Piston broke

My old mate Phil Clarke has come full circle. More than a decade after parting with his well-sorted 1970 Triumph TR6 in favour of GS BMWS and a Suzuki DRZ, he’s bought another one. It was a bit more expensive than his last one... but seems to be a good buy in original paint and soon fired up with fresh oil and battery. However, Phil’s experience told him not to swing a leg over the saddle without further investigation.

Everything looked good under the head, but it’s just as well that he decided to lift the barrel, too – because he found one top ring in bits and a slightly damaged piston ring groove.

‘Dunno if this is OK or not – is it worth trying to bash it back in shape?!’ he enquired, speculatively.

I think it needs a new piston. The damage is consistent with the ring breaking. As the piston rises and falls, friction makes any short segment try to somersault, gradually flaring out the groove at that point. You could perhaps get away with fitting new rings, but there’s a danger the aluminium is cracked as well as bruised and a bit could drop out. Also, the ring’s fit in the groove is important. It needs to be tight enough to give support, but still free to move. The clearance also needs to allow a little of the ignited gases to get behind and squeeze the rings against the bore – but not so much as to accelerate wear. Worn grooves also cause ‘pumping’ – ring movement squeezing oil up into the combustion chamber.

I’ve bodged pistons over the years and got away with it – but, in truth, these details are pretty important, so if new pistons are readily available it’s not worth taking the chance.

Get your wallet out, Phil!