Main bearing clearance?

Discussion in 'Street/strip 400/430/455' started by msamuel, Sep 15, 2021.

  1. msamuel

    msamuel Member

    I'm getting .004 Main bearing clearance measurements with telescoping guage and outside micrometer. The crank mains has been ground to 3.240" , the main bores torqued to 105 ftlb measure 3.244" with bearings and 3.440" without. General consensus on the internet says it should be in the area of .002" for a street build. But two machine shops , the one that did the work and another I talked to for a second opinion , say it should be fine. Will this be ok or should I ask to grind the crank .020 and try to get it closer to .020?
  2. sean Buick 76

    sean Buick 76 Buick Nut

    Yes that’s way too loose. Grinding crank won’t help You need oversized bearings.
    msamuel likes this.
  3. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD" Staff Member

    Way too loose. With clearances like that, you will lose a lot of oil pressure from the front to the rear of the motor. You'll also have low oil pressure at idle when hot. Do not listen to machine shops that tell you it will be fine. It won't.
    msamuel likes this.
  4. Schurkey

    Schurkey Silver Level contributor

    How accurate are snap-gauges? I wouldn't trust them any farther than I can throw 'em if I had something better. I'd want a dial bore gauge, reading in actual ten-thousandths.

    You're using a real 3"-- 4" micrometer that reads to the ten-thousandths to measure the OD of the crank journal, and to measure the snap-gauge? How does that mic compare to the insulated standard it came with?

    How much eccentricity is there from top-bottom versus across the parting line of the block main saddles?

    What does plain ol' Plastigage show for clearance?

    How many blocks and cranks have you measured that had acceptable clearance?

    .004 is WAY TOO LOOSE. You would need to re-grind the crank, buy different bearings, and verify that the main saddles have appropriate bearing crush and are in-line. But my concern is your measurement accuracy. IF (big IF) you're measuring accurately,
    • the crank could be ground to or beyond the "low side" of acceptable diameter for the specified undersize.
    • The block main saddles could be at or beyond the "high side" of acceptable diameter. This also screws-up your bearing crush
    Last edited: Sep 15, 2021
    alvareracing, chrisg and msamuel like this.
  5. msamuel

    msamuel Member

    Plastigage gives me the same results. The 3"-4" micrometer isn't super fancy but it zeros in with the guage it came with.
    I've only assembled 3 engines but all three had new cranks all measured with the same "fowler" brand micrometers and where within "all data" specs.
    I have not measured the eccentricity of the main saddles but the machinist said they were fine and did not need line honing.
    I do have a bore gauge I can recheck with.
    I did notice I only get a crush of about .001 when torqued from 20ftlb to 105ftlbs. I don't know how much it should be.
    It's hard to find block dimensions for this engine.
    I'll probably end up taking it to a third machine shop lol.
  6. telriv

    telriv Founders Club Member

    Use a .001" shim under the top bottom bearing shells. IF this is NOT a racing engine it will last for years at MUCH LESS cost.
    Just my thoughts.
    Tom T.
    Mart likes this.
  7. knucklebusted

    knucklebusted Well-Known Member

    OK, not super savvy in the measurements but you did see half of the difference on each side, right? If the difference between inside bearing and crank journal is 3.240 and the block is 3.244 for a difference of 0.004, wouldn't that be 0.002 on top and 0.002 on bottom?
  8. Schurkey

    Schurkey Silver Level contributor

    I feel better about your measurement system, and experience.

    You need to get the crank re-ground, and different bearings. If it's "ten under" now, I expect they'll clean this up at "twenty under".

    How are the rods? Clearance and crush OK?

    I think this is the voice of experience, and I have trouble arguing with what has worked for you.

    However, I gotta say that this does not give me a warm, fuzzy feeling. For one thing, the shim stock would need to be the same width as the bearing shell, and I expect that mostly shim stock will be too narrow.

    In MY shop, the crank would get re-ground properly, and bearings purchased to suit. The questions for me are whether the guy who screwed-up the crank grinding the first time, can be trusted to do it right the second time (and whether I should have to pay him a second time.)

    No. The upper and lower bearing insert ID when the caps are torqued (i.e., the bearings in the block) is 3.244. The crank main journals are 3.240. .004 total difference. Too damn much.

