[Question] Main and rod bearing clearances questions...454 build
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Main and rod bearing clearances questions...454 build

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    Off Road Badass Bonzen's Avatar
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    Default Main and rod bearing clearances questions...454 build

    Got my crank/rods/pistons back from balance, and im going to start puttin everything back together. But I need some info on the bearings and what clearances to run...I have a set of standard thickness King bearings for the rods and mains that were installed in the short block engine i bought, but Ive read in 100 different places that for a marine app you need to go with more bearing to rod/crank clearance. Something like .001 more then a standard automotive setup. So im looking on summit, and I see .001"undersize" bearings and .001"thinner" bearings. Are these the same thing or what?(I also see .010 and .030 undersize!) I can see if you had to grind a crank, or the inside of the rods and you removed material, you would need a thicker bearing to take up the space. I know it's hard to guess at this, but is .001 more clearance then standard stock the general rule of thumb for a marine setup that has not had a ground/honed crank and rods? Basically im trying to decide if i can run these bearings I have or if i have to buy a new "thinner" set.

    http://www.summitracing.com/search/b.../main-bearings

    So here is the kicker....I measured with two different sets of quality calipers the thinkness of the new King rod bearings and the thickness of the old GM rod bearings that came out of my old motor..and for the most part they are pretty much identical on average...they seem to vary between .001 and .0015 of each other....but in some cases the old worn GM bearings are thicker then the new Kings...Im totally confused here haha! Ive recieved alot of good advice from GN7, but I didnt want to bother him too much and so i decided to ask the opinions of the general population!

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    Platinum Member BigSteve's Avatar
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    Quote Originally Posted by Bonzen View Post
    Got my crank/rods/pistons back from balance, and im going to start puttin everything back together. But I need some info on the bearings and what clearances to run...I have a set of standard thickness King bearings for the rods and mains that were installed in the short block engine i bought, but Ive read in 100 different places that for a marine app you need to go with more bearing to rod/crank clearance. Something like .001 more then a standard automotive setup. So im looking on summit, and I see .001"undersize" bearings and .001"thinner" bearings. Are these the same thing or what?(I also see .010 and .030 undersize!) I can see if you had to grind a crank, or the inside of the rods and you removed material, you would need a thicker bearing to take up the space. I know it's hard to guess at this, but is .001 more clearance then standard stock the general rule of thumb for a marine setup that has not had a ground/honed crank and rods? Basically im trying to decide if i can run these bearings I have or if i have to buy a new "thinner" set.

    http://www.summitracing.com/search/b.../main-bearings

    So here is the kicker....I measured with two different sets of quality calipers the thinkness of the new King rod bearings and the thickness of the old GM rod bearings that came out of my old motor..and for the most part they are pretty much identical on average...they seem to vary between .001 and .0015 of each other....but in some cases the old worn GM bearings are thicker then the new Kings...Im totally confused here haha! Ive recieved alot of good advice from GN7, but I didnt want to bother him too much and so i decided to ask the opinions of the general population!
    So I guess your running your original crank ?

    Did you micro polish the journals ?

    I think I would drop in the bearings and crank and plastic gauge the clearance at this point

    If you were in the machine shop it could be measured a little different

    post up the results on the mains and rods

    PS be sure and bottle brush all the oil galleys in the crank and block
    Last edited by BigSteve; 06-19-2013 at 08:09 PM.

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    Senior Member lbhsbz's Avatar
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    GN7 will have the answers as to what will stand up to abuse...

    My BBCs seem happy with .0032-.0035" on the mains and .0018-.0022" on the rods with a good oil like Total Quartz Racing 15w-50.

    I'm not exactly nice to engines either.
    GN7: "If you were to have ever had two brain cells you could rub together and make heat, you be dangerous"

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    Some guy obnoxious001's Avatar
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    Different engine builders will work with different clearance numbers, and then to make it a little more complicated, choose to use different viscosity oils, and either a standard or high volume oil pump. I learned about the clearances I still use in the late 70's, while working at Gale Banks, based on using a high volume oil pump and heavier type motor oils.

    Perhaps more important than the actual clearance is consistency between journals, as well as making sure the crank pins are round. An engine is a combination of controlled oil leaks. There is a very interesting video on the Mellings oil pump website, showing the difference in flow between .001", .002", .003" and .004" clearances all on the same 4 cylinder engine with an electric motor running the oil pump.

