School Me In Flycuting Pistons!!!!

R

REEDSTR

SHADE TREE TECH
Joined
Dec 3, 2008
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Well Im Building A 12v And Im Wanting To Know All The Formulas And All That Good Stuff...... My Head Is Milled .010 And The Block Is Going To Be Milled As Well So Im Just Wanting To Know How Much Needs To Be Cut Off The Pistons. Ive Also Heard Ppl Are Making More Power By Dropping The Compression A Wee Bit, Is This True????
 
Your cam profile will dictate how much you need to flycut your pistons... or if you even need to. Just because you have your block and head decked, it doesn't mean you need to flycut your pistons. When you get a cam from a manufacturer, they will let you know if you need to and how much.
 
any one else want to add in on this i would like to know alot more then just that
 
it is when you machine the valve reliefs in the top of the piston for higher lift cams and larger diameter valves
 
REEDSTR said:
any one else want to add in on this i would like to know alot more then just that
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There is 2 kinds of fly cutting.

One is to drop compression, one it to clear the valves on a high lift cam.

If you drive your 12v you want to keep the compression over 15.5.

Over 70ish psi you may want to drop the compression.

You can make up for the decked head a block with a thicker head gasket. 0.010 off the block, 0.010 off the head, get a 0.020 over head gasket.

This is all you need to know.
 
fly cut 12 valve piston on top:
 

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JFlagg said:
On which motor? I think it varies between VE 12V, P-pump 12V, SO 24V, HO 24V and CR... I think my 2002 HO 24V was 16.5 to 1.
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HO trucks are 17:1, and SO trucks are 16.5 or 16.3:1.

I think he has a 12v though...
 
j-rod said:
HO trucks are 17:1, and SO trucks are 16.5 or 16.3:1.

I think he has a 12v though...
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Thanks... I knew the HO were about .5 to 1 higher than the SO and 16.5 was stuck in my head. And with that being said, I knew he has a 12V, but all he asked wa what stock compression was, nbt what stock compression for his truck was. All in all, he isn't helping the thread with his vague questions...

And to answer the OP's original questions, generally more compression will make more power... BUt there comes a point when you are forcing so much air/fuel into the cylinder that you start stressing the internals(rods and pistons) so you can drop the compression to help relieve those stress' and this also gives you more room for even more fuel and air. Though, unless you plan on breaking 1000 hp, I don't think you NEED to lower the compression.

And as far as how much you NEED toremove off the piston, find out how much needs to be taken off the block first, then work from there. Decking the head won't matter as you aren't changing the displacment of the cylinder, though you can effect injector and valve protrusion.
 
While we are on the subject and not to derail this thread. What is the stock specs for piston protrusion on a 12v?
 
be nice for someone to chime in that has actually done it maybe telling what bit they used on what mill and how they held the piston in the vice and if they tried to balace out the weight afterwards so that one piston does not weigh that much more then another. I have no idea what all goes into this so im just rambling. I am planning on doing this on my build but can not get good info on it.
Gonna have a buddy do it that was a machinest and would like to get some good facts on it instead of well you dont need to do this or how deep should it go.
 
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