Pistons too tall

R

Ralf

New member
I burnt a piston in my ITB Golf due to my stupidity and therefore had to put in new pistons.
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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
 
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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.
4602278797_6fe9f822d5.jpg
 
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.
 
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
 
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.
 
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.
 
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
 
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
 
Greg Amy said:
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
Click to expand...

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?
 
Gary L said:
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?
Click to expand...

Same goes for the "head saver" gasket. The material is free, but the compressed thickness must match stock.
 
Harlan and I ain't going to protest......you need every illegal point of compression to get past Gary's Blue Brick! See ya at the track!
 
Cheating is not something I'm willing to do. I know that I would probably never end up anywhere near the Runoffs or ARRC but I'm not going to build a cheater car so i can win regionally. This is why I'm asking here to get clarification.
 
I've got a stock head that hasn't seen any machine work that I'm going to try. Since the contact is only on one edge, I think it is only contacting the head by a few thousands.
 
Gary L said:
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?
Click to expand...

You got me looking in the book Gary.

"Cast or forged equivalent pistons shall provide the same dome/dish/valve relief
configuration, ring groove width and spacing, pin height relationship,
weigh no less than the factory standard bore pistons."

"Balancing and “blueprinting” of the engine assembly are
permitted. Lightening of parts beyond the minimum material
removal necessary to balance is prohibited."

So the way I read this is that the replacement piston can't weigh less then the standard size piston. Since the replacement pistons are oversize I would imagine that there is more material and therefore weigh more then the stock standard piston. Now since one can balance and lighten to the lightest stock piston, one can remove material from the new pistons. What prohibits one from taking material off the top of the piston to lighten? As long as you don't change the dome/dish/valve relief configuration, there shouldn't be a problem.
I'm by no means an engine builder/machinist, so I have no idea and that is why I'm asking. Better for me to be clear about something then to do it and be illegal later.

Give me your thoughts on the way I'm reading the book. Like I said, I have stock head I can try first so I may not have to touch the pistons anyways.
 
Weight/balance aside, seems to me you'd be changing the " ...dome/dish/valve relief configuration", if nothing else. And maybe the " ...pin height relationship", as well? I'm with you - not an expert - that's why I replied to the GA's post specifically. Greg, where'd you go?

Meanwhile, to Hell with this technical crap... you tellin' me we might have 4 (FOUR) ITB cars at MAM next week? They may have to give us our own run group!
 
Cutting the tops off is not legal. If the piston has a higher than stock squish band, and you matched it to the stock piston, you may be considered legal , by the result, not by the method.
Any head will hit the piston, cut or not cut. I said to use an uncut head to get near the legal compression limit, not to miss the pistons.
You need to measure the piston to head space by deduction. You can use the clay method or the washer method. Take a 030 thic washer, or a valve spring shim washer( Iuse a 5/16 grade 8 washer). Place on the piston squish zone. Set the head and gasket on, and bolt it on at 25# torque. Rotate the engine, without the cam in, and feel for any touch at TDC. Do both ends of the engine, one at a time. Take the head back off and look for hard marks from the washer. I use permanant marker on the head and piston to show interferance.
You can also measure the piston protrusion with a pair of the 030 washers and a straight edge. This is quick and easy and works at the 030 gap size, if you run less space(under 020), the actual space should be measured, IMHO.
Measure the head gasket. It will be about 070( @ 25#) for the Felpro. Remember that the HG wil be about 066 at full torque value.
You will need to cut the piston tops off to get any room. I suspect that the engine will not be legal.
I assume that you would want to be as legal as you could be within the spirit of the rules. The head saver gasket and the stock size head may be close to legal compression and you would make any cc test if you were to go fast enough to get taken apart. In reality ,the SCCA seldom takes any cars apart. My circle burner gets taken apart every time we run it.
As you get more races and start to go fast, find a good block and builder(me) and go fast legally. HTH, Mike Ogren/Protech
 
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