3.90 Crank piston question HELP!!!

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Oct 25, 2011 | 09:16 PM

Luke1834

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From: Rochester,NY

3.90 Crank piston question HELP!!!

Ok so i have the motor at the machine shop and i am trying to decide what to do and what pistons to order from MAHLE i am set on using there pistons because of budget. but i have a 3.9 eagle forged crank and eagle H-Beam rods 6.1 and im wondering exactly what pistons to use from Mahle. so my questions are

-can i use a 3.622 stroke piston with a 3.9 crank

-what will my C.I be if i have stock bored forged pistons and a 3.9 stroke crank

The reason i am staying with stock bore is because i am doing a light hone i plan on using a 200 shot of nitrous and this is on a 98 trans am block thinner sleeves

Thank you

Oct 25, 2011 | 10:27 PM

noice

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From: NW AR

9.24" should be your total deck height but this could change if the deck has been milled.
Now you have already picked a 3.9" crank so that is 1.95", plus your Eagle H-beam rods which I believe are not just 6.1" but are actually 6.125".

1.95+6.125=8.075

Deck Height - (Piston & Crank Height) = CH (Piston Compression Height)

9.24 - 8.075 = 1.165

So you have to find a piston with your bore that has a CH of 1.165"

I don't know of 3.9" cranks by Eagle, I know they make 4" cranks though. If that is the case I know they have Mahle off the shelf pistons for those, maybe with a thick head gasket or machining the pistons down a bit you could use the pistons they spec for a 4" stroke.

The reason you can't use the pistons from the 3.622" stroke crank is that they will have a CH of 1.314" which will be nearly a 1/4" out of the block at TDC.

Oct 25, 2011 | 10:47 PM

Luke1834

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i think i might just have the machine shop call ross or diamond to make me a set of custom pistons its a little bit more money but its to hard to find a set for my bore

Oct 25, 2011 | 11:37 PM

noice

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Quote:

Originally Posted by Luke1834

i think i might just have the machine shop call ross or diamond to make me a set of custom pistons its a little bit more money but its to hard to find a set for my bore

Mahle makes pistons for your bore, they even make some .005" over pistons. They make them for the 3.622" stroke, and the 4.000" stroke. Also with the .943" or a .927" pin.

Oct 26, 2011 | 08:10 AM

Luke1834

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But I don't have either of those strokes I have a 3.90

Oct 26, 2011 | 09:01 AM

slowride

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Looks like probe makes a cheap set of 3.90" stroke pistons, but I think they are for 6.125 rods. It would probably be cheaper to switch rods use these pistons rather than getting custom stuff made. http://flatlanderracing.com/probe-sportsbc07.html

Looking around flatlander's site you might be able to use some racetech pistons and have them tweek the compression height to work with your rods/crank for no extra cost.

Check around there might be more off the shelf ones and that will save you a lot of coin. You can offset the piston in the hole some with really thin headgaskets, etc. You'll have to run the numbers and see, but finding a piston for a 3.9" stroke and 6.098" is not going to be common.

Oct 26, 2011 | 09:45 AM

Luke1834

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I think my rods r 6.125 if that's the case then I can use those probe pistons with no problems , how will the probes hold up to like say 150 shot of nitrous