Legal head shaving comes in around 0.050" for 10.5:1 limit on early pistons. I don't remember the exact number off hand. So you will need to convert your MM to inches to figure it out. I would not trust a metal scale since you have the head off. I would do a check with calipers. Now if your first resurface took of 0.010" then you should be fine with another 0.010". If you think you are going to be close to 0.050" you probably should try to CC the head to besure you don't go over the limit. Remember the limit in the rules is 10.5:1 not a certain limit of shaving.

Per the factory manual.

Stock head = 24 mm + / - 0.1 mm
Stock gasket = 1.1 mm

The resurface limits are 23.6 to 23.8 mm. However or rules do not go by the factory resurface limits and we can go beyond that. Just not over 10.5:1.

Weston,

What Joe said- you've got enough margin to true up the head. When you take it to the shop have him check the flatness of the cam tower mating surface as well (if the head warped at some point then this surface will have warped also.) This should not exceed 0.1mm (~0.004 in) per Porsche specs.

Weston,

As Joe said, your measurements need to be more precise.

Here is a really easy way to tell. If you have done nothing to the bottom end and you have the early pistons, you will be under 10.5/1 if you can still see the squish zone behind the spark plug.

Stock seems to be about 0.045 below the surface of the head, and it takes a reduction of 8 ccs to bump the CR from 9.5 to 10.5.

Taking 0.045 off a new head will reduce the head volume by about 7.3 ccs.

Karl @ ART wrote:


Ok, I can see the area behind the spark plug and it sits a bit below the surface of the head, so it sounds like I don't need to worry. Thanks!

Weston,
If you are really not sure take the time to measure it closely or have it measured. The reality is that you as a competitor are required to have a compliant car. This means under 10.5:1. If you are 10.6:1 even if you did not know it your are non-compliant and DQ'ed. I expect you will be going to National and national is a place where stuff like this will get checked. You don't want to be guessing you are ok.

So take the time now to MAKE SURE you are ok. Once the head is on the car and all buttoned up it too late to be guessing. Resolve it now.

Dirks and I both have calipers .