cornishflyer

I'm trying to find which piston I need to order. Does anyone know where Sherco measure their pistons? On the piston crown it has 72.75 stamped, yet when I measure just below the gudgen pin holes, where I think I'm supposed to measure, I get a reading of 72.57. Even at the skirt of the piston it's not close to 72.75, perhaps that is the most worn part? I've discovered that the skirt is the widest part of the piston, they are oval and tapered to account for expansion at different rates. I've tried using feeler gauges (not the best way I know) to simulate a new piston + the recommended clearance of 0.02 and it's a very tight fit in the cylinder. If I am measuring in the correct location then that's a lot of wear, I might be good just going for an A or B piston.

Thanks everyone so far for the comments. I thought I read that ball hones are not ideal for nikasil because the coating is porous and the grit can remain in there. At this stage though I can't see how I could really make it much worse. I'm thinking of getting a D piston in there which is 72.78mm at it's largest diameter, I'm not having the best time measuring the cylinder but I figure there has to be enough clearance.If not (once I measure it with feeler gauges) I'll then have to to the re-plate. Hopefully having a larger new piston will stop the slap till I can afford the full re-plate. Cheers

Thanks Section one. Just pulled the top end apart, and sure enough there is quite a bit of wear (see pictures). From the state of the nuts when I tore it apart it feels like this is the first time it was disassembled so it's not unexpected that there would be some wear I guess. There is some scoring on the exhaust port side but most of it is below the port where the rings don't reach. I have smoothed the worst with some emery paper. There isn't really a ridge at the top of the cylinder. There is play in the little end so I will replace that, also I can feel a slight ridge on the gudgen pin. There is also quite a lot of play in the piston/cylinder clearance, way more than the 0.03 recommended in the specs that I found on the net. This is the A piston, I wonder if I could get away with just ordering another A piston or would I have to get a re-plate the bore and go for the B piston? I measured the piston and it's oval, I wonder if that is from wear or if they come like that. The big end feels pretty loose side ways, and it does twist a little bit but I can't really feel any up and down movement so I'll leave that I think. Given that this is an oldish bike I need to decide how much to spend on it, I'm hoping a new piston, rings, little end and gudgen pin is going to sort it out..... or am I chucking money away without re-plating that cylinder? Cheers

Hi all, First post here, hello! I know there are a number of threads already on the forum about top end noise, but I figure another can't hurt as it might help someone in the future. Before I strip the top end of my 2007 ST250 I would like to get the right parts ordered. What does this sounds like to you? I'm hearing the occasional pinging type rattle (not pinking). To me it's either little end on the way out or piston slap but I haven't discounted primary gear. The noise is unaffected by clutch movement, and it's pretty much the same as the engine warms up. Thanks, Dylan