2.0l pistons

[rich]

I just got some used 2.0l pistons and liners. My questions are; are all us mahle 2.0pistons dished? Also one piston got hot on the edge and a piece of the first compression ring was stuck in the groove. After carefully prying the piece out and cleaning the carbon out of the groove,(Some carbon some electrolisis) things seem okey. What is the theory on reusing this piston? during normal running will the piston retighten on the ring in this spot? Has anyone had experience with this?

[djs]

If I understand my 2.0L pistons, those destined for N. America were originally dished to reduce the compression ratio somewhat, I believe in response to emission constraints. European 2.0 motors in some (if not all?) cases used a flat-top piston, enabling a slighly higher compression ratio (8.0:1 vs. 7.6:1). You can still buy these new. Pelican sells them, and they actually cost a bit less than the dished ones, last time I checked.

In your case, if the ring was stuck in the groove due to carbon buildup, you ***could*** possibly reuse it. In a clean ring groove, you should be able to slip the ring in with no effort. If the ring goes in with any kind of tightness or resistance, something is wrong. The ring has to be able to move freely, as it is designed to push outward against the cylinder wall during combustion. That is the principle behind the compression rings. They use the pressure of combustion to seal tightly against the cylinder walls.

If the piston got hot, it could have distorted permanently. Normally, pistons are not perfectly round when cold. They are slighly elliptical. This is partially due to the fact that the piston is heated unevenly and the skirt area dissipates heat differently then the pin area. When the motor reaches operating temperature, the piston is designed to become round.

[Dave at Pelican Parts]

Some of the Euro-spec 2.0 pistons (8.0:1 CR) do have a dish in the top. The edges of the dish are curved, while the edges of the US-spec piston dish are beveled. The factory diagram has a drawing of a cut-away piston that illustrates this.

Some of the Euro-spec 2.0 pistons have no dish. I think (not sure) that the original-style dished ones are made by Mahle, and the flat-topped ones are made by Kolbenschmidt.

If you're worried about the ring lands, then measure them. They should be in spec, and even the whole way around. Clean the living heck out of them first, though!

--DD

[JP Noonan]

If you want to reuse those P/C's then soak them in a gallon can of carb cleaner (they sell them at FLAPS everywhere) overnight. Then mic across the skirts and across in the direction of the pin to see if the piston is within spec. Then use a bore fixture and a dial indicator to measure the bore and bore taper. If you use a piston with a tight compresion grove you may get blow-by once the motor is up to temp, or maybe break a ring or a chunk of the piston may come off. I think VW only used forged pistons (vs. cast or hyperutectic) and they expand and contract quite a bit over there opperating range.

The fixtures and tools to measure pistons and bore runout cost a couple hundred bucks (for decent stuff) so see what a local machinest will charge you.

Does it really make sense to re-use pistons? After paying the machinest, new rings, and asembly, do you really want to have to take the motor apart again because of a bad piston. Unless you just want to cut your teeth on this engine and have other motors as back-ups I would just get new P/C's (I know, I know, $500. Why do you think I have a 2.0L case, rods, crank, and heads just sitting around? I'm too cheap to buy the P/C's). Also before going too much further, make sure they aren't bus pistons, or else you may wind up with a 67 hp motor.

The other option is to go with a big bore 1.7 ($165 for the P/C's) but you will also need a 1.7L or 1.8L crank and rods.

Anyone who plans on rebuilding a VW motor really, REALLY, should buy Tom Wilson's book on how to rebuild aircooled VW engines.

[rich]

Daves explanation of the pistons was helpful. The pistons I have,have a curved dish. I'm on a very tight budget right now. Constuction is underway on the MANS garage! slab heat, 2"X6"walls Second floor work shop! So 500.00 makes me flinch just a little. If I lived on the gold coast I wouldn't need the slab heat and all that insolation! LOL! So after letting the car sit for a year I may have to finally order the 1.7 big bore kit and hold off on the 2.0 for now. thanks guys