Cylinder head cracks

[gammaxy]

Possibly because after 110 h I had burnt out 4 exhaust valves. The photo above is how they appeared before overhaul.
I have another theory and it relates to my Nikasil barrels that were “mushrooming” under the heads causing the valve clearances to disappear and creating their own brand of havoc. But that’s another topic.

Unfortunately the picture cut off the bottom half of the valves, but what I see of them looks pretty similar to mine. I'm doubtful that your problem is actually due to running too lean. Due to the uneven mixture distribution, it would be difficult to run all four cylinders too lean and still have a smooth running engine generating good power. Your temperatures are also pretty much in line with what everyone else sees.

My impression is that the residue in the combustion chamber (lead bromide?) causes mechanics without experience running 100LL to diagnose these sorts of problems as being due to a lean mixture.

What was the reason for sending the heads out to be rebuilt? Low compression? I suspect your hypothesis about the mushrooming cylinders causing the problem might be the real culprit. Even so, exhaust valve leaks seem to be fairly common. I've lapped a few exhaust valves, but at least two where on valves where the clearance dropped to zero between adjustments.

[RodgerC]

Chris

If you look closely at the photo you’ll notice near the copper head gasket (above in the photo) the recess in the head has been galled by the Nikasil barrels as I’ve attempted to remove the heads.

When doing my regular 25 h head torque and valve adjustments, I always had to tighten the head nuts 1/2 to 3/4 of a turn to return to 18 ft.lb. What I was in fact doing was mushrooming the barrels inside the head. I have no idea what the correct head torque should be for Nikasil barrels, but it ain’t 18 ft.lb.

If my reading of the online VW forums is correct, nobody else knows either.

If anyone reading this is still running Nikasil barrels, you may want to consult a skilled mechical engineer with automotive experience, for a torque setting (or better still a bolt strain) based on the E-modulus and coefficient of thermal expansion of aluminium barrels and steel head studs....But then it’s probably easier, safer and cheaper in the long run to simply replace the Nikasil barrels with iron and stop eating burgers and cake for a couple of months to compensate for the heavier components...And since you’ll need to fly-cut your post-Nikasil heads to remove the galling, why not fit a set of 2276cc barrels for a little more torque (HP)?

I hope that didn’t sound like a rant.

Regards
Rodger