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What pistons?

6K views 25 replies 8 participants last post by  dwilfong 
#1 ·
My motor is currently at the machine shop and found the knock was the #1 piston skirt destroying itself. All of the coating is gone and beat up but the bore is good.

My question is, Do I have them throw a set of forged pistons in and hone to fit or just toss some stockers back in?

Since the crank and rods are clean, i'll be re-using them with ARP main studs and ARP rod bolts. (Yes they will be checking the rods with the new bolts before installation)
 
#2 ·
I am going threw this now. I can tell you what I have found.
The block needs to be honed and checked to see if the cast pistons will still work with the clearance after honing.
Most likely thy will not.
I went with JE .005 over forged pistons as this was the best priced piston with a small over size. This way you are only paying for a hone job not a boar and hone.
Thy are lighter than the cast stock pistons so the assembly needs to be balanced.
I was told do not boar the block if you do not have to. The steel sleeves are thin in the stock alum block.
Cast pistons are .020 over at the smallest over size.
I was advised not to go there if at all possible.
I am also using the stock rods and new stock crank.
Thy have a pile of stock rods in the scrap heap. some times thy have to go threw a dozen to get a good round core.
Also the rod bearings are tricky with the stock rods. thy have to measure the crank and the rod for each journal to get them right. Takes lots of different size bearings on hand to do this. Then thy have to go on the journal thy match with.
Also the block will need to be line honed or board to set the main journal clearance with the soft bearings. The hard bearings come in more sizes but the cost is more than the soft ones and the hone job.
Not saying all shops do this....But well you know........
This is what I have bin told by the machinist doing my block work.
 
#5 ·
Thanks for the input so far. I have a 10 sec drag car already so don't need to go all out on this one but the forged rods do sound like an option if the labor to reuse stockers is high.

I'll have to see how everything measured up when I swing by the shop on Friday.
 
#7 ·
I have my eye on the 4.005 Wiseco pistons but would need to special order them a little less than 11.6:1 unless thicker head gaskets are an option. This is my first LS build so still learning.
 
#13 · (Edited)
#15 ·
Cometic Head Gaskets Domestic items in c5477 store on eBay!

Thin gaskets for the head with a 4" bore. Not a over size boar gasket like the stock ones are.

Use the thin ones and it is the same or more than cutting the head .016

You have to run the calculations. Also the exact boar of the gasket will gain some.

It may also change depending on the piston to deck measurements after the assembly.

run the number around in this. http://www.wallaceracing.com/cr_test2.php

Its bore man. Bore
 
#14 ·
Hone the block to 4.005 and get diamond 4.005 Pistons.
 
#16 ·
I wish I had the coin to go e85 since its available here, then I could worry less. I'm not afraid to take it a little over 11:1 but unsure of the pros/cons

I'll update tomorrow when I get to see the bare block and probably call Matt. Thanks for the gaskets links.
 
#19 ·
Remember the cylinder pressure is what maters not the CR.
The valve timing will change the cyl pressure with the same CR.
With a bigger cam you will need more CR to maintain the same cyl pressure.
Unless some one has the same cam and knows the CR it is hard to calculate cylinder pressure.
Don't think many even check there cranking pressure to tune a eng setup.
On out board eng making 300+ prop shaft HP it is every thing.......145psi is the max on 93 pump gas.
Thy don't have knock sensors to control detonation..........
Then if you are on the sensor you are leaving power on the table.......
So what are the cranking pressures on some different setups ???????
YES bore.............never said I could spell........LOL
 
#21 ·
It would be nice to see some cyl cranking pressures for different cam and CR setups.
What is the advertised cyl cranking pressure for the stock eng?
The one eng I ran in my 70 GTO was a 400 with fly cut TRW 12.5 forged slugs.
Came in at 12.1 CR. the cyl pressure on the gauge was 280 PSI with a 280 deg cam.
Needless to say that did not stay in the car long.........LOL
Got tired of dumping GEC in the tank and turning the distributor every week end to go have fun.
Now those where the days...........
 
#23 ·
I'm not used to having fun on the streets. The race car is "Somewhere around 14:1" since we never CCd the heads after milling. we just run it on VP C12 so I never worried about the calculations.
 
#22 ·
I'm still waiting on the block to get cleaned for final measurements. The #1 hole is visibly scored near the bottom of the bore which shocks me for a stock motor. The Coating is gone off every piston but the rest of the motor is more or less spotless. Builder says pistons were too tight in the holes from the factory by feel...I trust his spec more than GM's at this point.

The machine shop is a Wiseco dealer so its just a matter of honing to fit a 4.000 vs 4.005 I think. Crank needs a light polish and we are ready to roll.

Thanks for the tips on static CR.
 
#24 · (Edited)
280 psi is a good racing setup. If the cylinder pressure goes to high it will hold you back on top RPM's a little.
A simple compression tester is your best friend some times for tuning.
It will tell you if you have to much or to little CR for the cam.
The shop you go to will get the best pistons for the price for sure. I went with the JE because he gets them at a good price as this all thy use. $540 for the piston and rings is a great deal.
 
#25 ·
What Part # did you go with? I don't see any JE's that fit my bill but yea $540 with rings gets the job done.
 
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