98RedBird

iTrader: (77)

Pistons are sticking out of the hole .009" +- .001" on the new motor. It'll be N/A with maybe a small shot of nitrous later on.

Head chambers are 4.125 so that leaves me with Cometic for a gasket.

They offer 2 that'll work. either a .051" or a .045"

Which would you choose? .035 seems like it's getting on the tight side, and .041 puts me right in the middle of "ideal"

Also, are you shooting for a warm quench or a cold quench number? Logic would say that .035 would be good on an aluminum motor, let it warm up, and grow into the .041" range, rather than starting at .041 and growing out to .046"

Thanks

NEstyle

iTrader: (1)

I would do 0.035". It's calculated when cold.

spanks13

iTrader: (5)

.035 is too tight, go for .041.

The larger the bore the more piston rock you get. If you rev the thing you don’t want the pistons hitting the head.

98CayenneT/A

I run .033"

NEstyle

iTrader: (1)

Lots of people run 0.035".

Bazman

I run about 31. 35 too tight my ***. LS love it tight. To be fair, I don't mind it tight myself

Che70velle

iTrader: (6)

.035” here with a 4.155” bore.

spanks13

iTrader: (5)

Enjoy it guys lol. This isn’t something I’d slap together based on advice on the Internet. Check piston rock at TDC and make your own judgement call.

You don’t want the piston touching the head.

NEstyle

iTrader: (1)

He asked for advice on the internet and he got real world advice on the internet from guys that have experience. Isn't that the whole point?

Che70velle

iTrader: (6)

Piston rock will decrease as the engine heats up. Piston to wall clearance will tighten. I’ve seen .029” set up cold, iron block, and it went 500+ laps at Bristol. Tony Mamo tried .028” and it bit him, but only because he had one connecting rod a smidge longer than the other 7, and he missed it. .035” is fine with LS engines. Make sure out of hole is measured square with a dual gauge deck bridge for best results.

KW Baraka

iTrader: (7)

I run .035" and I regularly spin to 7K RPMs.

Go with .035".....and give that baby some compression while you're at it!

KW

G Atsma

How much are LS pistons USUALLY out of the hole, IF there is a common spec? Or are they all over the map? I had heard about .005-.006, and used with the .052 (I think) GM gasket gives about .046 quench. Am I figuring this right? I'm kinda new to figuring combustion clearances, so forgive my ignorance.

KCS

iTrader: (20)

The stock iron block engines I've checked were all about .006", but when you use aftermarket pistons and rods, and have your block machined, it could be way different.

Back in the 80's, MIT tested the effects of quench and found that it has less and less benefit as the compression ratio is increased. If you measure each hole correctly, you can get a way with .035" quench. If you're just guessing based off calculations, I would go with .041" to be safe.

Polyalphaolefin

I've run as tight as .024" at 6500 rpm without contact, but the part number etched into the piston top was readable in the carbon print on the head so it was coming extremely close. I would say .035" is more than enough clearance.

The bottom end also plays a factor in this. The heavier the pistons, the higher the inertia. The longer the stroke, the higher the inertia. Also lower the rod/stroke ratio, the higher the upward inertia.

G Atsma

So iron blocks usually about .006. Aluminum? I do realize aluminum expands more as heated, so out-of-hole should decrease more than iron.

AINT SKEERED

iTrader: (2)

I'm at .037 on my 427 turning 7900 rpm

tech@WS6store

iTrader: (2)

If it runs on the street and you dont get consistent fuel use the .041 (.051 gasket) I highly doubt youll see any difference worth the tighter quench vs fuel quality issues.

KCS

iTrader: (20)

I'm not sure on aluminum, I can't remember ever measuring a stock aluminum shortblock.

tech@WS6store

iTrader: (2)

Pistons grow also so keep that in mind too.
AND not all blocks have the same deck height side to side as well.
If you want to get that precise youre jumping down a rabbit hole sometimes.

Darth_V8r

iTrader: (4)

I had always read 035. When I was doing all the **** on the 346, I found the motor ran much better at 042 than 035. On the 428, I set it up for 041, assuming higher rpm would increase rod stretch.

My pistons were dead even on deck height on the 428, 006 out of the hole on the sbe 346. I measure deck height at the centerline of the piston where it moves the least when rocking.