QwkTrip

iTrader: (23)

Doing a head swap on my LS7. I think rule of thumb is don't go tighter than 0.035" quench.

Question: Is that a nominal measurement on the pin centerline axis, or does the 0.035" have be added on top of any possible piston rock?

I'm guessing the spec for 0.035" quench already takes into account piston rock plus other factors such as parts growth, and I should measure at the piston pin centerline axis. But I don't want to work on assumptions and have a piston kiss the cylinder head!

Thanks for the help.

KCS

iTrader: (20)

I measure the rock and use the number in between. So if one side rocks .020" above deck and .010" below deck, that's .030" of travel. Divide that by 2, and when either added to the below deck number or subtracted from the above deck number, you end up with .005" above the deck.

QwkTrip

iTrader: (23)

So basically you use an average value and not worst-case. Similar concept to checking on the pin centerline axis, but you don't have to know where is the centerline axis.

Che70velle

iTrader: (6)

A deck bridge with dual gauges is much faster, but it’s a tool that’s not worth the investment if you don’t build many engines.

QwkTrip

iTrader: (23)

Yep, I've got the tools. Just need to know how to take a quench spec and use it in the real world.

KCS

iTrader: (20)

I wouldn't think of the higher piston rock measurement as worst case because if it were rocked one way or another and it got too close, it would be able to give a little bit by rocking the other way. Where it is relative to the deck when the piston is square in the bore is worst case IMO, since it won't be able to rock if that measurement is too close at TDC.

QwkTrip

iTrader: (23)

I took a stab at this with measurements of piston #8. I'm guessing in this example I should use a value of 0.008 inch out of deck. This equates to a gasket thickness of 0.045 inch to get just a hair over 0.035 quench. And repeat 7 more times to see which piston is worst case scenario.... sound reasonable?

Measurement on piston centerline:
0.008 inch (left and right side)

Piston rocked down at top side:
top .002
bottom .011
Average = .0065

Piston rocked down at bottom side:
top .009
bottom .005
Average = .007

Frank O'Brien

Is there more drag on the outerside of the piston(down side, towards exhaust manifolds)? Causing the Valley side to be more of a concern? Piston pin offset? Honest question.

Che70velle

iTrader: (6)

If you have the tools, forget the math. It’s too easy to induce a mathematical error into this.
1.Bring the piston up to top dead center using your deck bridge.
2.Rock the piston each way, until the gauges read the same, both 12 o’clock, and 6 o’clock.
3.Re-check top dead center, with the gauges.
4.Record the number with a sharpie, on the deck surface.
5.Repeat 7 more times.

The distance out of the hole here is what you add to your gasket to achieve quinch. So whatever quinch your looking for, let’s say .040” for easy math, you’ll add the out-of-hole number to this. If your decks are perfectly square, which I doubt, but if they are, you take the .040” and add the out-of-hole number to this.
For instance, .040” (quinch)plus, say .005” (OOH) for your numbers (if decks are perfect...) would give you a .045” gasket.

If your decks are like most of ours, with 8 different numbers, you’ll take your piston the farthest out, and use that number. If your decks are terrible, it might require two different gaskets. Prolly not, but it’s possible.

I used to use the average number method, with means, as KCS is describing. It works just as well, until Tony Mamo talked me into a deck bridge. Easy peasy.

QwkTrip

iTrader: (23)

Ya, I have a bridge and 2 dial indicators. I abandoned my first method and redid everything with a single dial indicator at center of piston (it was the only location that didn't seem to be influenced by piston rock in any direction).

I got 0.006 - 0.007 for all even cylinders, and 0.003 - 0.004 for all odd cylinders except #5 which was lower at 0.002.

I will redo it again using your method and see what numbers it comes up with.

QwkTrip

iTrader: (23)

Did it again with dial indicators at 12 and 6 o'clock positions and leveling out the piston before taking a measurement. This produced less variation.

