Visiting from the C2 gang, hope there's some more practical BB experience over here.

L72 spec 427, .040 over, deckedand line bored block. Pistons are .017 out of the hole. Have been running composition gasket .039 compressed with .025 copper shim. The thing runs decent, but I'm guessing the .047 quench clearance isn't ideal. Any suggestions? There are MLS gaskets with the right thickness, but I don't think the head and block surfaces are good enough. Hoe about composition gadgets with .020 steel shim gaskets? Dumb idea?

Thanks for your help.

 

Are you sure those pistons are .017 above the block deck? What is your compression ratio with this quench height? They are pricey, but you might want to get a hold of cometic and get gaskets the exact thickness you want instead of screwing around with a gasket and a shim. Which gives you an extra fail point and a between the deck and the block. Plus using a shim as gasket.

 

Is your block debt .035 or oddball Pistons or a oddball rods?

 

Yes the pistons are .017 out of the hole. The block was decked at least .025 and the crank saddles were line bored. So much that I need to use a special short timing chain set. Why waste time with such nonsense? Have you priced a date correct 3866942, 4 bolt block lately? The better ones are almost unobtainium. C2 old guy crap...correct dates and casting numbers. Lol

With .064 worth of gasket, it's right at 10 to 1 compression. I used forged L36 pistons, the 390/400 HP ones to keep it streetable.

That said, yeah, Cometic makes the right thickness gasket, but everything I've read about MLS gaskets is the surface finish has to be just right or they won't seal. If that's not true, then I know what to get. If not, I can try something else.
.

 

I used a steel shim on my 350. Same sealing surface as an mls gasket. Cast iron block, aluminum head. All I did was scrape and clean block side the apply copper coat to both sides of the gasket and install. That was 10,000 miles ago. No leaks. I don’t know if MLS is pickier than steel shim.

 

I take it the Vin number is decked right off of the block? Then I would think don’t even bother with the date code. Put that block away for later and put any old 78 truck 454 block in it. Or gen 6 roller block

 

If you are not leaking you should try the .015 metal shim gaskets. I used to use them oN my small block. I was reading that some folks believe that the qwench isnt as necessary as once believed. HOw are your plugs looking? if they are uniformly tan then I dont see the issue. If you do get down ro a .035 qwench your compression is going to rise a bit as well

 

I was checking out what summit has and the fatties are all cometic and they dont carry shims that small in your cylinder siz...guess if you want to get smaller cometics your only choice

 

IF your swept volume increase from the .040 overbore (measured in %) is equal to the (%) increase in quench volume due to the thicker gaskets minus the .017 proud piston the CR is not altered.
Raising the head by using .064 total gasket thickness and raising the piston by .025 (from LETS SAY .008 below (it wasn't zero decked out of Tonowanda) to .017 above) STILL increased your Quench volume.

If your .040 390HP piston is a 10.5:1 AND you have OE piston to head clearance(steel shim head gasket) your CR would be in the 10.25 range.
However you have .050 more head gasket than an OE steel shim. Subtract the .025 difference in piston height and this leaves you with .025 increase in piston to head clearance.

How was your CR calculated?
Was every head chamber CC'd or is this a ballparking of CR?

Even with just the composite at .039 you have .022 clearance.
If you are worried about the compression... do the heads have any work in the chambers, especially rounding of any sharp protrusions? IF not this will increase the volume slightly, while cutting down on preignition sources. the correct work in the chamber will also increase the performance.
Are the pistons flat tops or slight domes? IF domes - slight massaging of the piston domes by rounding sharp edges will help with HOT SPOTS which are sources of preignition. WHILE also increasing the quench volume.

IF your car gets 10MPG and you drive 4000 miles per year.
Straight race would cost $4000 a year.
If mixing 25% race with $4/gal non ethanol your annual fuel costs would be $2200/yr for 4000 miles.

 

Firstgen, compression is not an issue. At all. I don't have any trouble on 93 octane pump gas. The combustion chambers were not cc'd. The heads are correct 3873858 castings with dates more or less in range for the build date of the car. The issue is the quench clearance is .047, and I think that's too big and making the throttle response a bit soggy.

