engine bearing clearance question
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engine bearing clearance question

  1. #1
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    Default engine bearing clearance question

    the clearance on my main and rod bearings are 3tho.mains and 3tho.rods,
    i fell that 3 on the rods is alittle much..bought a set of 1 under rod bearings
    and would use 1/2 shell to get the clearance down to 2-1/2 tho.--another builder friend of mine says to run it at 3/3 ---what are your thoughts
    thanks / maddog - p.s. engine is a n/a bbc

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  3. #2
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    Leave it at .003.

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    I second that!..

    Hey Bob, I see ya made it home ok!..

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    IMHO .003 is way too wide for the rods.
    I wouldn't go anything over .0025 and would rather have it closer to .0020.

    My cent and a halves worth anyway!


    Warp Speed

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    steelcomp was here
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    Quote Originally Posted by Warp Speed View Post
    IMHO .003 is way too wide for the rods.
    I wouldn't go anything over .0025 and would rather have it closer to .0020.

    My cent and a halves worth anyway!


    Warp Speed
    ditto
    If God is your co-pilot, change seats!
    Acts 2:38, the perfect answer to the perfect question.

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    steelcomp and warp speed --- would you suggest i do as planned ,,running the 1/2 bearing shell to bring it to .0025 or both to .002 -- that seems tight,,but those are my options..appreciate your advice //thanks---------------any and all chime in----

  9. #7
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    If this baby is a full on race engine and you need every bit, start going with other bearings to try and bring them down a little. If this is just a lake warrior, and your measurements are really consistant at .003, just go with that and don't use ultra-thin oils.
    It goes back to the idea of 'what is this for.' And until we know that, our answers will all be skewed one direction or another depending on our backgrounds.

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    thanks Wannabe -- i sold my drag motor and building this more user friendly
    462 bbc-lake warrior -good pcs.though--splayed cap block--forged steel crank--H beam rods -10.5 forged pistons - alot of work into 990 heads --roller cam etc..--won't bash the crank company but what a time getting the assembly balanced .man that crank was out.. ------ but its not a drag motor but i do "build them to beat them" -it mics.out perfect 3/3 across the board.. I never had a problem interchanging 1/2 a bearing from one set .001under to a std. set to close up the gap .0005 --- clevite 77 H series bearings-- this will be running in the 6000 + rpm range--- b cut berkely ----would you buy another set of rod bearings to try? Seems like swapping in a 1/2 of bearing is tabu ---i thought it was common practice,at least in has been in my garage for 35+yrs. especially on them old stock FE's--lol

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    here is some interesting info. on bearing clearance...

  12. #10
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    Quote Originally Posted by maddogeast View Post
    thanks Wannabe -- i sold my drag motor and building this more user friendly
    462 bbc-lake warrior -good pcs.though--splayed cap block--forged steel crank--H beam rods -10.5 forged pistons - alot of work into 990 heads --roller cam etc..--won't bash the crank company but what a time getting the assembly balanced .man that crank was out.. ------ but its not a drag motor but i do "build them to beat them" -it mics.out perfect 3/3 across the board.. I never had a problem interchanging 1/2 a bearing from one set .001under to a std. set to close up the gap .0005 --- clevite 77 H series bearings-- this will be running in the 6000 + rpm range--- b cut berkely ----would you buy another set of rod bearings to try? Seems like swapping in a 1/2 of bearing is tabu ---i thought it was common practice,at least in has been in my garage for 35+yrs. especially on them old stock FE's--lol
    The only reason you would run the .003 clearance in the rods is if you where stressing the big end so much that it is the only way you can keep the parting lines from scraping off the oil film (wedge). I doubt in your application you will be going this far with rpm and power even with stock rods (but I could be wrong).
    When running bearings this loose (especialy rod bearings) it is tough to maintain the lubrication wedge without running super thick oils. Remember, the rod bearing only gets a shot of oil once every revolution. The length (duration) and quality of this is dependant on main bearing grooves, main bearing clearnce and oil pressure. The important thing is to give it the best quality shot so the oil wedge will last until it's next shot. This is 180* of crank rotation away at best, and only really gets a quality shot for about 50* of rotation due to pressure loss at main bearing parting line.
    To do this we need to make the main losses as little as possible (through proper main clearance and oil grooving) and make sure what we do give the rod bearing stays put long enough to receive the next shot.
    The tighter the bearing clearance, the more pressure the oil wedge will withstand before breaking down (or disspersing through variouse leaks). This improves all the way to the point of failure where the clearance becomes to tight for the parts/rpm and the parting lines wipe the film away.

    In a nutshell, the tighter the better. and .0025 is a long way from too tight as long as the rods/bolts being used are even close to decent quality.

    Installing a half shell to tighten clearance? More than an excepted practice everywere from home garages to top end race shops. As a matter of fact, doing it as we speak. Well, as soon as I quit rambling!!!

    Warp Speed

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    thanks alot guys - i thought i was right with .0025 --i thought .002 was tight.
    and 1/2 shell it is,,thought i was doing something that was no longer acceptable .- after beers with some bench builders they had me double guessing myself......thanks again ---- great posting

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    Quote Originally Posted by maddogeast View Post
    steelcomp and warp speed --- would you suggest i do as planned ,,running the 1/2 bearing shell to bring it to .0025 or both to .002 -- that seems tight,,but those are my options..appreciate your advice //thanks---------------any and all chime in----
    Warp pretty much summed it up, but yes, the half shells are an acceptable and common way of adjusting clearance. Definately take the time to get the .0025. If nothing else, you don't need all that extra oil being thrown around in the crankcase. That's HP gained, right there.
    If God is your co-pilot, change seats!
    Acts 2:38, the perfect answer to the perfect question.

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    Never mind.
    Last edited by Fiat48; 01-21-2008 at 11:30 PM. Reason: Not constructive

  16. #14
    steelcomp was here
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    Quote Originally Posted by Fiat48 View Post
    Never mind.
    Hey now, come on Bob. You've always got something constructive to say about things like clearances. Even if it applies to blown alky engines, it's all good info. I was hoping you wold post something. You've got more experience here than most of us.
    If God is your co-pilot, change seats!
    Acts 2:38, the perfect answer to the perfect question.

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