CFI-EFI:

I noticed in the archives that you had zero-decked a block while getting an engine together and regretted it. What are your specific thoughts on zero-deck versus a 9.020" or a 9.010" deck height? I do not know the exact compressed gasket thicknesses available, but as I am shooting for a quench area of ~0.040" to 0.044" with a 383ci aluminum head SR and 219, what are your thoughts on the combination of deck height and gakset thickness?

In other words, you seemed to be against a zero deck (although this seems to be the easiest route to good quench: zero deck and a 0.039" or 0.041" gakset)?

I want to deck the block to make sure it's square (and to clean up the mating surfaces) and will probably use whatever 0.02-something gasket I can find that's common and reasonably priced along with a deck height of 9.015" to 9.020" in order to save room for another decking if necessary and good quench. What do you think- am I missing anything on this particular topic?

 

Every one has an opinion. I decked mine so that the piston is down .005 in the hole. with a .039 gasket the quench is just about right with D cup or flat top pistons

 

Depends on the quality of your rotating assembly and your intended RPM range, a good forged setup will have less stretch, so the pistons wont slap the heads, I have a cast crank and Hyper-pistons with Forged rods, I have .015 in the hole with a .026 gasket.

 

cast crank, steel rods, forged flat-tops with 2 valve reliefs. 5500RPM and less, so that's why I figured 0.040" or so and a combination of a medium thick gasket and the remainder in deck height.

Just curious as to opinions on either way of doing it.

 

For the most part I have found from the factory GM has the decks from 9.025 to 9.035 In the last three LTx blocks I have done, they were all between 9.030 and 9.035. They say its factory tolerances and that they are acceptable.

Most all piston manufacturers making off the shelf replacement pistons for engines will make pistons with a compression height for a block that is at 9.00". The reason is that they know most shops will deck the blocks, but where who knows. And they also know that 9.00 is about the limit, although some will go to 8.900 but not very many.

I set my decks depending on the piston design. The last engines I built were set so that the pistons were .005 in the hole. The reason being is that this would allow for a piston rocking in the hole, which they do. I use Ross Forged pistons almost exclusively. These are 2618 alloy pistons which require a bit more clearance. If I were to use a 4000 series alloy or a cast Hyper piston that would need less clearance then I would make the deck at zero. Also I won't take a block down to 9.00" because if I did, then there isn't any more "meat" to take off should someday I need to. I take them down to 9.015" to square and true the block. Doing this I have my quench or squish between .044 and .050 depending on gasket thickness. Ideally you want it less than .060".

Then I order a custom piston from Ross to get the the deck clearance I need. Yes, its a bit more money, but from Ross I can get custom pistons for what JE charges me for off the shelf pistons. Plus I consider Ross pistons equal in all areas to a JE and I get excellent service from Pat at Ross as well. Very knowledgable guy and won't BS you, and is always friendly and wanting your business.

 

Thanks TJ - good reasoning in your post. Part of the issue is that my search at summit revealed mostly 0.039" or 0.015" thick gaskets so the middle ground isn't too common... I want to avoid custom pistons but if I do wind up replacing my current set (already in a running motor) then I would go with the hyperutectic (sp?) which I believe are a tighter-fitting piston with less expansion properties? This is an all-n/a motor so the forged is a little overkill I think.

I know there are intermediate thickness gaskets out there, I just hate having to look TOO hard to find them.

I also hear bad things about shops not truely squaring a block or decking it to the wrong height... so a little leeway is always a good idea I think.

Thanks again.

 

www.cometic.com makes a .027,

SCE and Flatout are your only other options.

I would call Flatout first!

I have bought Cometic and SCC already in the past, supposedly the Cometic and Flatout do not need to be re-torqued, but I do that anyway.

 

I would have to reread my post to be sure of what all I said. My "regret" is that I ONLY had it decked for zero clearance. I wish I had had them take another ~.005" off of the deck. My Chevrolet Power Book specs .035" to .040" piston to head clearance, as the minimum. The piston to head clearance IS the quench distance. And as you correctly state, it consists of the deck clearance plus the compressed gasket thickness.

