Head gasket question

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MOPARMAGA

MOPARMAGA

" The other hard member"
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My 400 block is going to the machine shop next week, I'm using a 4.375 piston. This is a race only 470 & I'm trying to squeeze out all the compression I can.
Question is what bore cometic should I use
4.380 or 4.410 ? Oh by the way I'm using a 4032 forged so a low expansion forging
Thanks
 
MOPARMAGA said:
My 400 block is going to the machine shop next week, I'm using a 4.375 piston. This is a race only 470 & I'm trying to squeeze out all the compression I can.
Question is what bore cometic should I use
4.380 or 4.410 ? Oh by the way I'm using a 4032 forged so a low expansion forging
Thanks
Click to expand...
Something I learned about the cemetic gaskets after I bought one LOL Is that the require a certain kind of finish on the block and heads..
 
j par said:
Something I learned about the cemetic gaskets after I bought one LOL Is that the require a certain kind of finish on the block and heads..
Click to expand...
Lol, I learned that too. I have a nice set for my 5.9 magnum I put back in a box for a later more expensive day.
 
The only real way to KNOW if the surface finish is correct for Cometics is to use a profilometer. I don’t think anyone over here has one, and I don’t think anyone over here has a machine that is capable of machining a surface for a Cometic gasket.

I wouldn’t use either gasket. The small one is too small and the big one is too big. You are giving up compression and the crevice volume will make it very dirty.

Just my .02.
 
The last I researched it (within a couple of weeks), Cometic recommends a head gasket right at or very near (slightly larger) than the size of the bore in a high performance/racing application. But the 4.41 inch bore leaves a gap of .0175, or .445 millimeters on the edge of the cylinder back to the gasket. IIRC, they also stated that anything less than .020 is acceptable for a strictly racing engine. You didn't state what thickness you're going to run, but comparing the two on a .040 thick gasket by the website, you're only looking at .136cc of volume difference between the two, or probably a couple of hundredths of a point in compression. But what pistons and heads are you using? Is it positive or negative deck? If it were me with the 4-3/8 bore size I would be inclined to play it safe and go with the larger of the two given bore sizes unless the pistons were under deck by at least .015.
 
Garrett Ellison said:
The last I researched it (within a couple of weeks), Cometic recommends a head gasket right at or very near (slightly larger) than the size of the bore in a high performance/racing application. But the 4.41 inch bore leaves a gap of .0175, or .445 millimeters on the edge of the cylinder back to the gasket. IIRC, they also stated that anything less than .020 is acceptable for a strictly racing engine. You didn't state what thickness you're going to run, but comparing the two on a .040 thick gasket by the website, you're only looking at .136cc of volume difference between the two, or probably a couple of hundredths of a point in compression. But what pistons and heads are you using? Is it positive or negative deck? If it were me with the 4-3/8 bore size I would be inclined to play it safe and go with the larger of the two given bore sizes unless the pistons were under deck by at least .015.
Click to expand...

I have a 4.350 bore , using 4.380 cometic , .040 thick gasket set , after heads are torqued , the fire ring is about even with the bore all the way around . I would not use a smaller bore gasket ! Even with zero deck !
 
I'd use the 4.41 gasket. As far as block/head finish concerning cometic gaskets. We currently have 3 engines in our stable that we have cometic gaskets on. 2 blocks were never decked, all 3 sets of heads are OOTB (2 sets of Trick Flows, 1 Edelbrock). These are race only motors and we haven't had any issues. They are all 11:1 or under on compression.
 
yellow rose said:
The only real way to KNOW if the surface finish is correct for Cometics is to use a profilometer. I don’t think anyone over here has one, and I don’t think anyone over here has a machine that is capable of machining a surface for a Cometic gasket.

I wouldn’t use either gasket. The small one is too small and the big one is too big. You are giving up compression and the crevice volume will make it very dirty.

