Still, pinch has nothing to do with head shims. If an engine is getting stuck at TDC, more head shims is not going to help this situation. The engine just needs to be run in more. Sometimes you have to fight with high quality engines before they run right but in the end will still be worth it.

Brand new piston and Sleeve. The slime/gunk is from Castor re-forming. Castor provides Superior protection when used in 2-cycle applications due to the fact that when it is brought under extreme pressure (generated by friction/ or by extreme heat) its molecules reform/transform/combine into larger ones there by keeping expanding parts from making contact with each other. The draw back is the residue that becomes baked onto the hot spots. So based on this slime issue; yes you have excessive pinch and the engine is being run rich enough that your not "Baking the Castor" but getting close.
So maybe the guy running the synchronizer sneezed and in the process knocked over a couple of p.s. sets and the pistons fell out of there respective sleeves. Being that he is paid "piece work" he put back together which ever one was closest to the other. Somewhere out there, there is a person who just replaced his piston and sleeve. 1 gallon threw there engine its worn out.
You have not mentioned the manufacture? I have a piston and sleeve new in the bag that has never been opened. The reason is the piston is past the top of the sleeve to the point that the entire wrist pin bore is visible. Took pictures and called the importer...Sorry for the book.

it is a nova based engine. X12 Mugen engine.

tight
 

either return the engine or start honing the piston down with super fine sand paper. I know edward used to have a honing sleeve

LOL, then maybe I have your piston! I would return it. The old K&b outboard engines had to be lapped. We used extra fine valve lapping compound and worked the piston in and out in a figure 8 pattern until the bottom of the wrist pin hole was just visible at the top of the exhaust port. I would return it...

A very tight pinch can be caused by by a sleeve that is "egg shapped". This is not very common from the factory but it is very common from having a resize done. Pull your engine down and put the piston through the sleeve unitl it is tight. Shine a flashlight into the bottm of the sleeve. Now look at the top of sleeve and see if there is any small gaps between the piston and sleeve. I know there was batch of Novarossi and Mugen .12 engines with this issue a few years ago.

........so if he adds 1 inch of head shims the pinch will not be as tight?????????....the heads shims will only lower the compression slightly..........the pinch will still be there.........:weird:

OUCHHHHHHH!!!!!!! WHAT THE DAMN HAPPENED!?!? LOL:weird:

Look at it this way... he didnt have to disassemble the engine nor clutch to check for bearing smoothness... :D.

That must really hurt man, see an engine go out like that

Well, back to this subject. The piston/sleeve checked out for being round...no piston slap or nothing like that. Just very very tight. I'll try lapping it with something, but I'm more likely to sell the thing before I ever run it again. Just afraid to snap the rod in it.

you should never lapp a piston and sleeve. when you do this, you remove the cross hatching on the sleeve,
and ruin the fit and finish, also the cross hatching asists in allowing to oil to have better surface tension and
do it's job. it it has no cross hatching, it will be too smooth and the fuel will not work/movearound/and stick
to the piston sleeve surfaces as intended.

:eek:

I have seen people lap the piston leaving enough room for some break in. I've seen p/s sets that are so tight that it either ruined the rod or there was hardly any chrome left on top of the sleeve to create a good seal... You do need to measure before lapping and make sure you don't lap too far down the piston.

May I say something about lapping?
There is nothing wrong to lap piston sleeve as long as you use right tool and after lapping your parts are still round within tolerances (+/_ .002 mm).
Cross hatching is result of surface finish after using honning procedure and it is not intended to be there, just pricing of mirror finish is way to high to have it.
The oil will stay on the wall of sleeve/piston anyway without using cross hatching-it is happened on molecular level of chemestry and has nothing to do with cross hatching. One of the reason why castor oil in use in fuel is that forces between moleculas in castor oil are so great and way better then any known lubricant (with tech specs we need for running our engines).
Edward

Edward lapped a few of my engines and made break in much easier. The cross hatching is normally gone by the end of break in anyways and does nothing for the engine from a mechanical standpoint.

On regular gas burning engines with piston rings, cross hatching from honing helps seat the rings, but has no advantage with a ringless piston.