Lathe fixture for turning 911 heads

[Jim Williams]

I have this fixture posted on the 911 Parts For Sale Forum, but since the fixture is used for 911 engine work as well as being for sale, it seems to be fair game for being posted here as well. Here is the link:

Since there are so many engine builders posting here, if nothing else, the fixture should be interesting to look at.

[quint22cp]

Nice to see this I'm in the process of making one right now. On your fixture I can't see any adjustment for the positioning of the head, how do get the cylinder mating surface to run concentric so you can reface it ?

Also how much should I machine off the mating surface when refacing them ?
As little as possible or 0.25mm and use a 0.5mm base copper ?

[Jim Williams]

My fixture has holes for the location pins that are part of each head. These pins are same ones that locate the head on the cam housings. The holes on the fixture for these pins were located by careful measurement by the machinist that built the fixture. They used a head that I supplied them to make the measurements to do this. Every head I have put on the fixture has been a perfect fit and the head located perfectly by these holes. The center of the head lines up with the axis of the fixture that mounts in the lathe chuck, without any adjustments.

I don't know how the machine shop did it. I guess that is why they are paid a significant amount of money to do this kind of thing.

How much you machine is a function of measuring the depth of the cylinder mating surface. If there is no significant damage to any of the 6 heads, you use the head with the smallest measurement from the cylinder surface to the opposite side of the head, the side that mounts to the cam tower. Then you machine just enough from the other heads to match this number. Most heads will clean up with as little as 0.10 mm. Porsche says 0.25 mm is the max, other shops that have more experience than I may have a diiferent number.

What ever amount you take off, you should make a trial measurement and determine the deck height, and check the valve-to-piston clearances with your cam dialed in. Then at this point you decide what base gasket you need. These heights/clearances are discussed in many threads on this forum.

[quint22cp]

Thanks for the info' Jim

[john walker's workshop]

here's mine. one piece, machined from a chunk of stainless on the trusty jet lathe. the holes are ovalled for centering.

[cgarr]

These do work very nice, Mine is just like Johns and its in a 4 jaw for fine adjustmens, deadly accurate, here is a 0.001 cut just to clean it up..