Rebuilding a 425

Discussion in 'Big Block Cadillacs' started by Cadillac Kid 1, Apr 3, 2020.

  1. Cadillac Kid 1

    Cadillac Kid 1 Well-Known Member

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    You talk about being bored? While I am waiting for my 1970"binocular piston " 472 block to come back from the machine shop (being zero decked and cylinders honed) I started clearing out my garage and the low mileage 425 I pulled out of my '79 to replace with a large chamber 9:1 472 "jumped out" at me. Soooo, I am rebuilding it for something to do. Cylinders have been honed (stock bore with VERY little taper, new cam bearings installed and Moly rings ordered. I will post the progress, again this is to keep me from going stir crazy, so any conversation on this is welcomed.
    Greg Surfas
     
    Last edited: Apr 3, 2020
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  2. kzhurley

    kzhurley Active Member

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    I am rebuilding a kohler 12hp engine that doesn't need rebuilt for a mower that I will never use.. Lol
    No worky for me for a few weeks "officially". I'm sure it will turn into months
     
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  3. kzhurley

    kzhurley Active Member

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    Ive always wondered how well a 425 would do with some early small chamber heads and some cam. I know I'll never get to it since I have a couple 500s to go through first
     
  4. Cadillac Kid 1

    Cadillac Kid 1 Well-Known Member

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    I've got several 500's and 472's that are waiting their turn but this is a project that's just to be doing a project. I have absolutely no idea what I am going to do with the motor when its done.
    Cooling system differences (between 5200 block and 425) prohibit just swapping heads. Besides with the flat top[ 425 pistons and 76 cc's in the chamber you have something over 12:1.
    Greg Surfas
     
  5. 48Austin

    48Austin Safty First! Then pull the trigger!

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    Is the Kohler motor for a riding mower?
     
  6. kzhurley

    kzhurley Active Member

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    Yes. For a cub 72. Originally 7hp lol. Wanted a 16hp but couldn't find a good core for a reasonable price.
     
  7. 48Austin

    48Austin Safty First! Then pull the trigger!

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    Then there is no problem. Ditch the Kohler. Cad 425 more fun. . Show this guy who's boss:devil::devil:
     
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  8. Darius

    Darius Well-Known Member

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    This "engine sickness" is more pronounced than I imagined!!!

    Heaven help us!

    bro. "a life time sickie," d
     
  9. Darius

    Darius Well-Known Member

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    What the hell engine is in that thing?

    bro. d
     
  10. kzhurley

    kzhurley Active Member

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    "FUN".......

    Some sort of big block chevy
     
  11. kzhurley

    kzhurley Active Member

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    Aside from the cooling differences, which may or may not be major issue, just a rough estimate with a comp calc going from 96cc heads with 8.2:1 comp to 76cc would be more like a little under 10:1.... Very rough measurement.
    IF it did work out with cooling ports and whatnot, I think the larger ports of the "big" big cad heads and some kind of 230ish cam would make it for a decent revving and still have lots of torque to play with.
    Again, all hypothetical as I'll never bother to do it, lol. Especially with all the 500s I,need to use up first.
     
  12. 5one9

    5one9 Active Member

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    I don’t think that ‘small’ bore would handle much of a valve size increase. I agree I don’t think the compression would be that high.
     
  13. robocop

    robocop Active Member

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    A 72 cc cylinder head on a 428 Pontiac gives a nominal 10.75:1 compression ratio. But they weren't actually that high. More like 10:1.
    The Caddy early combustion chamber is a lot like modern LS motors. With a tight quench and a wide LSA cam it might work on pump 93.
     
  14. Cadillac Kid 1

    Cadillac Kid 1 Well-Known Member

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    Sorry
    In my calcs for CR I had the 4.30 inch bore of the 472. Correct CR with 76 cc heads is around 9.8.
    Greg Surfas
     
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  15. 48Austin

    48Austin Safty First! Then pull the trigger!

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    Just rehash the motor and dump it in the old John Deere. And have fun cuttin the grass!
     
  16. Cadillac Kid 1

    Cadillac Kid 1 Well-Known Member

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    I'm just doing the motor NOW because I am bored. It is eventually destined for my cousin's 1300 pound roadster.
    Greg Surfas
     
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  17. kzhurley

    kzhurley Active Member

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    Whats the build plan? Just re-ring and bearings or?
     
  18. Cadillac Kid 1

    Cadillac Kid 1 Well-Known Member

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    Moly rings, crank turned and main and rod bearings, cylinders honed, new cam bearings, probably an MTS #5 cam with Cloyes timing set, reworked oil pump, Flow kooler water pump, perhaps some head work and probably some more when I get to it. We have at least the month of April.
    Greg Surfaas
     
  19. Cadillac Kid 1

    Cadillac Kid 1 Well-Known Member

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    Well, Sunday seemed special so I did a bit more on the 425. The block is prepped. I have the crank that I was going to use in rebuilding the 425 before I decided to go to the 472. It was machined by MTS and the mains were turned 0.010 under, mikeing out at 2.490”. Installed the rear seal and laid the crank on the new bearings using GM Engine Assembly lube. Torqued the mains to 90ft/lb. with a piece of 0.001-.003” plastigage between the bearing and the crank. Results show a clearance of 0.001”.
    Enough for one day.
    Greg Surfas IMG_1769.jpg IMG_1782.jpg IMG_1783.jpg IMG_1784.jpg
     

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  20. Darius

    Darius Well-Known Member

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    LOOKIN GOOD!!!

    Funny how much gets done when we're bored.

    Best,

    bro. "keeping himself busy," d
     

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