The photo doesn't show it clearly, but the piston that's front and centre has some 'lovely' marks from some vice jaws. It has also been dropped, more than once. It was in my YZ125D when I first bought it - it was rattly as hell, and the lack of compression meant that the only way to make the bike go was to rev the hell out of it. Went alright once you gave it a big rev...
Bultaco Macca's ribbing finally convinced me to give it a freshen up after a couple of meetings, but it never blew up.
The piston on the left was from my DT175 racer. The bodgy air cleaner fell off during practice at Canberra. I didn't realise that this had happened until it stopped... After removing the spark plug, I discovered a small oblong shaped rock stuck between the spark plug electrodes. Put a new plug in (thanks Den!) and it went fine. Did three more meetings like that, including one at Clarence where the thing was pinned in top gear, trying (dismally) to clear the big table-top in the middle. Then I pulled it down to discover that my little oblong rock didn't begin life as an oblong - my poor little DT175 top end had made it into one...
The piston on the right of the photo came from my MX250. The "sponsored rider" (we'll call him Marty to protect the innocent - and incriminate the guilty) forgot the golden rule of "change up when the power flattens off" and tried to find more power by revving it harder... The poor old DT piston and its poor old dykes ring didn't like 20,000rpm and the top ring exploded. This one actually stopped - but only because the bits of broken ring ended up in the main bearings... There was no piston drama.
I managed to not get a decent photo of it, but the 4th piston is a DT175 piston with the holes very roughly enlarged to something like IT175 spec. It came in an IT175 I bought - I can't vouch for how many hours its done, but it clearly isn't new. Again, no sign of dramas because of the shitty piston.
Maybe the moral of this story is that Yamahas are built better than Rotaxes, but I doubt it. I think the real moral is that I shouldn't let Marty ride my bike... The secondary moral is that some corrosion
on the surface of a piston isn't going to worry it.
A good mate had a motor built for his rally car (Datto 180B). Proper motor it is too - it makes about 20kW at the wheels more than anyone thought was possible a few years ago. The engine builder is a talented dude who has a reputation for being a bit rough - this roughness resulted in my mate's new motor killing a head gasket before it had been run in.
My mate pulled the head off, to discover that one piston had multiple (incorrect) valve reliefs cut in it. He was almost hysterical, but was eventually convinced to put the bloody thing back together with the 'bodgy' piston. It made the power it was supposed to, and has given absolutely no grief to date.
