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92 Mustang LX
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Discussion Starter · #1 ·
I’m getting mixed results here. I’m reading that a rule of thumb is .025” will raise/lower compression by a half point or .5, but using 3 different calculators, they all show that a .025” increase/decrease changes it about 1.2 points, meaning 12.5:1 to 13.7:1.

So if .025” in gasket thickness will change it by 1.2 points, how comes going from a 4.100 bore to a 4.030 bore which is .070” reduced bore diameter only changes it by .08 point, meaning 12.5:1 to 12.58:1 ????

Right now I’m using a MLS, 4.100 bore, .041” compressed thickness. The thinnest MLS gasket I can find is a Cometic 4.030 bore, .027” compressed. Using the same calculators shows it going from 12.5:1 to 13.3:1. Does this seem right?
 

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what bore size and deck clearance are you at now?
 

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The thinnest MLS gasket I can find is a Cometic 4.030 bore, .027” compressed. Using the same calculators shows it going from 12.5:1 to 13.3:1. Does this seem right?
Supposedly the .027 is a little too thin because of piston to head clearance, if you're at 0 deck anyway, so might want to look into that first. I think the rule of thumb is -0.006 is -1 cc difference in combustion chamber volume.
 

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Discussion Starter · #4 ·

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Discussion Starter · #5 ·
Supposedly the .027 is a little too thin because of piston to head clearance, if you're at 0 deck anyway, so might want to look into that first. I think the rule of thumb is -0.006 is -1
Supposedly the .027 is a little too thin because of piston to head clearance, if you're at 0 deck anyway, so might want to look into that first. I think the rule of thumb is -0.006 is -1 cc difference in combustion chamber volume.
Right now I have .051 clearance (.010 deck height & .041 gasket) so going to a .027 should be ok as it’ll be at around .037” total. I always thought .030-.035 was safe, but I’m using older cast dome pistons and they do rock a bit when their cold so I don’t want below .035
 

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I show that a .01 drop in gasket thickness is worth about 0.5 points of compression, which stacks up pretty well with your figure of .025 being worth 1.2-1.3 points. I show that if you drop from 0.041 to 0.027 then you should be at about 13.2:1.

This is the calculator I use:

I dig your 302 setup.
 

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65 fastback pump gas .060 289 (297) solid roller, T5 all motor. 11.12 122 mph
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I ran the pistons in my last 289 .005 above deck with a copper fire ring felpro 1011-whatever. 4.10” bore. Quench should have been around .030”. Seemed fine at 7,500 rpm shift points.

The new 306 build will have the pistons at zero deck and something around a .038” compressed thickness and smaller bore diameter. Playing it safe since this one will want to pull a little higher and make more power.
 
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