Scooterotica

So, bought a second hand gt186 kit last year and done a little bit at a time and now am finally getting on with the build in earnest.

However, on measuring the ring gaps (top of the stroke etc) they are both near on 20 thou So, I've ordered and received a new set directly off Rich Taylor and just gone to fit them and check the gap again. These are just below 18 thou

So now im thinking the bore or the piston itselfs not right.

I know it's not the most accurate way to measure but on measuring the piston to bore clearance at the base of the barrel and the piston skirt I can get a 6 thou feeler gauge in there, and an 8 thou at a push.

I don't seen any evidence of piston slap and i know that these seem to have bigger tolerances but surely there's something not quite right and I just can't see it?

Any thoughts anyone?

well either way thats too big so the only option is to get an oversize piston and get it bored to match with the correct clearance.
I suppose thats always the problem with second hand parts, youve no idea of its history!

I once had a cast top end piston to bore clearance at 7 thou to save on the running in period, ran as sweet as a nut no issues

Admittedly it was just a temporary measure which got me out of a hole, but it's still in the shed if ever I need it again.

Good luck with it.

Sometimes it's best not to over think these things.
Build it and run it.
Next course of action will depend if it will rattles or not.

It'll be reet!

SF

LOL.

Thanks all, it does just feel like one thing after another with it.

I measured the bore at the top end and Im getting 63.73mm, then measuring the piston itself, again at the crown end but below the rings Im getting 63.57mm.

Thats only 0.16mm which is 6.2 though so definitly seems too tally with the feeler gauges :-/

I've a mate with all the proper equipment including proper bore gauges so will annoy him to check for me

Or buy oversize rings and file to suit.

Secondhand stuff...I never learn. Recently bought a "one season" used Rapido 225. Piston to bore 5 thou but rings 32 thou !!

Heres a low mileage asso piston with 7 thou clearance just about to break its skirt off

Sent from my GT-P5110 using Tapatalk

Donnie wrote:LOL.

Thanks all, it does just feel like one thing after another with it.

I measured the bore at the top end and Im getting 63.73mm, then measuring the piston itself, again at the crown end but below the rings Im getting 63.57mm.

Thats only 0.16mm which is 6.2 though so definitly seems too tally with the feeler gauges :-/

I've a mate with all the proper equipment including proper bore gauges so will annoy him to check for me

you do realise that pistons are cone shaped?
they are not as big at the top as they are at the bottom. you need to measure the piston at the skirt end!

Yep I do but for some reason I forgot, god it's ages since I built an engine and im paranoid on this one due to the reed valve and overthinking everything!

Right, just gone out and remeasured at the piston skirt which is 63.8mm and the bore at the cylinder spigot is 63.92mm so that's 0.12mm which works out at 4.7 thou.

Looking at that its likely ok then and I've been a muppet, however the new ring gaps surely still too big at 18thou still.

Is it ?
Surely the measurements are the diameter of piston and bore. Clearance at any given point is only 2.35 thou.
Or am I being a muppet ?