Mind the grind

Classic Bike (UK) - - WORKSHOP -

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

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