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too much compression

Discussion in 'The Hokey Ass Message Board' started by bct, Jan 23, 2012.

  1. bct
    Joined: Apr 4, 2005
    Posts: 3,097

    bct
    Member

    i sent my gm 151 head to the machine shop and hey found a crack....they informed me that they couldn't find another. so i had a look and found some on ebay for $100.....rebuilt.....i thought this was too good to be true but bought it anyhow. i could rebuild it i just need the casting.

    it was too good to be true ...the heads had been machined down .050 and the change i deck height was holding my valves open and compression on the cylinders was up to 100 psi with a wrench turning the crank, so maybe much more with a starter cranking/running

    How can i reduce the compression? i looked for thicker gaskets and thought briefly about doubling them up. any other suggestions? i was going to super charge this with a tiny b$m blower i got so i hope to reduce this compression quite a bit but need it strong. thanks
     
  2. Rickybop
    Joined: May 23, 2008
    Posts: 6,979

    Rickybop
    Member
    from Michigan

    Sorry to hear that, bct. And you'll be sorry to hear, that I doubt you have any other choice, but to buy another set of heads. And another thing. You don't just have compression issues. I imagine that because of the large amount of milling, the intake manifold will now not seat well against the heads...it's changed the geometry too much.

    Bite the bullet, and take solace that you've learned something here. Good luck, pal.
     
  3. KoolKat-57
    Joined: Feb 22, 2010
    Posts: 3,030

    KoolKat-57
    Member
    from Dublin, OH

    IT's a 4 cylinder head!
    I'm sure someone can figure the CR with that .050 removed.
    You will need .050 shorter pushrods to keep the valves from being held open, and you should check for valve to piston interferrence.
    I'm going to guess about 10.5:1 compression but don't quote me!
    No need for a supercharger!:)
     
  4. Dapostman
    Joined: Apr 24, 2011
    Posts: 294

    Dapostman
    Member

    Dish pistons maybe, or a different piston deck height?
     

  5. badshifter
    Joined: Apr 28, 2006
    Posts: 3,320

    badshifter
    Member

    Out of curiosity, if the change in deck height (not really change in deck height, but lowered volume of combustion chamber) is holding your valves open, how can you be getting any compression? Open valves and 100 psi is not possible.
     
  6. Rickybop
    Joined: May 23, 2008
    Posts: 6,979

    Rickybop
    Member
    from Michigan

    Woops. Scratch that, bct. Thanks, KoolKat-57.
     
  7. Retro Jim
    Joined: May 27, 2007
    Posts: 3,859

    Retro Jim
    Member

    Changing the pistons is the easiest way to go . Now you could get the thickest head gasket you can find and see how that works out but like someone else already posted , dished pistons would be the easy way out .

    Retro Jim
     
  8. sunbeam
    Joined: Oct 22, 2010
    Posts: 5,185

    sunbeam
    Member

    Felpro and Innovative offer head shims don't know about your app.
     
  9. bct
    Joined: Apr 4, 2005
    Posts: 3,097

    bct
    Member

    good question....only seemed to hold valves open on the two middle pistons....at first i thought it was the cheap head/valves but they are good....i put washers under the studs to space em up for comp test.
     
  10. bct
    Joined: Apr 4, 2005
    Posts: 3,097

    bct
    Member

    i used smog pistons that are dished already. could they be dished more.?? i guess it is a matter of material thickness ....and some custom pushrods.thanks very much for the info.
     
  11. Don's Hot Rods
    Joined: Oct 7, 2005
    Posts: 8,319

    Don's Hot Rods
    Member
    from florida

    I had one of those in a boat years ago and the guy who did the engine work on it for me built them for sprint cars. He used 283 pistons as I recall, so maybe you can find four of those with lower deck height.

    Just a thought.

    Don
     
  12. copper coat and the thickest gasket out there
     
  13. Retro Jim
    Joined: May 27, 2007
    Posts: 3,859

    Retro Jim
    Member

    If you have dished pistons , you can have a machinist make the dish circle a little larger BUT don't cut any deeper . If you do it might be too thin and well you know what will happen then . But even if you make the dish larger in size , that will only take is down maybe from lets say a 10 to 1 to a 9.8 to 1 , so in the end you are still where you started from but with a costly machinest bill now !
    Honestly the best thing to do is get a thick head gasket and a new set of pistons . That way you know things are correct and not a guessing game of parts swapping !
    Shorter rods will work too !
    BTW what engine are we talking about ?

    Retro Jim
     
  14. gimpyshotrods
    Joined: May 20, 2009
    Posts: 18,004

    gimpyshotrods
    ALLIANCE MEMBER

    I have seen (but not used myself) dead-soft copper head gaskets at a whopping 0.125".