    I don't think you will be capable of getting very accurate measurements with calipers, a micrometer is the more accepted measuring tool for crankshafts. Proper measuring tools get expensive if you aren't going to be building many engines.

    Crankshafts vary quite a bit, even brand new ones are not all the same size. If you need more clearance than "standard" bearings give you, there are extra clearance bearings (Clevite designates them as n "X" bearing). When you buy a set of .001 undersize bearings, that is for an undersize crankshaft, so the bearing is actually thicker.

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    Some guy obnoxious001's Avatar
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    A quick addition to my previous post. An old rule of thumb on clearance might be just over .1% of the diameter of the crank.

    Std BBC crank rod journal is 2.199",, so about .0022" ( I got a little extra clearance over that, .0025" or greater)

    Std BBC crank main journal is 2.749", so about .0028 (I use that kind of as a minimum on main bearing clearance, prefer .003")
    Last edited by obnoxious001; 06-19-2013 at 08:54 PM.

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    Off Road Badass Bonzen's Avatar
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    Ok good info....

    So to clarify for the record....an undersize bearing is thicker in wall thickness than a stock/standard bearing.

    A thinner bearing is exactly that...thinner in wall thickness for a marine application so to speak.

    Yes, the crank is stock, straight as an arrow and all journals have been polished at my machine shop.

    Im going to plastigauge a few of the journals/rods and see what it looks like with the standard king bearings that I have. Seems that i want around 2 and a half thou on the rods and mains, give or take a half thou....

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    gn7
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    Bonzen, you're over thinking this. You EVER seen a recommendation for "marine" bearing clearances? I have seen them for pistons and valve guides, but NEVER for a marine deal.
    Have any of you ever looked at the OEM or bearing manufactures specs for bearing clearance on a BBC? Its like .0017 to .0032 for the mains and .0012 to .003 on the rods!!!!
    If you are making less that 500 HP, I don't see the need to get all wound up about the bearing clearances. GM built some pretty bad ass engines using nothing but go/no go gauges. You think they pain stakingly measures every crank and every bearing with a mike? How many blew up??

    Not saying you shouldn't check, I'm saying I have bigger fish to fry than if the main bearing clearances are .00275. .003, .0035, or even .004
    I understand a full on blue print deal where you are trying to squeeze every drop of power from the thing and you are shooting for a given oil vicosity for the engine built. No need for 20/50 in a 500HP deal, so why built for it??

    I see no reason why a boat engine needs larger bearing cleances. Its not the same thing as cylinder bore or valve stems. Apples and oranges. The crank has no idea what the cylinder temps. water temps, exhaust temps, air temps, or any other temp other than oil temp.
    RPM, crank stiffness/flex, power level, and BLOCK STRENGTH, are all more important to bearing clearances than if its car, boat or airplane.

    Bonzen, if your mains come in between .002 and .003, and your rods come in between .00175 and .00275, put the damn thing together and run it.

    This thing was together when you bought it. I would LIKE to think that the person had some knowledge of what he was doing. I would be more concerned with things like cylinder bore clearance and valve guide clearnace and less about bearing clearance.

    Like Obnoxious said, if you are using calipers to make your measurements, you are pissing in the wind anyway.

    Like Big Steve said, use some plastagauge and check to see you are within the above numbers.

    If you don't have a GOOD bore gauge and mic set, it would be best if you don't try to get to tricky with the clearance. Just about impossible to get the bearings too tight on a low power deal, but its real easy to get them too loose.



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    Last edited by gn7; 06-19-2013 at 10:55 PM.

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    Off Road Badass Bonzen's Avatar
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    haha figured you would eventually find this thread....

    Honestly, I hear what you are saying. When it comes to mechanical things, I overthink the living hell out of them...it doesnt help that i work in the aerospace industry where .0001 will pass off a part or not. But yeah, im about over the stressing and ready to put this bitch back together. It was together when i bought it, but i couldnt bring myself to drop it in the boat knowing it wasnt balanced. Now that it is, I feel much better. I believe you on the factory clearances thing....there has to be alot of variation and all those cars and trucks run fine with pissed off teenagers behind the wheel on a friday night...

    I started putting the piston rings back on tonight, and that was at least, kinda fun...dimple dots on the rings face towards the top of the piston right?(I hope)

    I really appreciate all your (and everyones) advice and help. Im the guy that hates not knowing everything, so this whole engine build has been a humbling experiance for me. Hopefully there are some other noobs that will learn something too from all this.

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