[1969GPSJ]

So until I get aluminum heads for my 428 (16 casting, 72 cc, big valve, screw in stud heads), I have been considering using thicker Cometic head gaskets with my stock cast iron heads. Specifically I am looking at the 4.16 bore 0.080 or 0.098 gaskets. With the .080 compressed thickness gaskets, my static compression ratio would be around 10.06:1, with the 0.098, it would be around 9.70. Just trying to get the engine into the pump gas friendly zone to allow me to enjoy the car while I am getting another engine ready.


1) How does using a thicker head gasket affect engine operation outside of the obvious effect on the compression ratio?
2) What do you think the positive effects of the thicker gaskets would be?
3) What do you think the negative effects would be?
4) Which would be a better choice for my iron head - mostly stock fuel injected 1969 4 speed GP?



When I get the 72 cc aluminum D ports and a typical 0.041 gasket, compression would be 10.96:1 - should be pump gas friendly for aluminum heads.

[Goatracer1]

Are you racing the car? If not I don't see any problems using a thicker head gasket on a strictly street driven car especially for a short term solution.

[Johnny406]

Your #16 heads are likely going to be closer to 80cfm+ than 72cfm. If your block wasn't decked/resurfaced, you are likely to have your pistons .008-.015 in the hole as well. Measure both of those to verify and come up with actual compression instead of advertised. I'm betting you will be fine with what you have.

[Gach]

I would never go with that tick of a head gasket. .080….What you need to determine is what is your deck height is. Are they -.010 in the hole or more. Or is it zero deck. In order to find out what your true compression ratio is. What cc the heads are also.

I edit it because Way too much information for what he’s asking.

[steve25]

Ok, I’ll let it slide this time, lol!

[younggto]

Do your deck surfaces and heads have the appropriate surface finish for Cometics?

[hurryinhoosier62]

Originally Posted by younggto

Do your deck surfaces and heads have the appropriate surface finish for Cometics?

Best question I have seen asked! ALL MLS head gaskets require a specific surface finish on the heads and decks. If yours don’t meet the specs don’t bother installing MLS head gaskets.

[1969GPSJ]

Nope, the 16 casting heads I have cc'd have been consistently right around 72 ccs (low end 68 ccs, high end 74 ccs, but the low end may have had been resurfaced). Pontiac was pretty good with the RAIII and HO "d" port chamber machining consistency. Post 1970 heads may have had larger chambers, but the '68 through '1970 open chamber high compression D port heads used on the performance 428s and 400s did not.

[chiefbigb]

You can lay the eye brows back in the combustion chamber and get 3cc there. Most of the untouched 72cc heads I have checked are 74 to 75cc

[Gach]

Originally Posted by 1969GPSJ

Nope, the 16 casting heads I have cc'd have been consistently right around 72 ccs (low end 68 ccs, high end 74 ccs, but the low end may have had been resurfaced). Pontiac was pretty good with the RAIII and HO "d" port chamber machining consistency. Post 1970 heads may have had larger chambers, but the '68 through '1970 open chamber high compression D port heads used on the performance 428s and 400s did not.

Are you running pump gas now ? Or you mixing it. I had 69 Grand Prix 428, and never had a issue with runing 93 octane pump gas.

[1969GPSJ]

Originally Posted by 1969GPSJ

Nope, the 16 casting heads I have cc'd have been consistently right around 72 ccs (low end 68 ccs, high end 74 ccs, but the low end may have had been resurfaced). Pontiac was pretty good with the RAIII and HO "d" port chamber machining consistency. Post 1970 heads may have had larger chambers, but the '68 through '1970 open chamber high compression D port heads used on the performance 428s and 400s did not.

This specific 428 engine has not previously been apart, so the finish of the head and block would be as they were when they left the factory. I may end up using the Copper gaskets from Butler or another source.

Again, this is a somewhat temporary answer to getting the car back on the road. I have not dealt with Cometic's MLS gaskets in the past - I just have not needed anything that specialized, and the Fel-Pros worked just fine. I am just wanting a little more gasket to reduce compression from the factory 10.5:1 to 10 or 9.7.

