MTFCA Forum

Has anyone experimented with milling down a z head to get a little more than the 6 to 1 ? How much can be removed until it contacts stock aluminum pistons? I have an older one i was thinking of experimenting with

My experience with these heads is the combustion chambers are of a random volume due to poor casting molds and or poor machining.
Both that I installed hit the piston top in 1 or 2 cylinders. First step was to make the chambers the same depth. That process cleared all the pistons. @ that point removing .060" from the gasket surface would have been max when adding the head gasket.

The compression is actually less than 6-1. My calculations from CCing came up with 5.4-1. That said, that increase in compression greatly improves a T's H.P. Very noticeable in speed & hill climbing power.

To get a small gain in compression, you could machine off .100, then machine all 4 combustion chambers for piston clearance. You would, in effect be decreasing the volume of the valve chamber. This may require extra valve clearance.
Bottom line. With a bunch of machining you might get a very marginal gain.

John's input mirrors my experience with Z heads. When asked about them I refer the inquirer to forget the Z and buy a Prus head.

Allan from down under.

But then again, the Prus head has a similar low compression ratio - a far cry from the ads.

The Prus head is still the best one to use for performance, have used both, and prefer the Prus. The chambers are CNC matched machined, the Z is as cast. The Prus is also pressure sealed like modern alum heads to resist material corrosion.


Did my garage cc'd method, and the Z had 250cc chamber, and the Prus 210cc chamber, so its a bit more that makes a good difference IMO.

I agree the machine work on the Prus head is good but I object to the false advertising. Your measurement method is prone to errors since a small error in the fill height can easily give 10-20 cc error. You might try the method in the photo using a plexiglas plate. I measured 203cc on a Z head.

I use the plexiglass sheet method.
I found closer volume readings. Z=223 CC's Prus=220 CC's
I experienced & expect Z head readings to vary a lot between cylinders & different head castings.

I agree to the superiority of the Prus head.

As it happens, I have THE Jensen aluminum head that Kevin Prus used to make the mold he uses for his castings. I had a few thousandths milled off because it was warped but it’s been on my car for several years and it does run better than the Z head. And it has the Ford script on it !!

Ruxstel24 wrote: ↑

Fri Jan 02, 2026 2:59 pm

As it happens, I have THE Jensen aluminum head that Kevin Prus used to make the mold he uses for his castings. I had a few thousandths milled off because it was warped but it’s been on my car for several years and it does run better than the Z head. And it has the Ford script on it !!

By better do you mean smoother? that could be the result of the Z'z varying compression among the cylinders.
Is there a noticable HP difference? Have you CC'd or done a compression test? That would provide some comparrison #'s

speedytinc wrote: ↑

Fri Jan 02, 2026 3:05 pm

Ruxstel24 wrote: ↑

Fri Jan 02, 2026 2:59 pm

As it happens, I have THE Jensen aluminum head that Kevin Prus used to make the mold he uses for his castings. I had a few thousandths milled off because it was warped but it’s been on my car for several years and it does run better than the Z head. And it has the Ford script on it !!

By better do you mean smoother? that could be the result of the Z'z varying compression among the cylinders.
Is there a noticable HP difference? Have you CC'd or done a compression test? That would provide some comparrison #'s

No none of that, it just pulls better. With a Ruckstel and 3:1 gears, it’s noticeable on hills.
To be fair I did lap the valves, and properly adjusted them, honed the cylinders and new rings and added the OF Stromberg at the same time as it had been sitting for several years. But it runs better than before that.