Scooterotica

stripping motor to rebuild gearbox and noticed some scoring marks above exh port, can feel with fingernail. marks on piston to a similar effect but cant feel them, and dont recall seizing! so to me piston seems fine apart from some blowby, so have checked ring gap and clearance. The marks have appeared in the last 200 miles, as i took barrel off to do some porting work. Dont think its the chamfer, and alot of crap came out of the u bend for my fresco so have a horrible feeling my old exhaust has been shoving dirt back into the cylinder, or could just be open mouth carb but these have only been on for barely 200 miles.

Either way... I bought the kit (SR kit with asso piston) after it had been freshly rebored for a 63mm asso piston. So I have (wrongly) assumed that all tolerances had been checked!

When i took the top ring off it snapped, seemed quite brittle to me? Normal? checked gap and could fit my largest feeler guage in at .8 mm.

Piston to bore is somewhere between .15 and .2, i can get .15 down side of piston, but it wont drag piston up as i pull it out, but cant get .2 down at all, only a teeny bit.

really I want to reuse the piston, but im not sure if thats viable now. Ideally i just want to bolt it all back together and forget about it but hey....

So. Am I looking at a hone and new rings? Is the scoring bad enough to warrant a rebore, and is the piston bore clearance to large to consider anything but?

And as far as i can see, there isn't a whole lot of piston options; I couldnt in the brief look i had yesterday find any asso pistons in stock anywhere. Apart from rather expensive routes ie RT pistons or jap pistons plus rod change (which id love to but money) everyone is selling these no name indian pistons which i assume are copies of the sr piston....

either way first let me know if i need to bin this piston or not! AF sell rings for it, but those old rings hadnt even done 1000 miles so im stumped.
cheers all.

Was it running o/k before you opened it up?

yep. no problems. i could be imagining it but i feel at some point it lost a bit of compression but that could just be my imagination. Thing is its niggling me, and although strapped for cash i dont want to put it back together with clearances that i think are too big! Im seriously considering a suz 190 conversion but thats more money aha

Put new rings in it and carry on.(I come from the school of 'if it ain't broke, don't fix it' )

yes id love to do that, but wont the ring end gap be pretty similar with new rings? my thinking here is that theres not many miles on this kit, and i have a suspicion it may not have been rebored with piston to a good tolerance, or i have a soft liner. but i think im just catastrophising the situation. if anyone could confirm that my clearances are way out that would be useful!
Thanks so far!

Measuring with feeler gauges is not an accurate way to measure piston/cylinder clearance at all. You need to use an inside and outside micrometer. Leave the feeler gauges for measuring gearbox end plate gap and spark plugs.

After your porting work, did you hone the cylinder? If not then that's another problem. When working on the ports the sleeve will actually protrude in to the bore slightly at the port that was worked on. This creates all sorts of problems, like sharp edges and metal that will break away or cause abnormal wear to the rings. There are tons of videos on porting that explain this.

Before I'd spend any money I'd take the top end to a trusted two stroke machinist (not the same one you went to the first time) and have them measure the clearances. I also run a SR cylinder but with a RT 64mm piston. Still going strong 3k miles later and I ride it very hard.

Cheers for that mate. To clarify, i bought the kit already bored for the asso which came with it, hence my doubts as to wether this was done properly. I did also chamfer all the ports too, wasnt aware it would need a hone if ports wrre chamfered? Will get it professionally looked at and work out if i can keep current set up, ta!