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Old 06-28-2016, 05:20 PM   #1
tubman
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Default Combustion chamber "squish" question

Today I pulled the stock heads off of the used '51 Mercury engine I am getting ready to put in my '51 Ford club coupe. First of all more good news; barely detectable ridge, no odd colors, clean water passages, etc. I then proceeded to check the dimensions on my Edmunds heads. We did the drivers side first. and got a consistent .150 over the valves. Clearance over the pistons was (front to back) .065, .055, .055, and .055. So far, so good. The passenger side came in at.170 over the valves. Clearance over the pistons (front to back) was .068. .070, .080, and .072. These were used heads I picked up locally and seemed to be in excellent shape in every way; no broken fins, good tapped holes (plugs, water outlets, etc.), and they seemed to be dead flat when checked with a good machinists straightedge.

I have to believe that some time in the past, the drivers head was milled .015-.020 to correct some minor problem. My plan at this time is to have .015 taken off the passengers side head to even things up. How does this sound to you knowledgeable flatheaders out there? I want to run the Edmunds heads as the spark plug placement will allow me to run extended tip plugs, which I believe that will help efficiency. This engine is being built for crisp running and economy first, with getting some extra power out of it as well. I will be running a converted vacuum over mechanical SBC distributor and a small base 2GC on a bored out aluminum Merc manifold (to start out; I have a couple of vintage 4BBL setups to try later if I'm not satisfied with the 2GC.

Last edited by tubman; 06-28-2016 at 07:15 PM.
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Old 06-28-2016, 09:48 PM   #2
Kahuna
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Default Re: Combustion chamber "squish" question

Hi Tub
How was the clearance checked?
Was a compressed gasket used?
Were the measurements taken with the compressed gasket thickness or no gasket?
Essentially you want around .045" above pistons with the gasket installed. No more than .050". Some go as low as .035".
As you mentioned, it's a great idea to equalize the clearances on both sides of the engine with the gasket installed.
This will give you the best economy & performance.
Jim
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Old 06-28-2016, 10:13 PM   #3
tubman
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Default Re: Combustion chamber "squish" question

We put aluminum foil balls on the pistons and valves held on by dabs of grease. I used the gaskets that were on the car with the stock heads. They were the original "Mercury" script gaskets, and I believe they heads have never been off the engine before. I had thought that the ideal "squish" was between .050 and .060. If your numbers are correct, it seems like I should be taking .010 off of the drivers side and .025 off of the passenger side. this would give me clearances of .055, .045, .045, and .045 on the drivers side and .043. 045, .055, and .047 on the passenger side. As you see, if I do that, the variance would be.012 (.043 to .055). Do you think this will be a problem? Since I don't have any machining capabilities, I would be reduced to modifying the chambers with a die grinder. It looks like the chambers have never been touched, and I don't feel like getting in there free hand.

Thanks for your comments.

Last edited by tubman; 06-28-2016 at 10:19 PM.
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Old 06-28-2016, 11:35 PM   #4
Bassman/NZ
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Default Re: Combustion chamber "squish" question

Sounds like a plan. I doubt you need to even up the chamber volumes, it will make negligible difference on a basically stock street engine. My squish averages out about .050 and each chamber is different. Runs fine.
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Old 06-29-2016, 01:59 AM   #5
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Default Re: Combustion chamber "squish" question

Tub,
If it were mine I'd do the .010 off the drivers side and. 025 off the passenger side. If you could confirm the thickness of your head gaskets and that they are same thickness side to side. If there the same, go for it.
Martin.
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Old 06-29-2016, 06:41 AM   #6
JWL
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Default Re: Combustion chamber "squish" question

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What is meant by "front to back" measurements? Is that the locations of the measurements or the order in which the measurements are displayed? How many "balls" were used on each piston? I use 8.

The most important area for tight clearance is at the outside with head torqued on compressed gasket.
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Old 06-29-2016, 09:27 AM   #7
Ol' Ron
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Default Re: Combustion chamber "squish" question

I use aluminum balls to check clearance. I feel this is more accurate than clay.
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Old 06-29-2016, 09:36 AM   #8
tubman
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Default Re: Combustion chamber "squish" question

I used five foil balls, one in center and four equally spaced around the outside of the piston, about 1/2" from the edge. All were within a couple of thousandths (plus or minus) of each other for each cylinder. It looks to me like Edmunds did a pretty good job of conforming their combustion chamber to the piston top, so I figure it's good to go. The measurements given are for the center of the piston front to back on each side. I am trying to keep anything I do to this setup to be minimally invasive, since all the components are used. I'd rather give up the last 5% gain if I don't have to start grinding on the heads. The head gaskets on the engine, though compressed to .055, look to be in excellent shape, and in a pinch look like they could be reused. It appears no sealer of any kind was used in the initial assembly, as the heads, and then the gaskets, lifted right off the block with no transference of material. I am going to install my new Fel-Pro composite gaskets the same way and hope for the best. It also appears that the factory applied sealer to the threads on the short head bolts only. I was careful and used thread chasers instead of taps and dies to clean the threads. I was able to turn the chasers by hand on the great majority of the bolts and holes. Since this will be the second assembly of this engine, I am planning on using thread sealer on all of the head bolts.

i'l say it again; it seems like I got incredibly lucky with this engine. It had to happen to somebody after all the disappointments we all have had over the years; I'm just glad it was me.
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