[1969GPSJ]

Originally Posted by Gach

Are you running pump gas now ? Or you mixing it. I had 69 Grand Prix 428, and never had a issue with runing 93 octane pump gas.

Car has not been on the road for quite a while, but it was very, very octane sensitive - even with the best (non race) pump gas available, so much so that I had to dial back the timing so much (from 9-10 degrees advanced to 1-2 degrees retarded) that it ran poorly, and was very sluggish. With higher octane fuel, I could run up to 12 degrees advanced, and the car's throttle response was night and day different. Maybe with a cam change it will be less sensitive to the octane. It has the original 067 in it now.

[Gach]

If it’s that sensitive my guess is you got something else going on. Really I ran mine with no issue on 93 octane, for years. Here’s a possibility and I’ve seen this a more then a few times. Some times your running what you think is total timing 32-36 degrees, but if there’s issues with the distributor, for example you put a time light on it and say at 1500 rpms. Time in while it says 32-36 degrees But ! You bring it up to say 3000-3500 and it keeps advancing, I’ve seen the advance up to 50 degrees. But you don’t know if that’s happening say your first instinct is kicking it back 10 degrees then it feels sluggish. Just something did you want to check out and make sure that it’s not advancing. Honestly I never had any issues with my 69 Grand Prix 428. Running on 93 octane. Just a thought something you might want to check out.

[Gach]

What do you mean by the original oh 067 ? Do you know what number the heads are on the motor. Another question what octane pump gas fuel was you running

[1969GPSJ]

Originally Posted by Gach

If it’s that sensitive my guess is you got something else going on. Really I ran mine with no issue on 93 octane, for years. Here’s a possibility and I’ve seen this a more then a few times. Some times your running what you think is total timing 32-36 degrees, but if there’s issues with the distributor, for example you put a time light on it and say at 1500 rpms. Time in while it says 32-36 degrees But ! You bring it up to say 3000-3500 and it keeps advancing, I’ve seen the advance up to 50 degrees. But you don’t know if that’s happening say your first instinct is kicking it back 10 degrees then it feels sluggish. Just something did you want to check out and make sure that it’s not advancing. Honestly I never had any issues with my 69 Grand Prix 428. Running on 93 octane. Just a thought something you might want to check out.

Hi Gach, in my first post of the thread I mentioned that the heads are 16 casting big valve, screw in stud 72cc (nominally) heads. I have not had the heads off of this engine. The engine, being a '68 428 originally in an automatic car came from the factory with either the 9779066 or 9779067 camshaft and the 1111959 distributor. When I got it, it had a 1970 RA III stickshift quadrajet - which was rather odd, but a windfall.

In the past, I put a timing light on it (I have a dial back light, so I can check the timing throughout the range) and it was in spec. During an inspection it was obvious that there weren't any missing bushings or pins in the distributor. Springs were stock, weights were stock. Vacuum can was fine and functioning. Could not find any problems with the advance mechanism at the time. Checked the balancer with a piston stop, and the balancer had not slipped (at that time). I don't now have faith that the elastomer is serviceable, and I expect to change the balancer as part of getting the engine back to running and reliable service. Again, it ran well with a blend of race gas and 93 octane, but sounded like ball bearings in a tin can with just the 93 at 9-10 degrees of timing. To drive it on 93, I had to retard the timing by 12 degrees from stock. Not fun.

[Gach]

Wow you’ve done your home work. Here’s a possibility the automatic cams usually are in at 108-109. Being its a 4 speed car ( sorry can’t remember what 4 speed cams are 112 ? ) that could be part of the issue. I mean you could try one of the new generation pump gas cans more suited for a four-speed. The compression part, I mean hoping those 16’s are 72 cc. So ok my 68 GTO 4 speed 400 had number 16 heads. Those motors rated at 10.75 compression.

It sounds like you’re going to have the heads off anyway so lets see what those 16’s check put ass cc wise. I may be wrong but 16’s on a 400 rated at 10.75 compression vs being on a 428 may be more true 10.75 then on a 400. Anyway I would definitely go with a cam change and definitely see what cc those heads are then decided from there what head gaskets to go with.

