LD85

How much piston rock is typcial? is there an average?


When i chekced them there was @ .004"

Is this @ the average?

clem zahrobsky

it depends on the "piston to wall clearance" more clearance more rock :chevy

LD85

.030 over 4.155 bore.

66427-450

you have forged pistons? cast? or?

forged pistons expand more, so you build with more cold clearance...... so more rock when cold.

norvalwilhelm

You don't check piston rock you check piston to cylinder wall clearance. I mike the piston, then with a bore gage or snap gages measure the bore diameter. The difference between the two is the piston clearance. Cast pistons are alot tighter in the .002-.003 range and some tighter. With forged pistons I put them in at .0045 on a big block and tighter on a small block. I also know an engine builder you uses a feeler gage between the piston and cylinder wall. I do not like this method. Also be carefull as to where you measure the piston. I measure at right angles to the piston pin and just below the pin.
I also use a good micrometer and a calibration check with the supplied rod with the micrometer and everything at the same temperature.

LD85

The reason I ask is for the quench, I measured the top of the piston at several points, using depth micrometers, and took the hi and low and averaged them.

The "rock was @ .0035" so I am considering a close to zero deck so I can use .039 gasket this go around instead of .022 in the hole and .021 copper gaskets.


This would keep me in the .040 range for quench.

norvalwilhelm

I did the same thing. I decked my block for Zero deck height and used a .040 copper head gasket.

CFI-EFI

Chevrolet reccommeneds minimum piston to head clearence, of .035"-.040". With a zero deck and a .039" gasket, you should be at the "edge", but OK. Good luck, and...

RACE ON!!!
1984 Crossfire
Member NHRA

gkull

Gaskets are never as strong as the block. I would only deck a block if the surface was uneven. I have had some trued during blue printing. This last block and build up with my old crank, rods, and pistons was @ .022 in the hole. the .021 copper head gasket is still doing it's job

bence13_33

George:
I agree with you. I could have "0" decked my block when it was at the machine shop, but there really was no need to. The machinist said that the surface was true. So with my standard deck height taken into consideration, I then chose a .020" steel shim head gasket to get my .040" quench.

LD85

Yeah, the first time they did a kiss cut on the deck and that left @ .022 average, I added the SCE .021 copper and it sealed fine.

Now that they are redecking the block I told them to deck it at .005" so with an .039 gasket I should be @ .044"

This is ok for me and I wont have to re-torque the heads like I did with the .021 coppers. :thumbs:

gkull

ID85 - with a cut to .005 I would be tempted to go with a .030 head gasket.

I would have liked less than my .043 The problem is gasket thickness choices and being aluminum friendly.

LD85

I think I will look for some, that are reasonable, I would even settle for .035

I just checked the DD200 and a reduction of .010" is only good for @ 2-HP - 2-TQ.


[Modified by ld85, 8:16 PM 6/10/2003]

CFI-EFI

DD2000 nor any other simulator can show the benefits of a tight quench area. These programs don't take the benefits of swirl and knock surpression into account. The miniscule HP gain from the insignificant compression increase is not why we strive for a good quench area. Good luck, and...

RACE ON!!!
1984 Crossfire
Member NHRA

LD85

Then it should be easier to just do a zero deck, is there no fear of the piston contactin the heads?

black bart

Larry I have a zero deck the only clearence is just the 039 gasket.
The pistons were custom made by Ross they are D shaped inverted dish. After they were all mounted on the rods they were checked for overall length in a special fixture. The block was decked to match the shortest one yes there is a few thousands difference in overall length but once I had one the right length use dial indicator to check top of block and piston then removed that one put it in the fixture and got the reading then turned off the rest of the pistons so all were perfectly flush with the deck when installed. As far as rock I have some Ross recommends 20 thousand clearence on their blower pistons. I'm into my fourth year with this engine no problems so far :cheers:

clem zahrobsky

.020 piston to wall clearance????? :chevy

black bart

No on the top ring.
I would need to go back over my papers to be sure but I think the wall clearence was like .04 it is a little noisy when cold I always let it idle for a while before driving it.
Sorry for any confusion after reading my post again I didn't make things as clear as I should have. :yesnod:
I remember when I read the instructions I could not believe the clearences that they where recommending but these things are built really thick.

norvalwilhelm

black bart. .020 clearance on their blower pistons? I run forged blower pistons at .005.
There must be alot of piston slap, oil consumption and short ring life?

CFI-EFI

Reccommended piston to head clearence: .035"-.040". How you get there is of little importance. The math is a little easier with a zero deck, But a tight quench and sufficient clearence is what your shooting for. If you have the piston, down the hole and a gasket you trust, that makes the clearence you want...go for it. In that case extra money spent decking the block just to say it is zero decked, is a waste of money. Piston to head clearence equals, deck height plus gasket thickness. I have seen engines with the piston as far as .005" OUT of the hole. In that case you subtract the .005" from the gasket compressed thickness. Good luck, and...

RACE ON!!!
1984 Crossfire
Member NHRA