tore down motor, got a couple questions about my pistons.

[mk3-4-me]

ok so i tore my motor down because i got another RK.

Anyways i take it all apart and look over everything. i have some questions about my pistons.






1. what is the "clean" part on top of the pistons from. I know the dot in the middle is from the spark, but what about the outside edges that are cleaner? theres no indication on the head from impact, so i dont think its banging on that.

2. Is the supposed to be gasses passing the first ring to color it like the top of the piston? The part inbetween the top ring from the combustion, and im wondering what thats from and if its normal....

thanks!

 

[nofx230]

how old are those pistons and rings?? and did you install them yourself?? were the rings gapped to spec just throwing out ideas I'm a lil confused but got a few ideas what could be causing it

 

[mk3-4-me]

pistons arent to old. i got them in my motor i bought and it had about 10k in there and 2k in mine. i rebuilt it and put them in myself. Yes i gapped them, well i checked and they were all within spec from the box.

these piston rings are only 2k miles old and was planning on reusing them.... what if the bore is out of round or something and the ring cant seal properly...? it was bored n honed from the PO and when i rebuilt it, i could still see the cross hatching. I can still see it now the second time apart

 

[nofx230]

dude never trust a PO's word iv seen cross marks in cylinder walls that had 25, 40 even 50k miles on it mic the cylinder walls and get new rings rings are cheap anyways why risk using old ones ya know

 

[nofx230]

is it just those two pistons or are all 6 dining the same thing also how does the HG look?

 

[hvyman]

Should always be able to see the cross hatch unless something horrible went wrong.

One of my motors with 200xxx miles still had cross hatching. They don't just go away.

Also I would always use new rings and be best to get the cylinders lightly honed to break the rings in better.

 

[nofx230]

cylinder i wouldn't doubt is egg shaped but I'm trying to tie in how that would lead to blow by, and why aint there oil build up on top of the piston (scratching head)??
also is is there still that lip of carbon build up on the very tip of the cylinder wall?

 

[KTM530]

hello just my 2 cents, I'm NOT ASE cert or anything and not the most knowledgeable fan of the 7mgte either. But judging from your piston pic you have very little exhaust gas getting past your rings i can't quite see where the wrist pin is but looks pretty clean which in my opinion is a great thing. Like hvyman said "Also I would always use new rings and be best to get the cylinders lightly honed to break the rings in better." a very good idea while your down there and pretty cheap for a pice of mind. as for the hone job just go to Granger or there web site and order a new hone for your boar size, like 20 quid. throw it on a drill with some oil and have at it. there is a proper speed and technique to this but its not rocket science, check out you tube.
Also when i have experienced a true "rod knok", not to say that you didn't, its more than likely the main and or rod bearings. but could also be worn wrist pin or piston slap but i really doubt it. and to answer your question the different colors are from the rings doing there job and keeping the blow by to a min.
if anything said here is not true, someone please chime in as it will help out fellow MK3 owner but also enlighten my self as well.

 

[hvyman]

If in fact your cylinders are egg shaped your piston is still round thus leaving a hole for blow by.

Highly doubt all 6 were egg shaped tho.

If the timing belt tensioner bolt is over tightened it can cause the number 1 cylinder to get egg shaped tho.

Also there are 2 compression rings for a reason. Not every thing is going to be sealed in. And you also need the gap. Too much gap and the piston will seize.

 

[Nick M]

If the piston was knocking, it would be obvious and have scoring. Same with the cylinder. Some carbon on top is not abnormal. Especially if you never blow out the cobwebs, so to speak.

 

[nofx230]

If in fact your cylinders are egg shaped your piston is still round thus leaving a hole for blow by.

Highly doubt all 6 were egg shaped tho.

If the timing belt tensioner bolt is over tightened it can cause the number 1 cylinder to get egg shaped tho.

Also there are 2 compression rings for a reason. Not every thing is going to be sealed in. And you also need the gap. Too much gap and the piston will seize.

but if the case is it not sealing and getting that blow-by effect, why aint that piston oil covered?
so i guess the real question is what cylinder is this happening too?
just start checking everything and seeing if everything is in spec check your valves and seats too could be a intake valve not fully seating and a lil extra air slipping by, idk just throwing out ideas and symptoms I'm by no means an expert

 

[Nick M]

A picture of the deck of the cylinder head and block would be nice.

 

[mk3-4-me]

I'll get other pics when I get a chance.

But it's on all pistons. I know carbon build up is normal.

This thing ran healthy as hell before I tore it down.
15 psi I dyno'd 391 whp.

These piston rings are new. Only 2k miles on them. Lol and Im not worried about money.

I'll take it in to get rehoned for peace of mind I guess.

 

[mk3-4-me]

And the head was rebuilt by PO. But I may rebuild one of my other heads I have.

 

[nofx230]

And the head was rebuilt by PO. But I may rebuild one of my other heads I have.

do you know there previous owner personally?? or is he some joe you met on craigslist? PO of my car said this and that and bla bla bla and i could tell just by looking at him that he was a hack, but didn't really care cuz i had every intention on rebuilding the motor cuz the body and interior of the car was in such great that i just couldn't pass it up. my personal experience is never take anyones word, unless your wallet can handle it

 

[mk3-4-me]

Ya I know. My wallet can handle it to... So far. I spent about 6k just last year on it! Lol it's gettin tricky though when I start paying for other stuff.

But the head looks like it was gone through. Everything was a lot cleaner than my other heads. I got it off here, but I never trust anyone to begin with. But I know it was all rebuilt before I got it.

I'm concerned about the pistons. Is there usually non of that blow by of gasses past the first ring like that?

 

[mk3-4-me]

Any ideas? Should I just get it rehoned then?