MTFCA Forum

Anyone know the depth of the compression chamber from the gasket surface on a high head from factory?
Got a customer wanting a head decked .100+ for high compression pistons. Don’t want to remove too much.
Wanting to find a head with the least already taken off.
Thanks Dan

Dan,

He wants to take .100 off the head AND use high compression (domed?) pistons?

I don't know the feasibility of doing that, from a clearance point of view, but I do know of a guy who milled off .125 and later had one of the compression domes break away from the head and push up into the water jacket. A mess. Variation in core shift when the heads were cast, plus 100 years of corrosion, can cause variations in wall thickness that might make taking off .100 a bit risky. Or... maybe not...

I am running an engine with .030 bored, .100 off the head with high compression pistons with no issues. On another engine that is .060 over I had trouble with the outer edge of the piston dome contacting the head. Went with a stock head on that one.

This engine is .060 over. I have to check that.
That is reason I am looking for that measurement. So I know how much to take off a head.
Anyone know what that measurement is? Thanks Dan

Dan Hatch wrote: ↑

Fri Oct 25, 2024 1:37 pm

This engine is .060 over. I have to check that.
That is reason I am looking for that measurement. So I know how much to take off a head.
Anyone know what that measurement is? Thanks Dan

Dan,

I believe that would be a very hard thing to quantify. Since you can't know exactly where the piston might contact the head, there is no one place to measure the combustion chamber depth that is going to help you predict the clearance.

Here is my suggestion...

1. Mount the head on the block with no gasket.
2. Test assemble the engine, with the crankshaft & rods & pistons with no rings.
3. Turn the engine over, bringing each piston to TDC.
4. Remove the rod cap from each rod while at TDC.
5. With a dial indicator, measure how much further you can push the piston up, into the bore, before hitting the head.
6. That's how much clearance you have.
7. Remove that amount from the head. Then, with a gasket installed, you won't have any clearance problems.

Jerry I use the modeling Clay on top of piston method.
I am wanting the stock measurement so I could determine if and how much any heads I got have been decked.

Dan, I know you'll get an answer before I can give you one, and I know you want an answer as soon as you can get one to keep from delaying your work on the customer's engine. I have a 26-27 high head that has already been milled .100, but it is in storage, 140 miles one way from where I'm living. I wish it were closer so I could help.

Might be a challenge to work out what head you are working with.

If I read that chart correctly, 26/27 heads have a smaller compression chamber than earlier heads.
Therefore higher compression?
Didn’t the Tulsa club do a test on compression chambers size?

Dan Hatch wrote: ↑

Fri Oct 25, 2024 3:00 pm

Jerry I use the modeling Clay on top of piston method.
I am wanting the stock measurement so I could determine if and how much any heads I got have been decked.

Dan,

I have heard it mentioned that measuring the thickness from the gasket surface to the machined bosses of the head bolt holes can be an indication of past resurfacing/milling. Of course, the question then becomes, what should that dimension be on an unaltered head.

Forgive me if I'm not "getting it", but the way I see it, it doesn't matter how much was milled away already. What matters, is how much more can be removed without interference. The method I mentioned will determine that. My method was not intended to see if a piston will hit, (your clay method will do that). It was intended as a method of determining how much more stock can be removed from a given head, regardless of what may have been already removed.

That gasket to bolt measurement would be nice, but not sure how accurate it would be unless it came from a ford print.
Years ago I was brought an engine with some stripped head bolts holes to fix.
Guy had an Aluminum head that he wanted to use the repro SS head bolts with. They were stripping the block holes. Turned out as I knew can’t use them with an aluminum head. Also, his Aluminum head was thicker on that measurement than one I had on the shelf.
If the new guys making heads can’t get that measurements the same what could Ford have done?

The curvature of the high compression piston is not the same as the inside of the head. As you increase the bore you must check clearance all across the bore especially at the edges.

From a head removed from a 26, the center depth measures 1.19" and at a point next to the plug hole, 1.35".

Stetch where measurements were taken with a vernier from a straight edge. No gasket.

Thanks for the info

Hi Dan,
From the broken shaved heads I have seen the question is not how much you can take off but how much is left when you are done.
All the broken heads I have seen where a result of too little material left around the combustion chamber. Maybe your customer would
consider a low head I have heard of good results with that combination. I have noticed that there may be 2 dome sizes in HC pistons also.
Craig.

Your welcome

These HC pistons look to have a radius cut on them at top where dome starts. I will take a picture and post when I am okay to get back in shop.
I do have a fresh decked early head, aka low. I will try it too. Thanks Dan.

Craig Leach wrote: ↑

Sat Oct 26, 2024 12:43 pm

Hi Dan,
From the broken shaved heads I have seen the question is not how much you can take off but how much is left when you are done.
All the broken heads I have seen where a result of too little material left around the combustion chamber. Maybe your customer would
consider a low head I have heard of good results with that combination. I have noticed that there may be 2 dome sizes in HC pistons also.
Craig.

Craig, I think that I have noticed a difference in the domes on High Compression pistons, also. I have a set of Egge's and a set of newer Snyder's. It won't help with the post, but the next time that I get a chance, I'll take measurements and post them.

Dan, what was the problem with the stainless steel headbolts? Were they too short? A customer had the stripping problem when fitting a Z head using standard bolts. The Z heads seem to vary in deck height and by the time you add the washer under the bolt head, there can be a problem with the depth of engagement of the bolts.

With an alloy head I use head bolts from a GM Holden six. These are too long. I fit the alloy head without the gasket and run in the bolts until they bottom out to find out how much to trim off the end. Needless to say, this is done after the block hole threads are cleaned out and the threads chased.

Allan from down under.

I like to use tin foil balls to test clearance between pistons and heads. I use several foil balls per piston to get an idea of clearance around the combustion chamber. Get a used head gasket that is already compressed, use a dab of grease to hold each small ball of tin foil, bolt the head down, and rotate the crank over a couple times. The tin foil will squish down to the clearance. Pull the head back off and the thickness of the squished tin foil is easily measured. This will tell you without question how much clearance is currently available, and what can be milled off a head. Don't go less than 0.050 piston to head clearance. I don't have modeling clay around the house, but every kitchen has tin foil.

Allen: At one time there was a footnote In catalogs and on websites about not using SS 26/27 head bolts with Aluminum heads. Not sure if it still applies but it did at one time. Dan

Stainless bolts may be too hard and slick, and they may have undesirable expansion properties for that application. Another issue might arise if you screwed a stainless bolt into a Helicoil thread. Stainless bolts and nuts are a no-no. They can gall and seize, even using ordinary hand tools. Stainless into stainless NPT joints require a special lubricant, or they will likely seize before they make-up.

Fred, you raised a laugh for me. While staying with model t friends in Spokane one trip the lady of the house was also using some tin foil. I asked why aluminium foil was called tin foil and she couldn't tell me. A couple of days later she was canning peaches, in glass jars!

Allan from down under.

I suspect that most people living today have never seen tin foil. It was widely used in radio and TV capacitors up until the end of the vacuum tube era, and it may have been used in cigarette packs and some other types of packaging. I believe that "tin" cans were tinned steel almost from the beginning, and I'm pretty sure I've never seen a tin pot. Every "tin roof" I've ever encountered has been tinned, or "Galvanized" steel, and Tinned Beef has come from steel cans in my experience. My Grandmother had a hand cranked machine for crimping lids on metal cans, and I remember seeing stacks of new canning lids. But for the most part, she used canning jars.

I bet most people don't know what a tin lizzie is.