Here's what I figured out over the last several weeks dealing with machine shops.
First, If your machine shop doesn't work on your type of engine on a regular basis be sure you know all the specs of everything you have and know exactly what you need in the end. This goes for the type IV as well as a flat 6, Subby, or water cooled V8.
I had a shop repair my heads. The 928 has a lug on the end of the heads cast on for measuring the thickness of the heads. The stock thickness 24mm. The minimum thickness for machining is 23.6 mm. When you cut the heads you need to move from the stock gasket at 1.1 mm to a 1.4 mm thick gasket.
I failed to measure the heads before I brought them into the shop. Based on where they ended up I can only assume the heads were cut once before they went into the shop. They were repaired and cut the heads.
I again failed to measure the heads before I installed them or ask the shop if they checked them. I assumed they were in spec. My fault.
The engine ran about 200 miles maybe less. I had a ticking sound from the time it started. When it failed a valve snapped off at the retainer, dropped into the cylinder, broke again, then the piston drove it into the head. This broke the piston and block.
I brought my heads into a second shop to get an estimate for repair. The shop called me and told me it would cost $ 900.00 plus parts.
Now because I just had these heads done I had a wierd feeling about cutting them again. I asked the shop how thick the head boss is. He didn't know and or didn't really know how to check them. I managed to give him clear enough insturctions over the phone. He check them and called me back. He told me one head was at 23.5 mm on one end and 23.6 on the other. Then told me the head that had the broken valve was at 23.1 mm (remember the minimum is 23.6 with a 1.4 gasket) I used the stock 1.1 mm gasket...
Originally I thought I may have damaged an oil ring during installation. Based on my findings the oil ring was probably fine. But ultimately I still killed my own engine.
Anywho, These heads are gold. Larger valves, larger ports, smaller combution chambers. The US engine is rated by Porsche at 218 hp the euro "S" engine I have is rated at 300 hp. That's pretty good from a 4.7 L
I ordered a set of used euro "S" replacement cores for $ 1000.00 I figured that would need a valve job. Turns out these heads were also cut to 23.1 mm.
I'm returning the replacement cores to very reputable vendor no questions asked.
I'm back to repairing my heads. I'll need to cut the notches in the pistons a little deeper and degree the cams but at least I know what happened so I don't repete this problem.