    I know, THICK, and I did not believe it, until I saw it. 0.050" should be do-able.
     
  15. knotheads
    Joined: Jan 4, 2007
    Posts: 499

    knotheads
    Member

    seems like a felpro permatorque head gasket should make up most of that . 050 milling. .045 on the felpro gasket comes to mind.
     
  16. bct
    Joined: Apr 4, 2005
    Posts: 3,097

    bct
    Member

    its a 151 iron duke ...1/2 of a 302 v/8.......i'll ask about the pistons and gasket.
     
  17. dblgun
    Joined: Oct 24, 2009
    Posts: 348

    dblgun
    Member

    Have you checked with a gasket maker like Cometic? Seems like they could make a MLS gasket to your thickness requirements. Won't be cheap but probably cheaper and easier than some of the other options.
     
  18. KFC
    Joined: Jul 17, 2008
    Posts: 407

    KFC
    Member
    from UK

    solid copper gasket as mentioned before
     
  19. Dapostman
    Joined: Apr 24, 2011
    Posts: 294

    Dapostman
    Member

    @ .050 mill, I would start to question head deck strength. Was it angle milled or straight milled? You could try a long duration cam to lower dynamic compression.
     
  20. Truckedup
    Joined: Jul 25, 2006
    Posts: 4,040

    Truckedup
    Member

    There are a few heads that were used on those engines. Is it a later 151 with a corssflow head like used on the S10's?
    With dished pistons the stock compression ratio is around 8.7. I don't know the combustion chamber cc's but thinking it's similar to a SBC head should be about 64 cc's. Using an online compression ratio calculator and typical SBC numbers it comes out to 8.35.Usually compression ratio is listed higher than actual to allow for this or that.Shaving .050 off the head should bring it up about .75 to around 9.25 to 9.5 actual. This won't cause any real problems or anything close to what you're having.I believe you have a mechanical interference problem..
     
  21. I'd be looking for another head ... the head IS the cause of your problems right? Why mess around with pistons etc to compensate for "sumpin that just ain't right" to begin with.

    It's not likes it's an Ardun, or a Ford SOHC, there must be other good/uncompromised heads out there somewhere.
     
  22. ray-jay
    Joined: Feb 23, 2008
    Posts: 200

    ray-jay
    Member
    from Buford GA

    Can you have the pistons machined so that the outer rim is removed ? In other words make them flattops that are way down the hole.

    Somebody do the math and figure out how many cc's is in a .050" thick 4" bore gasket. As mentioned earlier, a .050" milling job shouldn't raise the comp excessively.

    You could also maybe unshroud the valves and get some cc's that way.
     
  23. Don,

    Some times a couple of inches is important. I would guess that you were running a 153 cubic inch Chevy II and not a 151 Iron Duke (different engine, just looks about the same on the outside.

    Charlie Stephens
     
  24. Truckedup
    Joined: Jul 25, 2006
    Posts: 4,040

    Truckedup
    Member

    Iron Duke is the later engine..intake and exhaust on opposite sides of the head.153 has both on the same side like a typical inline 6
     
  25. HEATHEN
    Joined: Nov 22, 2005
    Posts: 7,591

    HEATHEN
    Member
    from SIDNEY, NY

    The early 151s had both intake and exhaust on the driver side as well; '77 through mid '79, I believe.
     
  26. 39 All Ford
    Joined: Sep 15, 2008
    Posts: 1,531

    39 All Ford
    Member
    from Benton AR

    No way you can jump to conclusions without cc' ing the heads and pistons, and running the piston height, gasket thickness etc..

    100 with a wrench SOUNDS high, but with a heavy coating of oil or assembly lube on the cylinders and new everything, there is no way you can assume that you have "too much compression" judging by this reading alone.
     
  27. bct
    Joined: Apr 4, 2005
    Posts: 3,097

    bct
    Member

    good point, 39 all ford. it is a 1980 head with the cross flow casting # 552 i believe. the ebay deal was the only head i could find .... thanks for all the responses,.. with them i have several options at the moment ....i'll start with the free ones and see what happens.
     
  28. like the others have stated from what you have said you shouldn't have too much compression the motor is assembled and there is oil in it install the starter turn it over without the plugs to get rid of excess oil then install a compression gauge and check before doing anything
     
  29. CTaulbert
    Joined: Apr 8, 2007
    Posts: 1,206

    CTaulbert
    Member
    from Detroit


    A gasket with those dimensions would yield a volume of 0.63 cubic inches per cylinder or 10.30 cubic centimeters per cylinder.
     

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