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Technical Double head gaskets?

Discussion in 'The Hokey Ass Message Board' started by '51 Norm, Aug 12, 2018.

  1. '51 Norm
    Joined: Dec 6, 2010
    Posts: 713

    '51 Norm
    from colorado

    I am currently screwing together a Pontiac 455 and with the current combination it has a compression ratio of right at 10.9:1. I'm thinking that is a bit too high for a regularly driven street engine.

    I see that there are extra thick head gaskets available to reduce the CR. This got me to thinking--dangerous I know--would putting on two standard thickness head gaskets achieve the same goal?

    My sometimes faulty calculations show that two gaskets with 0.042" compressed thickness each would result in a compression ration of about 10.3:1 something that I think I can live with on a street engine.

    I would like to hear from anyone that has made the attempt, good bad or indifferent.

  2. the only time i saw two head gaskets used, it was leaking.
  3. I have never done it, and would not recommend it
    If you want a thicker gasket and it's available, get it

    Sent from my VS996 using The H.A.M.B. mobile app
  4. greybeard360
    Joined: Feb 28, 2008
    Posts: 1,543


    Ford used to put 2 steel shim head gaskets on some vehicles that were going to be exported to countries that had pretty low grade fuels. No idea how long they would last that way. Look into copper head gaskets, you can buy them in different thicknesses.

  5. lumpy 63
    Joined: Aug 2, 2010
    Posts: 1,347

    lumpy 63

    I wouldn't do it ...but I seem to remember reading that the hi po chevy 409s came double gasketed, steel shim of course. And you could remove one set to raise the compression for racing. With too much gasket it also screws up the quench ...
    onetrickpony likes this.
  6. Felpro makes a copper head gasket shim called a head saver. I had resurfaced the heads and block on my 390 Ford, and with 10.5 - 1 pistons I thought I would end up with too much compression for a good street engine. I used these shims and over 50,000 miles later I still think it was a good idea. I don't remember how thick the shims are but I'm sure Felpro can tell you.
    deadbeat likes this.
  7. yellow dog
    Joined: Oct 15, 2011
    Posts: 435

    yellow dog
    from san diego

    The factory 340Hp/327 utilized 2 steel shims in 60's. I replicated their use successfully twice, but failed on third trial and would never again try. Not as many gasket options back then to manage CR, PV clearance, etc. Calculate your dynamic compression considering your cam profile may be considerably reduced from 10.9. New engine tech w/ variable cam timing are now routinely 13-14 and I've read reports of near 20 CR to be introduced.
    Joined: May 5, 2015
    Posts: 782


    I have a Atomic Four gas engine in my sailboat that has a factory recommendation of using two head gaskets. If I remember right they were steel shim. I have not had any issues with that setup, not sure if that principle would work on a motor vehicle.
  9. drtrcrV-8
    Joined: Jan 6, 2013
    Posts: 1,354


    Lumpy 63 is correct : It will screw-up your quench : you need roughly .045 quench for the motor to run correctly.
    Unkl Ian and G-son like this.
  10. G-son
    Joined: Dec 19, 2012
    Posts: 691

    from Sweden

    It is sometimes done on off topic engines when adding a turbo. It kind of works, but leaking gaskets are common and fuel economy goes down the drain w/o proper quench.
    For a cheap toy you'll use a few hours, sure, for something you're actually going to drive around I wouldn't risk it.
  11. Joe H
    Joined: Feb 10, 2008
    Posts: 966

    Joe H

    Don't just go by what Pontiac claimed for compression, they were always a little high. What heads are running?
  12. '51 Norm
    Joined: Dec 6, 2010
    Posts: 713

    '51 Norm
    from colorado

    I'm curious how much change in compression ratio there would be by having two steel shim gaskets since they are usually pretty thin---around 0.020" I think.

    Thanks for the information on the Felpro gasket, I'll look into that. The leaking gasket thing is what I'm mostly worrying about. I'm not figuring on making this my daily driver but then changing head gaskets isn't a lot of fun either.

    I'm not seeing a lot of quench area in my Pontiac heads to begin with so I don't think it's going to matter.

    Thank you for the link. I'm running a RA IV style cam and if it is "hot" enough the compression issue may go away.

    So I guess that the bottom line is I can buy the expensive thick gaskets--around $230-- or put on an extra set of $40 gaskets and risk a weekend changing them out.
  13. Engine man
    Joined: Jan 30, 2011
    Posts: 3,476

    Engine man
    from Wisconsin

    We often used more than one steel head gasket on small block Chevrolets to reduce compression or provide valve clearance. It might not work so well with multiple composition gaskets.
  14. HuskerNation
    Joined: Dec 28, 2010
    Posts: 276

    from Montana

    I used double gaskets on a 69 Pontiac Bonneville on the rebuilding of its 428ci engine due to the compression. Ran it for years with the double gaskets without a single problem, sold it around 1991.

    From my experience 30 years ago, I wouldn’t be afraid of doing it.

