Andrew Clink

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Eurovan TDI Conversion: Piston Protrusion

18 September 2015

Rebuilding a 1Z, AHU, or ALH? Using oversized Nüral pistons? You're going to want to deck your block. I believe they're cast shorter than stock pistons intentionally with the assumption that you will be milling the block shorter.

On TDIs, the head is perfectly flat. The pistons also protrude out of the block very slightly, projecting into the head gasket. Volkswagen specifies three different head gasket thicknesses depending on piston projection, and each step is within 0.003" of the next. Some lazy mechanics will just throw a three-hole (the thickest) gasket on rather than measuring, assuming all is well. However, this effectively lowers compression, sacrificing economy and making for harder cold starts.

Gasket Min (mm) Max (mm)
1 hole 0.91 1.00
2 hole 1.01 1.10
3 hole 1.11 1.20

From my reading, new Nüral pistons and a stock deck height often make for a piston protrusion of about 0.030". The minimum per Bentley for a 1-hole head gasket is 0.91mm, or 0.0358". A two-hole head gasket is specified at 1.01mm or 0.0397". All my pistons are measuring at right about 0.030". At least none of my rods are bent.

I've read that each 0.010 of additional piston protrusion counts for 1 point of compression loss. Given that the minimum is 0.036 and I'm at 0.030, that's puts me at about 18.44:1, but all that loss is in the critical "squish" area. Put in these terms it seems so inconsequential, but when I think of how far I want to drive this thing (i.e. hundreds of thousands of miles over years and years) it seems prudent to handle it now. I don't want to be camping in Alaska and be unable to start the bus because of a known defect during the build.

Since I haven't torqued the rod cap bolts to yield, it'll only cost me a new set of main bolts and $100 for decking. There's also a small scratch between cylinders 2 and 3 that would be nice to clean up. It seems like much ado about 0.006", but when VW found it important enough to specify three different gasket thicknesses 0.003" apart and I'm 0.006" under the minimum, it looks like I'll be removing the crank & pistons and sending everything back to the machine shop tomorrow.