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Technical 390 Cadillac heads on a 331 Cadillac

Discussion in 'The Hokey Ass Message Board' started by Mike Paul, Jul 14, 2021.

  1. Mike Paul
    Joined: Oct 10, 2003
    Posts: 936

    Mike Paul
    Member

    I have a good pair of 1960 390 heads that I want to install on my 1950 331 caddy. I did a thread search and messaged some members but still have conflicting answers. Some say that they are a direct bolt on with the 390 pushrods and some say the 331 block has to be bored to 4” like the 390 because of the wider valve center in the 390 heads. That would give me cylinder wall thickness concerns. I’m just looking for a definitive answer of what needs to be done to bolt my 390 heads on my 331. Thanks
     
  2. lumpy 63
    Joined: Aug 2, 2010
    Posts: 1,432

    lumpy 63
    Member

    Did you search the 331 365 390 Cadillac thread on here? Tons of good info.
     
  3. Mike Paul
    Joined: Oct 10, 2003
    Posts: 936

    Mike Paul
    Member

  4. Mike Paul
    Joined: Oct 10, 2003
    Posts: 936

    Mike Paul
    Member


  5. cretin
    Joined: Oct 10, 2006
    Posts: 2,954

    cretin
    Member

    We put some 390 heads on a 331 at the shop. It was not bored to 4". If I recall correctly, we had pushrods made, I'd have to check on that. The engine has only been broken in at this point, but there have been no issues.
     
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  6. Mike Paul
    Joined: Oct 10, 2003
    Posts: 936

    Mike Paul
    Member

    Thanks for the reply. Has it been bored out at all or is it still the stock 3 13/16” bore? I dry fit my 390 head on one side and the intake valve clears the cylinder wall but the exhaust has interference at a little more than max lift. I plan to bore it .060 so hopefully that will give me enough room
     
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  7. cretin
    Joined: Oct 10, 2006
    Posts: 2,954

    cretin
    Member

    I don't remember off hand, and I don't want to give you incorrect information, so I'll look into it when I'm back at the shop this week.
     
  8. ckh
    Joined: Jul 1, 2013
    Posts: 19

    ckh
    Member

    It is my understanding this will not work on the 49-54 331's. It will on the 55 331. And later also.
     
  9. cretin
    Joined: Oct 10, 2006
    Posts: 2,954

    cretin
    Member

    The engine we did it on is a '49. I'll try to get some more info.
     
  10. cretin
    Joined: Oct 10, 2006
    Posts: 2,954

    cretin
    Member

    Ours was bored to .030 over. Not for clearance, just because we were building a fresh engine and that was the next step in bore.
    We don’t quite remember what we did for push rods. Although looking at some photos, the push rods don’t look new, so maybe we did have and use the 390 pushrods.
    I suppose someone else will have to chime in on that, or you’ll need to do some measuring.

    I’d open my 390 and help you with pushrod lengths, but with my current situation I’m unable to. Hopefully someone else here has some they can measure.

    Hope that helps.
     
  11. ckh
    Joined: Jul 1, 2013
    Posts: 19

    ckh
    Member

    In 1955 Cadillac changed the rocker arm ratio. The same ratio is used on the 390's. That's why a complete 390 head assembly is a direct bolt on for a 55 331. You must also use the 390 cam, pushrods, and lifters to make it all compatible. I fail to see how 390 heads are going to work properly on a 49-54
    block. Am I missing something?
     
  12. cretin
    Joined: Oct 10, 2006
    Posts: 2,954

    cretin
    Member

    You'll have to excuse our improvised run stand, but here is a '49 331 running 390 heads.
    If the '55 331 has the same rocker ratio as a 390, I'm going to assume (correct me if I'm wrong) that the 365 rocker arm ratio is also the same. We have also successfully run 365 heads on a 331.

     
  13. Mike Paul
    Joined: Oct 10, 2003
    Posts: 936

    Mike Paul
    Member

    Cretin, thanks for the video. What are you running for a camshaft? I’m leaning toward the 270 Isky
     
    Desoto291Hemi likes this.
  14. southerncad
    Joined: Feb 5, 2008
    Posts: 774

    southerncad
    Member

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