MTFCA Forum

I would like to hear others' experience with and/or opinions around milling an aluminum Z-head to increase compression.

By my estimation, there is currently some 0.150" space between the pistons and the cylinder head "horseshoe" squish area. This is including the head gasket (photos below)

There looks to be ample meat in the head casting to mill, say 0.050 — 0.075 off the head.

The engine is a 1924 block with aluminum pistons, SBC valves, Stipe 280 cam, Simmons SuperPower carb and enhanced splash oiling, coupled to a period cast-iron Warford, in a medium-weight speedster.

Most times less is more.

15 years ago on my first and current T speedster engine I milled the stock head 0.100. It did not improve performance noticeably if any.

A year later I put a 7:1 Lizard head on because of the advertised and perceived better performance. This was supposed to be because of higher compression and supposedly better spark plug location. Was it really the best head for the application? In hindsight, my opinion is, NO. Did it provide better performance than a stock or milled stock head, yes. Did it cause other problems, maybe.

I then put on an out of the box 6:1 Z head on after that and have been running it for over 12 years. I feel this is the best performing head I have used. I feel that they knew what they were doing when they designed this head. And I mean both compression ratio, combustion chamber design, and other factors.

Please remember less is more.

Terry

My experience with Z head was some of the pistons caused interference, needed to grind away some alum at the lip inside the Z head. The Z combustion chamber is ‘as cast’, and each chamber is slightly different. Shaving the head may cause issues.

Changed out the Z and installed a Prus head, really significant change in performance and smoother running. The Prus head has CNC machined combustion chambers, all are identical. My preferred hi—compression head now.

Just “academically”, how much of an increase does that .050-.075” make in the compression ratio ?

I had to mill .034 off of my Z head to make it flat and it caused no problems and it didn't seem to change performance any. If you mill it now and need to later you may be out of luck, I would say leave it alone.

Thanks to all who replied, although I was expecting a livelier discussion.

With the number of Z-heads in service and the general interest by so many T'ers in improved performance, I'm surprised more folks have not chimed in with direct personal experience.

Seems to me if the bottom end is in good shape and any clearance issues resolved, that 0.060 to 0.070 off the head can only improve power output. Even with a head shave the compression ratio would still be lower than the Plus head, which is used frequently with, AFAIK, no bearing longevity issues.

As has been noted, milling of a head with a Ricardo-squish type combustion chamber is limited by clearance over the piston. The book "Souping the Stock Engine" says better performance is achieved if the clearance does not exceed 0.090". It needs to be at least 0.040". You can easily calculated the CR change due to milling. I put some examples up at the Tulsa website years ago - http://mtfctulsa.com/Tech/head_design.htm. Milling a Z head 0.050 increased the CR from 5.3 to 5.7. The stock combustion chamber is so cavernous that milling even 0.125 increases the CR from 3.8 to only 4.2, but you still have a very poor combustion chamber design. You could fly cut the squish area above the piston, but if you go too far, the bottom deck will be too thin. The other idea is a custom popup piston, as has been mentioned. Why doesn't someone offer a true 6:1 head? Instead we have all the false advertising about 8:1 heads. - Larry Young

Several makes of heads that have different combustion chambers and claim high compression. A Waukesha Recardo, Sherman and a Prus.The Prus is on the right in the combustion chamber comparison photo. As you can see the Sherman has very high compression. I was told not to run it on a stock lower.

Couple more heads.

We named those Lizard heads Yapp Crapp for a reason. I cut mine in half with my chop saw and threw it on the iron pile so it would get hauled in for scrap instead of causing problems for somebody else.

IMHO or NSHO the Prus Iron head is the best of the new heads. The PRUS Aluminum head is next. Of the old ones, the Haibe Highpower - which is the same as the Giant Highpower - is the best of all. I've had a couple Waukasha heads, virtually every one I've ever found on an original engine since I started looking for them in the 50's came off a Baldwin combine power engine. I wasn't much impressed with the ones I ran.

Wanna go Zoom?? Haibe or Prus. Z - not bad

Thank you gentlemen. That’s the sort of lively and helpful discussion I was hoping for.

If you check the Tulsa MTFCA club's site, they have a entire section dedicated to cylinder heads and testing. I think they tested a 'stock' Z head and one milled .050.

Thanks Terry, that's very helpful.

Further up the same page the author states the preferred clearance above the piston is 0.050-0.080". What I have now is double or more this ideal.

https://mtfctulsa.com/Tech/head_design.htm

for the fun of it :

if you added material instead of milling, you would need a theoretical "disc" of metal 3.75" diameter by .130" thick to bring the compression ratio from 5.3 to 1, to 6.0 to 1.