The tighter the quench, the more effective it is. The only limitation is the physical contact of the piston to the head. The benefits of a good quench, diminish gradually, as the distance increases. At about .060" piston to head clearance, the benefit of a "good" quench is gone. Therefore, "less is more"

Rather than focus on the deck height of the block, and subtracting the spec dimensions of the parts from that, I measured the actual piston down the hole dimension. This was a NEW assembled long block that I purchased. It was ready to run. I simply took it apart to "check it out". I don't recall any (mis)representation as to the deck height. I simply didn't like seeing the piston .007" down the hole and I sent the block out to be "zero decked". The machine shop didn't like going to zero. They wanted to leave .002", but they said they would go zero, if I insisted. I did. Getting them to go another ~.005" might have been tough. In retrospect, I'll just live with it. It really isn't all that awful. I haven't put it back together, yet. I am hoping that when I do, I'll find a few thousandths "error", with the piston out of the hole.

Quench doesn't care if it is achieved by deck clearance or gasket thickness. For reliability's sake I am pretty much stuck with the .041" thick Fel Pro 1003 head gasket. I'd like to find a "good" .030" gasket, and "push" Chevy's spec a little. I've heard of others doing that with no ill results. I would be especially be inclined to push the minimum with your 5500 rpm limitation.

If I were you, I'd do a trial assembly, measure the deck clearance and then send the block out for a final haircut, based on the gasket you plan to use.

Did I cover everything? Did I contradict myself? I hope not. Good luck, and...

RACE ON!!!

 

You did JUST fine. Thank you everyone for your imput. I think I'll try to err on the low side of 0.04_" quench then, esp if I go for the hypereutectics.


EDIT: I checked to make sure "Err" was a word, and I was right.
err ( P ) Pronunciation Key (ûr, r) intr.v. erred, err·ing, errs
To make an error or a mistake.
To violate accepted moral standards; sin.
Archaic. To stray.

 

I don't think that is an "err". I'm thinking Chevy's spec is pretty much for race engines. I think .040" is "good", but I wouldn't get upset if it came out less...even a lot less. Especially with hypereutectic pistons. If you go with Keith Blacks, just remember that while they fit tight in the bore, they require wider than normal ring end gaps.

I wouldn't be too concerned about a block "decked to the max". On future builds, the block could still be trued, and if necessary, the new pistons, shaved.

RACE ON!!!

 

My only thoughts are that piston to head and quench can be two different animals if domed pistons are used. The knuckles on BB racing pistons come to mind.

Also, as TJ already said, piston to wall has to be factored in when running a really tight quench clearance due to piston rock in the bore. The piston doesn't remain square with the bore.

.035 to .040 is the general recommended clearance but it can be run tighter if the other variables are accounted for. The performance difference is small though.

I've run pistons as much as .010 out to a point of where the piston was actually kissing the head. Dangerous game though.

Jake

 

I hope you didn't do that on MY account. I KNOW it's a good word. I guess I shouldn't have used it, or quote marks, in my response. Damn!!! I must be the most misunderstood guy on the forum. I meant NOTHING by it.

RACE ON!!!

 

No worries - I didn't take it as a slam - but the quotes did make me question my usage so I checked, and decided to edit my post *just in case*

JAKE: I'm using flat-tops with only two valve reliefs to maximize the quench area as much as possible while keeping a clearance, with normal sized rings as well so I'm not anticipating much piston rock. I also wanted a little extra room for valve-to-piston clearance (CYA), so I'm not going to purposefully be going below that piston - head 0.040" mark.

 

I know of no advantage of running a dome, close to the combustion chamber. Only the flat part of the top of the piston where there is a corresponding flat of the head to match up with, in a wedge design head.

The performance CAN be significant. The quench squeezes the the burning air/fuel mixture around the combustion chamber resulting in a faster burn. The ignition lead can be trimmed back. In fact best power is made with less total timing in a tight quench engine. The less the spark lead needed to attain max power, the greater the combustion efficiency. The fast burn and reduced timing is tolerant of higher compression ratios, too. This is often referred to as knock resistence.

When I mentioned the benefits of a tight quench to a buddy that had built and raced quite a few engines, a light bulb went off. He had an engine that ran exceptionally well, but never knew exactly why. He recalled that when he tore it down, the part number stamped into the top of the piston was readable in the deck surface of the head. It never made a peep, but it was evident that the piston to head clearance was at a minimum. Then, HE got it.

RACE ON!!!