Yes, the VIN and engine code stamping, whatever they were, had been decked off at some point. They were restamped. The NCRS Gestapo is perfectly happy with that, in the for what it's worth department. You guys might imagine that restamping is a sore, sore subject on the C1/C2 side of this forum and I'm not going to open that can of crazy over here.

The questions I'd like answered are (1) is there any other single gasket choice (Cometic, etc.) that comes in .055 to .060 thickness that will seal on an ordinary machine shop decked block and heads, or if not, (2) what can I use instead. It's clear enough the .025 copper shim and .039 composition gasket combo works. Not my favorite, but I'm not changing blown head gaskets, either. So far I haven't found any thinner copper shim gaskets. Can I used a .018 or .020 steel shim with a composition or other gasket? If no one knows, that ok, I'll just leave it alone. It runs well enough and made over 450/motor on the dyno during run in and testing.

derek: funny you mention a '78 truck block. I used to have a '65 convertible that was converted to BB. Had a mongrel motor, '74, 2-bolt 454 pickup block with a 427 truck crank, oval port heads and a factory L72 cam. I have considered something like that again. FWIW, L72 cam or something like it works great with oval port heads.

 

I did not explain well enough, my fault.
OR I got too long in the reply and the point was lost.

The block was cut .025 with a rotary cutter, this leaves a great surface for ANY gasket. THE deck is true flat with milling cuts which RETAIN the gasket,
IF the heads were cut as well, --- TRUE FLAT mates to TRUE FLAT.
Run which ever design in the thinnest compressed height as you feel safe with.
IF you can find something compressed at .043.
You cut the quench to .030 which AT MOST is going to raise the compression by a tick over half a percent.
IF the pistons were slightly under and you were to remove the dowel pins and then slowly lap the head to the block you could run linseded oil on the mating surfaces.
WE did this on precise, high pressure industrial valves used with extremely heavy pieces of machinery quite often. NUMEROUS racing engines have done this as well.

 

firstgen, this is a big, big help. Thanks. Cometic makes an MLS gasket with just the right thickness. I just wasn't confident they'd hold seal. I could probably get away with .035 quench, but .040 is a good number.

 

Try SCE gaskets? they have (and can make) any type or thickness you need. They are in Santa Clarita Ca
Yes MLS is best with a specific finish. Will they work with what you have? perhaps Im no expert

Id use a regular old composition gasket in the thickness you need no stacking gaskets.

 

Stock quench was in the .023 range.
.015 steel shim+ piston .008 under the deck at TDC = .023

I was an apprentice for 3 days a week in a race engine shop - 30 years ago. Bigblocks With aluminum rods you set the pistons .025 under deck to accomodate for the rod stretch.

 

.023 would be just about perfect with just a single .039 compressed composition gasket, but that sure sounds too tight. Was that for cast pistons with .002 or so wall clearance? I'm running forged ones at .005 wall clearance. The rods are factory forgings w/ 3/8 knurled shank bolts, so I'm not overly worried about rod stretch. A bit of piston rocking, that's another matter.

 

I have never dealt with pistons above deck height so piston rock could be possible, however if the pistons are stock height I doubt they are going to rock much, rock and slap usually occurs when the compression height (wrist pin to top of the piston) and the skirts are short due to clearance issues on strokers.

With the deck cut .025 was the intake milled or were the china walls cut to facilitate the correct aligment of the intake manifold.
Also relating to your shorter timing set... cutting decks .025 changes the cam geometry re: were the push rods shortened in order to keep the roller rocker contact in the center of the valve stem?

 

I sure did check the rocker geometry. It's perfect. The .025 shim didn't hurt, in fact it put the heads back where they're supposed to be relative to the cam center line. I have Comp roller tip rockers. They fit under stock valve covers.

cv 67, I did get in touch with SCE, thanks for the suggestion. They make a copper gasket with built in o ring combustion seals, they call theirs ICS gaskets. Choices are .050 and .062. .033 squish makes me a bit nervous, about the .050 gasket but that may be what I wind up doing. Even a too loose .062 gasket is better than what's on there now. They also have em in .043 if you don't have a patched together mongrel like I do.....lol

Sure is good C3s share chassis parts with us worn out old geezers. You guys have been a big help. If I had to do it over again the correct casting number nonsense would have been a no go from the get go, and I wouldn't have spent the last quarter century messing with this POS!