pyropete125

Where do I measure the thickness of the head to see if it has been milled?

I have an 8R head that is going on our lemons car and it has a machinist stamp of 03 on it. I don't know it it means that 0.003" or 0.030" was taken off.... or it was done at Porsche or something

Since I am doing the milling on my machine, its technically not costing anything so it is lemons legal, so my question is how much can I take off before things get too close? I like the idea of raising the compression and since it brings the cam centerline down it retards the cam a bit for a bit more top end.

It is going on a good 1987 bottom end, and we have the later DME with a weltmiester chip. It has a wideband o2 to keep an eye on it and it has a AFPR, plus the 6 different setting on the DME screw.

Thanks all,
Pete

V2Rocket

pyropete125

Thank you that's exactly what I needed

I know that the minimum is there but I know for spec racing you can mill the heads on older engines to get to 10.5:1 Which I am assuming makes the head less than the minimum thickness. Any thoughts?

ddombrowski

I can't say I have any experience milling the head, but running in Chumpcar with similar rules I'm very interested to hear your results of this so please post how it turns out.

I've always thought that 10.6 pistons and milling the head, while not SPEC legal, would be pretty interesting in a budget endurance racer and a reasonable path to take.

mikehayes

I'm far from an expert on machining engines, but could you install the cam in the head, rotate it so that one of the valves is fully open, then install it loosely on the block with the old head gasket, and put some shims on the cutouts of the piston that are the same thickness as your desired clearance, and rotate the crank until they touch? be an awful lot of guess and check but it would be one way to verify.

V2Rocket

i would think you could mill a little more but youd want to consider deepening the flycuts in the pistons for valve reliefs.

951and944S

Pretty sure if you are teched for proper head, they will use the dimension for "installed" thickness measurement with head gasket.

Rules allow for using the thicker head gasket to bring a head back into service margin that was machined past minumum uninstalled height, so it's logical that only installed dimension matters -

"Minimum thickness for installed heads is 0.929in (23.59mm) for 9.5:1 pistons and 0.965in (24.51mm) for 10.2:1
pistons as measured to the surface of the block from the factory reference location as show on factory manual
page 15‐16a dimension A. This installed measurement includes the head gasket thickness and allowance for some
variation of head gasket crush and measurement."

T

951and944S

You just know your measurement from the factory unmachined material height vs newly machined height, figure the decreased valve to piston clearance and have that known amount machined from the valve reliefs.

And to a previous poster, I do not know that 10.6 Euro piston compression is legal at all (though I think I have seen "SPEC" pistons sold at that ratio), the 1988 vs '87 and prior is 10.2:1 USA.

Put .4 : 1 compression increase into an engine configuration = output tool on the web, all other know variables of a 944 engine in the inputs.
The increase on less than 1/2 a compression point on a 140 bhp engine is so minimal, you won't even bother.

You'll get 3 times that increase from a properly designed ram air pressurized air box (and doesn't show on a dyno test) and another 3 times that on a thoughtfully designed exhaust with scavenging in mind.

T

pyropete125

What you do is install the head with soft valve springs, and 2 feeler gauges set to the compressed hg thickness in place of the hg. Then clay the piston to measure or put a lash cap on the tip of the valve of the desired thickness and see it it touches.

On engines that you can put a dial indicator on the valve spring retainer, I find that the easiest.

This is a lemons car, not a SPEC or anything that has rule.

Thanks all​​​​​​

pyropete125

Interesting point. We run no headlight and a custom cone filter box in place there to the meter and a 2.5" custom exhaust no cat.

Pete

​​​​​​this is the cars 18th lemons. It has been in fire, engine blown and smashed into too many times to recall. We did get 12 races on one engine, then only one or two races on every other engine

V2Rocket

curiosity made me measure the 944 tubular manifold pipe diameter the other day...
it's about the right size to chop off at X length and slip a new piece of 1-5/8" ID over and welding it together...probably need to heat the new pipe to get it on (something like 1.65" OD for the 944 tubes)

might be useful for getting some "stepped tuning" going...

pyropete125

Just thinking about this and i was wondering what is stopping people from angle milling? As in taking a 0.00" to say 0.040" (or greater)cut from intake to exhaust side? I have had it done to and heads. It's a great way to take out more in the chambers.

Seems like an easy way to cheat the head thickness measure test.

V2Rocket

for one thing the timing belt covers probably wouldn't fit anymore and maybe issues with the belt itself?

it's worth a try though but how much angle might be necessary to get a decent gain like they found on old SBC?