    Sent from my iPhone using H.A.M.B.
  15. Bandit Billy
    Joined: Sep 16, 2014
    Posts: 7,050

    Bandit Billy

    Blowing a head gasket, towing the car back home, tearing the top end back apart, buying new gasket set, new coolant, new oil change, not to mention at the very least a fresh bottle of Crown Royal...all to save $190.00?????
  16. 25 years ago, a customer had a 40 dodge coupe towed in with a leaky head gasket, i pulled it apart and found two head gaskets, i checked the head and altho it looked liked it was planed before, it was a little warped. i thought someone had tried to compensate for the warped head by using two gaskets. i had the head planed and and got a new gasket installed but the motor wouldn't turn. i thought; did i leave a rag in the motor? or did antifreeze leak in and hydro the pistons? i loosened all the head bolts, turned the fan and watched the head "dance". i took it apart and noticed the pistons actually protruded out of the block .080". turns out some one had used chrysler pistons in the dodge block and altho the bores are the same, the wrist pin is in a different spot. i removed all the pistons, had them turned down .080" and i reinstalled them and the head. ok, let's add this up: pistons even with the top of the block, and a head that had been cut at least two times..........fastest 6 cylinder i ever drove, you would swear it had a v8...............just wouldn't restart when it was warm......
  17. '51 Norm
    Joined: Dec 6, 2010
    Posts: 713

    '51 Norm
    from colorado

    Thank you for your experienced input. I was hoping to hear from someone that had actually been there.

    Yep. I like to live dangerously.

    The neat thing is that I have managed to blow (single) head gaskets in Pontiac V8s before and actually don't consider it a big deal.

    Now that's the sort of thing that I want to avoid. At least he didn't have a problem with too little quench!
  18. KoolKat-57
    Joined: Feb 22, 2010
    Posts: 3,006

    from Dublin, OH

    Call Butler performance, they are the Pontiac engine gurus!
  19. old sparks
    Joined: Mar 12, 2012
    Posts: 414

    old sparks

    have run double gaskets (fel pro ) composite doing the usual grease around water jacket passages etc. not a problem. did it because minor interference and wanted at least .040 clearance. this big block chev was static 13 to 1. that being the case teardown every season was mandatory maintenance
  20. Cometic multi layer shim (MLS) head gaskets are the answer if there available for your application. There made to order in a verity of thicknesses. These thing are not cheep, as I spent over $400$ to put them in the Hemi Dad and I built.
    Unkl Ian likes this.
  21. Budget36
    Joined: Nov 29, 2014
    Posts: 4,884


    How about doing some combustion chamber work then CC'ing the heads. Can do a little port work while you're in there.
  22. '51 Norm
    Joined: Dec 6, 2010
    Posts: 713

    '51 Norm
    from colorado

    Yup, that's how I found out that I have a 10.88:1 compression ratio. I just rounded it up to 10.9 'cause it sounds so cool. In order to get below 10.5 I would have to take out an additional 5CC or so and that is a bunch of cast iron.

    As far as port work goes, I was appalled at the casting flash and misalignment in the exhaust ports. The intakes were pretty good, something that makes you go how come is that?

    That is where I found the expensive thick gaskets that started the thought process.

    That is pretty much what I was thinking of.
    My experience with Pontiac water passages is that the water holes in the heads don't line up exactly with the water holes in the block. The water holes in the heads are a lot bigger than the block holes and that can result in a very thin place in the head gasket between the water hole and the cylinder.
    That is something that will get checked very carefully if I go the double gasket route.
  23. mcsfabrication
    Joined: Nov 26, 2006
    Posts: 887


    My factory fresh (in 1968) '63 Corvette factory fuelie, never opened up from it's birth, had double steel head gaskets.
  24. Simply polish your combustion chambers and CC to the largest volumed one. Two head gaskets is asking for problems. Also a bigger cam can often reduce compression because of overlap.
    porknbeaner likes this.
  25. I worked with a certified gas fitter in the old field in the '70s. He was an old guy and he said that you always wanted to make as few joints as possible. I asked him why and he said, "Every place you have a joint you have a leak."

    I wouldn't say this on a public forum because I would not want the world to find this out but @Old wolf is pretty sharp if you catch my drift.
    Bandit Billy and Old wolf like this.
  26. Fordors
    Joined: Sep 22, 2016
    Posts: 3,159


    Pontiac chambers are fully machined, they are not as cast.
    Deuces likes this.
  27. You can still polish them. The polishing removes any protrusions that can glow (hot spots) and cause preignition.
  28. I can't let this one go. So sorry in advance.

    Unless they are billet heads or a forging they are cast. They may be finished after the fact and not as cast but they are cast. GM does not CC the heads on an assembly line motor, it is cost prohibitive, and the heads are not cast without combustion chambers and then cut them that too is cost prohibitive. So they may be finished but that are cast. :D :D :D
    Old wolf likes this.
  29. wheeldog57
    Joined: Dec 6, 2013
    Posts: 1,917


  30. this was my thought at first also, but with the big motor [lots of heat], even dialing the advance back and running high octane fuel, he may have a problem with "street-ability".

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