I am sure this has been discussed before. I am running a set of alm. pistons that are not supposed to be HC but they hit a Z head. Presently running another HC head from one of the vendors and am having pre ign knocks unless I run high test gas. Would like to change to the Z since it has better combustion chambers. What clearance do I need, will have to do some grinding.
To help you determine for sure what pistons you have, here is a thread showing what the high compression pistons look like:
And here is a thread showing stock height pistons:
Hope this helps.
I've seen others posting about clearance problems with std alu pistons and Z heads, so it's not unheard of. Is it #1 and #4 hitting?
If you grind the combustion chambers just until the pistons clear when the head is mounted w/o a headgasket, then the clearance will be OK with the gasket in place, since many head gaskets are about 0.045" thick when compressed and optimum piston to head clearance is 0.030" to 0.060" over the squish area for best preformance. (.030" may be a bit tight in a T engine?)
Here are some old discussions on the topic:
Of the 3 I've installed, 2 needed some "adjustment". Some Prussian Blue for marking and set the head on without a gasket and the bolts in place but very loose. Slowly crank the motor over to mark any spots that need to be ground a bit. Grind down those spots on the head and repeat as needed until the pistons no longer bump the head as you turn the crank. This may not be the most sophisticated method but it works.
As always, clean the head bolt holes and check the head bolt length with washers in place but without the gasket before final assembly.
The pistons I have are like the one in Bud's pic. Last time I had the head off I laid a Z with head gasket on and it bobbed up and down, was in a hurry so put it back on the shelf. Guess I will have to do a lot of grinding or milling.
Watch out for the valve clearance too. I bent four valves once installing a Z head on a car that had oversized 1.66" diameter valves. Bad day.
The same problem came up locally here last week. After an few measurement we decided the head had been rather aggressively decked when new valve sets were added.
The solution was grinding the head, seems fine now.
I just heard a local vendor has quite a few Z heads that were damaged in transit, but repaired. Offering a good price. Don't know the kind of damage as yet, or where. Anybody wanna offer an opinion or two on whether to even look into one?
I should have written that the "block" was aggressively decked, sorry.
Tim, I think nearly everyone will agree that the Z or other new aftermarket heads are the easiest and cheapest way to add a few extra ponies for a stronger running T.
It's worth checking into
I'm surprised there isn't a MTFCOA club sponsored dvd on all aspects of the Z head and other high compression heads to include some discussion of the true advantages for different engine configurations, if a "other than stock" crank shaft is required, etc. Sort of a one stop shop for someone to learn about what you really get out of such a project and what is required/involved in doing it. I know there are some books out there that discuss building horsepower into a T engine but I think they may be a little before the Z head and others? It would be neat to know if putting an early low(higher compression) head on a T with a stock crank is worth the effort. I think the forum consensus is it's not advised but it would be cool to have a book that captures all the forum knowledge in one place without having to key word search for a day. This seems to be a frequently recurring topic.
A video would be nice
Until then there's lots of good info collected at the Tulsa Model T club's website: http://mtfctulsa.com/Tech/heads.htm
Missing from the Tulsa pages are the currently available Prus and Sherman heads, but there are several good discussions about them here to read, like this recent thread: http://www.mtfca.com/discus/messages/411944/430626.html?1400000847
You're right, this is a common topic and it looks like you've been here long enough to have seen the obvious answer to your inquiry. A good deal of modern performance info is readily available in the technical page of the Tulsa Chapter of the MTFCA.
And I think that your conclusion that z-heads are not advised with stock cranks might not be as universally accepted as you stated.