compression height

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compression height

Post  kjett on July 16th 2012, 11:40 pm

Ok, someone teach me someting. I'm looking for a new piston set and want to make sure it's correct for what I am doing. Looking at the Probe p/n 14781-080 which is a 4.440 bore, 4.15 stroke, with a 6.7 rod, and a compression height of 1.525. This is really for a 3.975 stroke crank with a set of 6.800 aluminum rods. I'd like to find a 4.420 bore piston, but looks like I will be going .080 to get the correct compression height.

So the question is, knowing that I need to leave something for the aluminum rod stretch, am I safe with the 10.32 factory deck height and a .040 head gasket? Or am I not leaving enough space?
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Re: compression height

Post  dfree383 on July 17th 2012, 12:26 am

Call Lem and Order what you need.
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Re: compression height

Post  DanH on July 17th 2012, 12:59 am

check the rod center to center lenght , aluminum rods can be shorter the spec. . built in extra deck clearance

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Re: compression height

Post  kjett on July 17th 2012, 1:15 am

DanH wrote:check the rod center to center lenght , aluminum rods can be shorter the spec. . built in extra deck clearance

I did, I have 2 sets that came with the asseembly. One is 6.750" and the other is right at 6.800"
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Re: compression height

Post  LivermoreDave on July 17th 2012, 8:40 am

If you have not measured your block, do so. You will be ask that dimension and it is very important as well as your intended operating engine speed especially with aluminum rods. Remember aluminum rods require frequent service!

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Re: compression height

Post  kjett on July 17th 2012, 8:49 am

Block was square decked and is at 10.31. So to verify my math, the 3.975 crank (x .5 = 1.9875), with the 6.75 rod, and a 1.54 c/h piston will leave me .0325 in the hole. Top that with a .040 head gasket and I get a total of .0725 room for growth. Stack height should be 10.2775 Is that too much or am I right where I should be? I can take a little more off the block if need be, just don't want to go too much. My machinist is letting me in to work on this myself under his guidance/teaching, so I want to be right the 1st time if I can.
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Re: compression height

Post  Mark O'Neal on July 18th 2012, 12:50 pm

kjett wrote:Ok, someone teach me someting. I'm looking for a new piston set and want to make sure it's correct for what I am doing. Looking at the Probe p/n 14781-080 which is a 4.440 bore, 4.15 stroke, with a 6.7 rod, and a compression height of 1.525. This is really for a 3.975 stroke crank with a set of 6.800 aluminum rods. I'd like to find a 4.420 bore piston, but looks like I will be going .080 to get the correct compression height.

So the question is, knowing that I need to leave something for the aluminum rod stretch, am I safe with the 10.32 factory deck height and a .040 head gasket? Or am I not leaving enough space?

At 10.310 Block Height and the 6.800 rods you'd be sticking out of the hole .002. With the 6.750 rods you'd be in the hole .048. So....with a .051 gasket with the long rod, or an .002 gasket with the short one Very Happy.

IF you pm me an e-mail address, and have tell me which version of excel you have, I'll send you the file I use to compute this stuff when I'm on the phone.....

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Re: compression height

Post  Mark O'Neal on July 18th 2012, 12:53 pm

kjett wrote:Block was square decked and is at 10.31. So to verify my math, the 3.975 crank (x .5 = 1.9875), with the 6.75 rod, and a 1.54 c/h piston will leave me .0325 in the hole. Top that with a .040 head gasket and I get a total of .0725 room for growth. Stack height should be 10.2775 Is that too much or am I right where I should be? I can take a little more off the block if need be, just don't want to go too much. My machinist is letting me in to work on this myself under his guidance/teaching, so I want to be right the 1st time if I can.

That's .075 piston to head, which is more than I'd like...but the math is right.

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