Pistons too tall

[Ralf]

I burnt a piston in my ITB Golf due to my stupidity and therefore had to put in new pistons.


I cleaned out the bores with a hone and replaced the old pistons with a new set of Kolbenschmitts. Ran the motor and was getting what I thought was lifter noise. Replaced the lifters with a new set, and got the same result. Took off the head and thoroughly cleaned the oil passages thinking they may be clogged from piston debris. The result was still the same. Took the head off once more but this time as I was cleaning the bores out, I noticed some marks on the top edge of the pistons. That's when I realized that the lifter noise was actually the piston slightly tapping the head. Good thing I didn't go above 3500 RPM.
So the question I have is do I need to get it machined slightly or can I just shave a bit off with a belt sander? Looking at the old pistons, they were also ground down a bit. I think just removing .010 in off that edge would keep it from hitting the head.
Thanks

[Lael Cleland]

What is the part #? I have not seen that happen......

[Sandro]

somewhere along the line did you deck the block or head? you running a stock gasket?

[Ralf]

Not sure and the head is temp installed right now, so can't get the numbers off the pistons. (Can't really read the numbers on the old pistons, but they are the same.)
The ridge that you can see on the burnt piston is actually the high point of the piston, not a relief cut. It is actually higher on the new ones then the old pistons.
Here is a picture of one of the old pistons. You can see some pitting from detonation.

[Ralf]

It was a motor I bought from a Prod guy that had it in his ITB car. I'm assuming the head and block were decked. I was told it was built by a reputable shop. I'm using a metal head gasket.

[Knestis]

Too funny! We had exactly the same thing happen with an engine we had built for a rally car back in the '80s. It sounded like a diesel when we started it up...

The solution was for the guy who built it to pull the head and whiz off a few thou from the tops of the pistons with a 3M pad on a die grinder. I was appalled but he bolted it back up and it ran strong for several years...

K

[joeg]

That piston in the first Pic looks pretty well damaged. I am not sure a post-relief cut, shave or grind will save any damage to the ring lands, but you never know.

If it were me and that happened, I would bite the bullet for new pistons, relief them before final assembly and rebalance.

[Ralf]

I have new pistons in there now. Its the new pistons that I'm having trouble with. Those pictures are of the old pistons that I pulled.

[Greg Amy]

Given the unknown history of this engine, I would:

- Measure the distance the pistons are protruding from the block, and have them shaved by that amount plus a margin to account for stretch and slack; I'd have to look it up, but I think .003" is the norm

- Remove the pistons and rods, and have the pistons machined

- Replace the rod bearings; you probably damaged them with the pistons hitting the head.

- After block assembly, do a full cc of the engine to ensure that the compression ratio is legal. This block has obviously been machined, and it doesn't take much to increase compression by the legal 1/2-point; I'd bet a dollar that your compression ratio is illegal for Improved Touring.

GA

[Flyinglizard]

The guy that built that engine, usually used the cheap, 1 pc thrust bearings. As said, replace the rod bearings, check to make sure that you are using the 6 pc thrust bearing. Pop out the pistons, cut and balance. The felpro head gasket is about 066. thic. Figure for 030 head to piston gap, min.
Try doing a search for "head saver" head gasket. I have never used one on a VW or know if anyone makes them.
Deburr the sharp edge off of the top of the block and the edge of the head, match to the gasket..
An uncut, stock head, should be close for the compression numbers.IMHO. MM

[Gary L]

Given the unknown history of this engine, I would:

- Measure the distance the pistons are protruding from the block, and have them shaved by that amount plus a margin to account for stretch and slack; I'd have to look it up, but I think .003" is the norm

- Remove the pistons and rods, and have the pistons machined

- Replace the rod bearings; you probably damaged them with the pistons hitting the head.

- After block assembly, do a full cc of the engine to ensure that the compression ratio is legal. This block has obviously been machined, and it doesn't take much to increase compression by the legal 1/2-point; I'd bet a dollar that your compression ratio is illegal for Improved Touring.

GA

So I think I missed something... where in the ITCS is the allowance to machine (or otherwise modify) the tops of the pistons? Or is this a normal repair/replace procedure that is outlined in the VW shop manual?