Just my .02.
Click to expand...
Thanks for your valuable .02
Ken, my builder is successful with the cometic for years, that's all he uses.
I believe those are the 2 bore sizes available unfortunately
EDIT: Options that work for me
 
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Garrett Ellison said:
The last I researched it (within a couple of weeks), Cometic recommends a head gasket right at or very near (slightly larger) than the size of the bore in a high performance/racing application. But the 4.41 inch bore leaves a gap of .0175, or .445 millimeters on the edge of the cylinder back to the gasket. IIRC, they also stated that anything less than .020 is acceptable for a strictly racing engine. You didn't state what thickness you're going to run, but comparing the two on a .040 thick gasket by the website, you're only looking at .136cc of volume difference between the two, or probably a couple of hundredths of a point in compression. But what pistons and heads are you using? Is it positive or negative deck? If it were me with the 4-3/8 bore size I would be inclined to play it safe and go with the larger of the two given bore sizes unless the pistons were under deck by at least .015.
Click to expand...
Thanks Garrett, hg thickness is tbd, as it stands just on measurements alone, the pistons are .002 under deck, but yet to square up the decks, I measured all the pistons at tdc when I disassembled this 400, if I remember correctly the pistons were tiny bit deeper towards the back.
So I gotta wait to mock it up for piston to valve clearance before figuring thickness.
 
famous bob said:
I have a 4.350 bore , using 4.380 cometic , .040 thick gasket set , after heads are torqued , the fire ring is about even with the bore all the way around . I would not use a smaller bore gasket ! Even with zero deck !
Click to expand...
Okay that's something to think about, which I didn't. Thanks
 
I have a 4.38" bore and ordered the 4.41 Cometic gaskets for mine. The one thing that surprised me was the bore shape. I assumed they would be round, but they actually still have the scallops near the valves, just like the Fel Pro composition gaskets have. I guess the bore of the block could be slightly notched there to aid in airflow from the intake valve, but I have not decided if I will try that or not. There is not much room up there above the ring pack.

I think it was @pittsburghracer that said he calls 4secondsflat.com when ordering Cometic gaskets, and they help him get what he needs. Maybe that would be worth a call. I did not know about them back when I got mine. I just ordered at Summit, from what they showed available. I now understand custom bore diameters and thicknesses are available for most applications.
 
Yes Don is great to deal with. If you are going to order Cometics do so ASAP because they are and have been running weeks behind due to Coronavirus. I think mine took 5-6 weeks and were drop shipped straight here.
 
Yes, the 4.41 bore gasket is your friend. Thank you for the heads up @famous bob, I've been curious about how much "real world experience" people have had with the compression of the Cometic gaskets.
 
yellow rose said:
The only real way to KNOW if the surface finish is correct for Cometics is to use a profilometer. I don’t think anyone over here has one, and I don’t think anyone over here has a machine that is capable of machining a surface for a Cometic gasket.

I wouldn’t use either gasket. The small one is too small and the big one is too big. You are giving up compression and the crevice volume will make it very dirty.

Just my .02.
Click to expand...
Cometic has relaxed there surface finish requirement to 50um or less after extensive testing. Most shops can get that if they try.
 
Those FelPro gaskets are very inconsistent. Technology has moved on and those things are pretty much obsolete IMO.
 
For years both Edelbrock and 440source recommended not to use the 1009 felpro gasket with their heads. Many of us shifted to the marine gasket many years ago with success. Even the 1039 wouldn’t clear the chamber.
 
I never have a problem with them (1009)
I race a 400 Roadrunner in NHRA Stock Eliminator
and we believe we have the fastest 400 Stocker in
the country! However, it is Stock Compression but
I run them in several of my Edelbrock Headed Bracket
Cars @ 12 to 1. Never tried them at real high compression.
The stocker has Edelbrock Alum. NHRA approved replacement
heads. What is the Marine Gasket sizeand compessed thickness?
My engine is 060 (4.40 Bore) - I had no Luck with any other
gasket in the car = I would actually get corrosion around the
water passages and destroy the gasket and get a leak situation.

If you are curious =

Goggle John Irving Drag Racer

To see it and a few of the other car.
 
Also, starting a 400/470 build. Please let me know what you decide for head gaskets and how they fit up once you get them if you dont mind.
 
80fbody said:
Also, starting a 400/470 build. Please let me know what you decide for head gaskets and how they fit up once you get them if you dont mind.
Click to expand...
Yes sir I will do so asap ! Good luck with your build too
 
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