[Gach]

Or you could spring for a set of aluminum D-ports and a cam more suited for a 4 speed, definitely aluminum head would end compression issues and definitely motor with right cam will definitely make a ton more power. I would definitely not consider 080 head gasket. Otherwise you’ll be chasing your tail.

[1969GPSJ]

Originally Posted by Gach

Wow you’ve done your home work. Here’s a possibility the automatic cams usually are in at 108-109. Being its a 4 speed car ( sorry can’t remember what 4 speed cams are 112 ? ) that could be part of the issue. I mean you could try one of the new generation pump gas cans more suited for a four-speed. The compression part, I mean hoping those 16’s are 72 cc. So ok my 68 GTO 4 speed 400 had number 16 heads. Those motors rated at 10.75 compression.

It sounds like you’re going to have the heads off anyway so lets see what those 16’s check put ass cc wise. I may be wrong but 16’s on a 400 rated at 10.75 compression vs being on a 428 may be more true 10.75 then on a 400. Anyway I would definitely go with a cam change and definitely see what cc those heads are then decided from there what head gaskets to go with.

The difference between the 428 10.5:1 and 10.75:1 engines was a piston without a dish. The same was true for the standard GTO/Tempest 400 and RA III - all at 10.75:1 and RA IV vs the 350 HP GP engine. The GP engine had the dished piston and was 10.5:1 - almost all of the other specs matched the GTO/Tempest standard 350 HP engine.

I have considered using the 9785744 (RAIII 4 speed cam) or 9794041 cams for both more power, but lower dynamic compression. I still think that thicker head gaskets would help too. You can hear part of knock, but not necessarily detonation, and I don't want to go down that path with a stock engine and cast pistons.

[1969GPSJ]

Originally Posted by Gach

Or you could spring for a set of aluminum D-ports and a cam more suited for a 4 speed, definitely aluminum head would end compression issues and definitely motor with right cam will definitely make a ton more power. I would definitely not consider 080 head gasket. Otherwise you’ll be chasing your tail.

Yep that's the plan for stage II. Right now I just want to get the car together and be driveable, safe, reliable and comfortable to drive. Then I will do the engine rebuild and aluminum heads, then a TKX, a twin disc clutch and rebuild the 12 bolt.

[HWYSTR455]

Ok, elaborate on the comment ' I put a timing light on it (I have a dial back light, so I can check the timing throughout the range) and it was in spec. '

What's 'in spec'?

What's your initial, total, and timing at 3000 rpm? And how much is the can adding?

When/where does it ping? Rolling of a dead stop, under load at 1800-2800, or above 3000?

If it's not been apart, the pistons are def in the hole, how much is the question.

If you haven't had the heads off, you don't know what the heads CC are.

Rated static CRs are exaggerated.

'sounded like ball bearings in a tin can with just the 93 at 9-10 degrees of timing' . You're obviously talking about initial, which doesn't mean jack.

Definitely need more (accurate) info to make any type of recommendation. If you're willing to change gaskets, you can pull the heads and determine exactly what you deck height is and head volume. Without that basic info, I don't think anyone can give any valid recommendation.

Now, just to say, most shops finish deck and head surfaces to 50 RA or better as SOP, which supports an aluminum head/iron block combo with MLS type gaskets. If it's iron/iron, it's a lot more forgiving, and you could possibly get away with an MLS gasket. Or, you could try 'smoothing' the surface some if you feel brave and lucky.

Another option is you could stack composite gaskets, that too would be if you feel brave or lucky. Neither would be advisable with a performance build, but if you're just trying to 'get away with it' for a short period of time, you could take a shot at it.

This could all be moot if whatever cam in there now is compromised or there are other factors that could be contributing, like heavy carbon build up. You could go through the trouble and try to get 'lucky', but you may end up having it fail and be forced to rebuild.

Can't cheat physics, might be better to just ride it out until you can go through and properly rebuild.



.