Even side, all at 0.006 except #4 at 0.007
Odd side, all at 0.003 except #3 at 0.004

#3 and #4 share the same crank journal, right? That would explain why those two are both taller by a tad.

QwkTrip

iTrader: (23)

The way things are patterning out I could run a different gasket thickness right and left hand side. Anybody ever do that?

Darth_V8r

iTrader: (4)

Its not crazy common but it does happen

pantera_efi

iTrader: (1)

Hi Frank, I agree with your observation, the Pin Offset effect.
The Side/Side is opposite with an off set pin.
The piston is MORE balanced with the OEM Pin Offset, GOOD for Turbo use.

My Squish measurement is completed AFTER the engine is raced.
I "scribe" the cylinder # on both of the Head Squish Pads, each side.

After the race I can read the head cylinder number on the piston crown !
This is when the head is removed, though a Bore Scope could do the same read.

Lance

QwkTrip

iTrader: (23)

I called Cometic tech line today. They have some catalog options of 0.036, 0.040, and 0.045 compressed thickness. They tell me they can make a 0.042 but can't do anything between 0.036 and 0.040 (Apparently that's not a stack height they can come up with). Cost of a 0.042 is same as other catalog options.

So I could do a 0.045 and 0.042 gasket combination and even out the sides perfectly with 0.038 quench. Or I could do a 0.042 and 0.040 combo and nearly perfectly even things out at 0.036 quench.

Do you think the LS7 can handle quench as tight as 0.036?

Che70velle

iTrader: (6)

Mine is .035”

ColeGTO

iTrader: (3)

In my opinion .005"-.010" additional quench is not going to make a significant difference in power or tuning.

If it's an aluminum block it will grow with heat, so your .035" cold is more like .040"-.045" at running temperature.

If it's a drag race only engine that operates at a much cooler temperatures for much shorter durations the engine can handle a tighter quench.

For a "street car" that has the potential to see elevated temps (200°F+) on an occasion, I'd stay closer to .040" as the minimum.

If it's too tight, you find out the hard way with a pinched ring, broken ring land, broken piston, hole in the block, etc. Not worth it in my opinion unless you are trying to set a world record and need every last .5 HP.

Again, just my opinion.

QwkTrip

iTrader: (23)

Good points. This engine has cost waaaaay more than I told my wife it would and I sure as hell can't take a chance ruining anything. This is probably the last time daddy gets to spend money like this for a long time.

Got a note from Tony Mamo and he advised that a stock LS7 short block can go as tight as 0.040 gasket if the tallest piston is 0.006" or even 0.007" out of hole. That's some pretty tight quench!

Tony @ Mamo Motorsports

I've staggered gaskets many times on engines that I can do so. Thing is if you have one side at say all .007 - .008 out of the hole for example and the other side has three pistons .004 out and one .007 out, your really stuck running the same thickness gasket on both sides because you have to cater to the tallest piston on each bank of the block!

Occasionally I do get blocks that one side is a few thou lower across the board and I will order say a .034 thick and a .036 thick etc., just a hypothetical example

Naturally this assumes your confident in your measurements but most LS7 OEM shortblocks are .003 - .007 out of the hole. Fact is with an aluminum block that grows .008 - .010 hot (that's why valve lash changes so dramatically when the engine is warm with an aluminum block) AND the fact that OEM blocks have very tight piston to bore (so minimal piston rock)....all this allows the tighter quench without fear of slapping the piston into the heads and IMO they all run a little better with tighter quench....are less prone to detonation and have a slightly more effective burn. Its not going to make a big difference but its one of those "detail situations".....and those kinds of things tend to add up....especially when you focus on ten items just like that!



Hope this helps!

-Tony

QwkTrip

iTrader: (23)

It more than helps, Tony. Thank you!

I've got the time to sweat the details with this engine. I'm not very experienced so I'm going through learning curve of what that means but I'm definitely putting in the effort this time.