Danju

my 3.3 has good compression but it doesnt have any "pinch" when the piston reaches TDC.
if i take the glow plug out, i could turn the flywheel so easily that it barely feels like the piston hits TDC. theres no "pinch". but when the plug is back on, compression is very good...
so is it alright to have no pinch but good compression?
thanks

6speednissan

Sorry, but I've never understood what pinch means. Could you please fill me in?

ttoks

when cold the top of the piston sleeve tapers to a size smaller then the size of the piston, it's fine as long as you have good compression, pinch can be hardly notesable or not existent once you have properly run the engine in.

DaveG55

Sorry ttoks, on the pinch you are wrong.
Every nitro rc engine, unless it's ringed, has pinch. It has to have pinch to have compression. You can have pinch but no compression (like a scratched sleeve) but you cannot have compression w/o pinch. You may not feel it as much as when the engine was new but it's there. It will be more noticable when the engine is cold but, unless the engine is worn out, it's there when warm/hot too. Both the sleeve and the piston are tapered (think of two drinking glasses stacked togeather, one slips into the other but only so far). In fact, if you pull the sleeve and piston from your engine you can verify that you still have pinch by trying to push the piston through the sleeve. The piston should stop between 3/16" and 1/8" from the top of the sleeve. If it goes further then you are close to the end of life on that engine.
As for break-in, the purpose of that is not to reduce pinch (though that is a small side effect) or elimminate it, it is to seat all the parts togeather for proper seals and good mating surfaces. That's why it must run rich during break-in, to provide extra lubrication to flush away the excess metal particles that are worn off of the new, rough surfaces.

ttoks

well thats why i said not notesable, obviously the pinch is there, it's one of the basic pricibles of a nitro engine, but once run it you may not be able to notes it as much or at all.

sorry if i didnt explain that enough.[:-]

DaveG55

But you said "not existant" which is not true.
And if the pinch has decreased enough that you cannot feel it, or feel it very little, then I'd bet that it is approaching inadiquate.
If I could not feel the pinch by turning the flywheel with one finger I'd pull the engine and check it visually.
I've got engines with three and four gallons through them that still have a very tight pinch. My TM, close to two gallons, has enough pinch that it still gives the ez-start fits when it's cold.
Proper break-in should NOT make the pinch hardly noticable or not exist, though it will ease it a bit.

ttoks

non existent as in it's very difficult to feel, again sorry for not explaining that properly.

IOWAREVO

Here's a visual showing a sleeve that no longer has any pinch just as Dave describes. The best thing (for state of mind at least) is to pull the sleeve & piston, and do the check. It will give you an idea of where the engine is in it life.

http://www.misbehavin-rc.com/pit-lan...worn-motor.asp

Danju

you guys are making me nervous...when the plug is out and i move the flywheel, there is very small resistance i can feel. i havent done the pull-out-piston-and-sleeve test but can you guys tell if its out of compression? ive only had a little less that 2 gals through my 3.3 and your telling me its almost dead! lately ive been having ridiculous problems with my engine (i just put in a new pipe).

EDIT: i just reread DaveG55 post #6...and now im really nervous to the point of sweating..i think i have a worn down engine... please tell me my engine is still workable.

IOWAREVO

No...I'm not saying your engine is shot, but we can only troubleshoot so far without being there. I thought my 2.5R was too loose and it was acting like it was worn out. But I checked the piston in the sleeve and it was fine so I looked else where and found a leaky front bearing. You just have to eliminate the possible causes. I would never sugguest replacing the engine until you checked out ALL the possible causes of a problem.

Danju

im still having troubles with my 3.3 with the new THS pipe....is this because my engine is worn or is it the pipe that makes the tune off...right now, i feel like throwing my car to hell.

BAMF90

My Feedback: (1)

When you pull your glow plug out, you are supposed to be able to turn it over by hand.

Put the glow plug back in, and then try to turn the fly wheel. If there is some resistance then you are OK!