    When running with an oil wedge, yes there'd be .002 at the block side, and .002 on the cap side--in fact .002 all the way around, except for whatever eccentricity there is near the parting line. But .002 all the way around isn't how things traditionally get measured; and it still adds up to .004 clearance which is still too much.
    Last edited: Sep 16, 2021
    srb likes this.
  9. 436'd Skylark

    436'd Skylark Sweet Fancy Moses!!!!!

    It would be cheaper to align hone the mains than grind the crank again.
    msamuel likes this.
  10. msamuel

    msamuel Member

    The rod big ends where resized and the crank pins were ground . 020 . All rod bearing oil clearances are at .0018.
    I work in a auto repair shop but this is my personal project. Today I called the machine shop that did the work , from the shop, for a quote on a customers van that also needs crank work, but they told me they don't have anyone that can grind cranks at the moment ,...but they just did mine a week ago??
    Anyways I think there's a shortage of machinists that are capable of this kind of work in my town. They did a good job with the rest of the block work but I think I'll have to look in the next city.

    Btw thanks for the advice the second machinist almost convinced me to run as is

    BUICKRAT Torque Rules!

    There is a shortage of machine shops, they have been regulated out of business.
    Super Bald Menace likes this.
  12. 83T-type

    83T-type Well-Known Member

    Can’t say I have experience with what will happen at that clearance, but after looking at my notes from the last time I assembled my motor I had .0027”-.0029”.That being said I think most guys here said to keep it under .003. I have about 60 on cold startup and the lowest at idle it’s ever went hot is 20 psi. So if you ran .004 I feel like it could drop to single digit oil pressure while hot…
    msamuel likes this.
  13. 436'd Skylark

    436'd Skylark Sweet Fancy Moses!!!!!

    The rod bearings get their oil from the mains. The more that leaks out in the mains from loose clearances the less the rods get. We all know the 7/8 issue..
    msamuel likes this.
  14. 83T-type

    83T-type Well-Known Member

    Funny you mention the 7-8 rods. Before my 430 I had a 455 that developed a small knock. Just had to move it out of my garage and the next thing you know I had one of those see thru engines. I’d run a .001 oversized bearing if available at the least. Like others said .004 might work but it’s likely asking for issues.
    msamuel likes this.
  15. msamuel

    msamuel Member

    Got it, So the rod bearing's are first to go in a low pressure condition. That's what happened to this one. It spun #7 rod bearing.
  16. Bens99gtp

    Bens99gtp Well-Known Member

    That's the journal that typical goes first.....seeing the rods get lube after the mains and 7/8 are the farthest from the source, sometimes there just isn't enough oil flowing to get enough back there
  17. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD" Staff Member

    There is a reason Buick built them with tight clearances. Larger clearances bleed oil pressure from front of the block to rear. #7 and #8 rod bearings are the last to get oil.

    My engine was built with an oil balance line. A braided line runs from the front oil sender hole to the rear oil galley, so the engine oils from the front and rear. Overkill for most 455's, but my engine got one.


    msamuel likes this.
  18. msamuel

    msamuel Member

    So do you need a high flow oil pump with this setup? it would be nice if the line coming out the back was steel at least to outside of the bell housing area so you won't have to remove the engine to replace that hose
  19. Jim Weise

    Jim Weise EFI/DIS 482

    Ok, let's get to the heart of the matter here.

    Your main housings are too big.. the main spec is 3.4380 to 3.4390... if you measurements are correct, then your a full thousanth over.

    Main housing size, and the size with the bearings is nearly a direct correlation.

    All my cranks are done to high spec (which yours is) and my main housings are done to mid spec (3.4385), which, with Clevite 77 mains will produce a main clearance of .0025, slight variations in housing size from journal to journal, or of the bearings will only change the clearance by a tenth or two, which will not matter.

    Find someone to fix the block.

    That old saying "Don't take your Buick to a Chevy machinist" stems mainly from engine failures due to oversized/out of round main bearing housings. Our motors are some of the biggest in diameter for the mains in the auto industry, and require the larger diameter honing mandrel, that many machine shops don't have.

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  20. Schurkey

    Schurkey Silver Level contributor

    THANK YOU for looking up the saddle (main housing) and crank journal dimensions. I don't have easy access to that info, and gave incorrect advice (grind the crank, buy new bearings) as a result.
    Max Damage likes this.

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