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Old 08-07-2018, 04:33 PM   #1
Ol' Ron
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Default Piston to head clearancing

I get this question allot, thought I just post this information for those that might not understand why this is a good practice.

Understanding why this is important will give you the reason to do it. Modern engines have a very hi compression ratio, and still run on regular gas. This is due to the very hi turbulence in the chamber just prior to ignition. This is done by an area called the Quench area. The Flathead doesn't have such an area, so we have to make one. Thus the .045/.050" piston to head clearance. I measure this with little alum balls I make from aluminum foil. A bout 1/2 3/8 in in diameter. I stick them in the area to be measured. Now if your not sure about this, have someone else help you. You must mill the head to get this number, which is usually just an area around the perimeter of the piston. I relieve the valve clearance at to top of the cylinder head with a small die grinder, so as not to remove to much material which would lower the CR. Doing this will not raise the CR very much, but cause a hi degree of turbulence in the chamber condensing the AF for better and complete combustion. Moer power, economy and better throttle responses. And for very little cost.
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Old 08-07-2018, 05:40 PM   #2
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Default Re: Piston to head clearancing

I agree with Ron about this. I spent a lot of time getting .050 clearance on my last two engines. They both run very well and are "crisper" than those I built previously. It is well worth the effort.
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Old 08-07-2018, 06:51 PM   #3
Ross F-1
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Default Re: Piston to head clearancing

I've heard about this and when the time came for a rebuild, I "clay'd" the clearance to see what would be required. (see below) The numbers are with a used head gasket torqued down.
I asked my machinist (who has done hundreds of mild-to-wild flatheads) what he thought about milling ~.070" off a head; his jaw literally dropped open! From what I've read here and elsewhere, I think my numbers are pretty common for uncut heads.
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Old 08-07-2018, 09:52 PM   #4
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Default Re: Piston to head clearancing

I would agree.
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Old 08-07-2018, 09:57 PM   #5
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Default Re: Piston to head clearancing

What's stock piston to head clearance?


When in doubt. Mill .050.
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Old 08-07-2018, 10:36 PM   #6
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Default Re: Piston to head clearancing

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I think that measuring is the only way to do this. I only concern myself with the area around the perimeter of the piston. However a check should be made in the center for safety. Just milling off a bunch could be expensive
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Old 08-07-2018, 10:38 PM   #7
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Default Re: Piston to head clearancing

I agree Ron. Was wondering about stock clearance.


Lets say you have stock heads you "think" have never been milled. How do you check this? I don't know.
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Old 08-07-2018, 10:48 PM   #8
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Default Re: Piston to head clearancing

Has anyone used a 4 1/8" stroke crank with stock 4" stroke pistons? This would put the piston .0625" closer to the head, and in the case of post #3 would be close to ideal with a little hand work.
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Old 08-07-2018, 11:38 PM   #9
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Default Re: Piston to head clearancing

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Originally Posted by 40 Deluxe View Post
Has anyone used a 4 1/8" stroke crank with stock 4" stroke pistons? This would put the piston .0625" closer to the head, and in the case of post #3 would be close to ideal with a little hand work.
That was standard practice in the olden days.
The pistons then came to deck height.
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Old 08-08-2018, 05:48 AM   #10
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Default Re: Piston to head clearancing

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Originally Posted by Ol' Ron View Post
I think that measuring is the only way to do this. I only concern myself with the area around the perimeter of the piston. However a check should be made in the center for safety. Just milling off a bunch could be expensive
I'd second checking in the centre too. Some pistons (French for one) are more "pointy" than others. I had a problem with stock French pistons hitting an unmilled Offenhasuer head. They only touched in the middle.

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Old 08-08-2018, 06:45 AM   #11
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Default Re: Piston to head clearancing

When measuring for the 45 - 50 thou clearance is it done with a gasket in place or without a gasket?
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Old 08-08-2018, 07:02 AM   #12
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Default Re: Piston to head clearancing

With a used gasket, torqued down.
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Old 08-08-2018, 07:29 AM   #13
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Default Re: Piston to head clearancing

what would be the difference between flat and domed pistons as in the way clearance is measured?
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Old 08-08-2018, 07:51 AM   #14
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Default Re: Piston to head clearancing

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Originally Posted by Ross F-1 View Post
I've heard about this and when the time came for a rebuild, I "clay'd" the clearance to see what would be required. (see below) The numbers are with a used head gasket torqued down.
I asked my machinist (who has done hundreds of mild-to-wild flatheads what he thought about milling ~.070" off a head; his jaw literally dropped open! From what I've read here and elsewhere, I think my numbers are pretty common for uncut heads.
Yup, your measurements are what I normally find on uncut heads and OE Ford pistons.
Martin.
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Old 08-08-2018, 08:27 AM   #15
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Default Re: Piston to head clearancing

I milled .045 and .055 off a used set of edelbrock heads to get the clearance I wanted. I bought it as a stock unfired rebuild. looked like the deck had been skimmed but there was .010 difference per side.



seems to run good. definitely sounds different than my last motor which had about .070
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Old 08-08-2018, 02:19 PM   #16
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Default Re: Piston to head clearancing

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Originally Posted by 40 Deluxe View Post
Has anyone used a 4 1/8" stroke crank with stock 4" stroke pistons? This would put the piston .0625" closer to the head, and in the case of post #3 would be close to ideal with a little hand work.

I suppose it's too simple an idea to just increase the compression height a little (say, .030" - .040") on an otherwise stock piston blank? That would help a lot.
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Old 08-08-2018, 03:02 PM   #17
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Default Re: Piston to head clearancing

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Originally Posted by marko39 View Post
what would be the difference between flat and domed pistons as in the way clearance is measured?

No difference. In either case you put strips of clay or little foil balls on the piston tops.
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Old 08-08-2018, 03:07 PM   #18
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Default Re: Piston to head clearancing

Thanks for posting this Ron.

I was getting ready to ask questions about this regarding new after-market heads (Offy, Edelbrock, etc.) to verify that these also need to be measured and/or milled.

As I understand what you are saying and what others have posted, I think the answer is yes, regardless of the CR these manufacturers may advertise.
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Old 08-08-2018, 03:17 PM   #19
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Default Re: Piston to head clearancing

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Originally Posted by Heard View Post
Thanks for posting this Ron.

I was getting ready to ask questions about this regarding new after-market heads (Offy, Edelbrock, etc.) to verify that these also need to be measured and/or milled.

As I understand what you are saying and what others have posted, I think the answer is yes, regardless of the CR these manufacturers may advertise.

the squish has nothing to do with actual compression ratio. the idea is to keep the combustion area as small as possible. if the piston to head clearance is large this becomes another portion of the combustion chamber even when the piston is all the way up. think of a smog head when they used dished pistons to get lower compression. the motors pinged and never ran right. all because of a large chamber volume. in a flathead you actually want the combustion chamber to be the transfer area only.
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Old 08-08-2018, 05:57 PM   #20
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Default Re: Piston to head clearancing

I to followed Ol Ron’s advise last winter when building my 59ab . The machine that
Milled the heads is still talking about my request .
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