like ice on a pond; if it’s thick you can stand on it, if not you get wet. Grinding the hardening is like thinning the ice – it reduces support below and can result in the surface cracking and flaking off. Damage like this means you need a new crankpin. A light grind, say 5 thou, to remove slight wear is OK, but the next problem is obtaining oversize rollers – they’re a thing of the past and hard to find. If you’re incredibly lucky, you may get away with just fitting new standard rollers – in a crowded roller bearing they rub together and wear faster – but I think the Red Hunter has a caged bearing that makes this less likely.
So would I trust anybody who offered to regrind a big end? Maybe but I’d want to strip it and decide if it was within limits myself. Probably consulting Ariel suppliers about obtaining a new big end is the best answer.
Light wear may be ground out; pitting (right) is terminal