Different size main bearings ?!

[cuzzi23]

Hi all<br /> 1989 4.3L 262 cid v6 Mercruiser<br />Engine overheated; found water in oil; decided to take whole engine apart.<br />Of course, found blown head gasket, between 1 & 3.<br />Other 4 cyl look good. Today I finished taking things apart. This is what I found at the crankshaft: four different size of main bearings . I'm a machinist ( altough not an engine machinist ) but not a mechanic so have no experience with this. I though this was a nono .<br />Could have this been done at the factory ?<br />This is what I foud :<br /><br />Bear Pos _Bear Size _Journ Dia _Journ Spec<br />Front _.006 us _2.4485 _2.4479-2.4488<br />Second _ nominal _2.4483 _2.4481-2.4490<br />Third _.0006 _2.4482 _2.4481-2.4490<br />Thrust _.001us _2.4479 _2.4484-2.4493<br /><br />Clymer book refers to first, second, etc bearing and I an assuming first is the thrust bearing.<br />Also I've never heard of .0006 us bearing, but that's stamped on bearing (GM .0006us).<br />Sorry for long post. Any Ideas ?<br />Thanks guys !!

 

[rodbolt]

Re: Different size main bearings ?!

hello<br /> its completely typical. the engines are produced with select fit bearings.<br /> however for service use the selection is somewhat more limited.<br /> the thrust bearing is the aft one with the flange. it absorbs the fore and aft motion of the crank.<br /> you must check the radial clearence as well as the lateral clearences on that bearing. if the bearings are not damaged they can be reused, you can check all the radial clearences with plasi-gauge.

 

[cuzzi23]

Re: Different size main bearings ?!

Thanks rb<br /> I think bearings need to be replaced. <br />Given the dimensions I've got on the crank, I was thinking about going with standards except for the thrust (.001"us). What do you think? <br />I also can check clearances with mics ( have id and od mics ). What clearance is acceptable?<br />Clymer book gives values for service and production clearances <br />Thanks

 

[mkast]

Re: Different size main bearings ?!

You might consider getting a Mercruiser engine manual, well worth the cost.

 

[tommays]

Re: Different size main bearings ?!

cuzzi23<br /><br />the clymers manual will just make your head spin<br /><br />your going to use plastic guage to check each one if you reuse them or go new it gets mushed in the bearing when its at the correct toqure then removed and compared to a chart for the correct clearence<br /><br />tommays

 

[cuzzi23]

Re: Different size main bearings ?!

Thanks guys <br />Have the clymer book and I understand most of the stuff ( except for the info that I need )<br />See, the book gives two set of clearances for main bearings 1,2,3,4 . Production and service.<br />Which one should I go for?<br />Also, which main is #1, the thrust or the one at front?<br />tommays, why do you think I should use plastic instead of micrometers to measure clearances ?<br /> Thanks again

 

[Dunaruna]

Re: Different size main bearings ?!

Plasti-gauge is much more accurate - no room for incorrect interpretation. The thrust is part of the rear main bearing (closest to the flywheel).<br /><br />Aldo

 

[cuzzi23]

Re: Different size main bearings ?!

Hi Aldo<br />But I think I am not explaining myself properly<br />( sorry )<br />I know which bearing is the thrust bearing. <br />But is that the #1 or the # 4 bearing ?<br />See, specs at book are for #1, #2, #3 and #4 bearings. It does not specify which one is thrust or if #1 is at front or at rear of engine. I would assume is at front, but....

 

[Don S]

Re: Different size main bearings ?!

Aren't the bearing caps marked with a number and a corrosponding number stamped in the block? #1 is supposed to be at the front of the engine, but you need to make sure the proper cap is on the right journal.

 

[tommays]

Re: Different size main bearings ?!

in there confusing verison of a manual i believe they mean that the front is 1 and rear is 4 but with a clymer its allways hard to sure<br /><br />i cant tell you the best available book for rebuilding but can say that the clymer would best be used for cleaning oil spills from the floor <br /><br />and i realy mean that i am not makeing a joke i think they hold some type of record for misprints<br /><br /><br />tommays

 

[Dunaruna]

Re: Different size main bearings ?!

O/K, I misread your post. The front of the motor is always the reference for #1 as in #1 piston, #1 spark plug, #1 connecting rod etc and #1 main bearing. Therfore the thrust is #4. The bearing caps should be stamped 1,2,3,4. If they are not stamped you should stamp them so that there is no possibility of mixing them up.<br /><br />Aldo

 

[Don S]

Re: Different size main bearings ?!

The best thing you could do right now is order an OEM Mercruiser manual. The best place to get the manual is from Mercruiser, see link below. You will need the serial number off your engine, but if that is missing (common on older engines) use 0B773243 as a serial number, it will get you service manual #7.<br /><br /> Mercruiser Service Manuals <br /><br />Then take that Clymers manual and give it to someone you really dislike.

 

[Buttanic]

Re: Different size main bearings ?!

The manufactures use the odd sizes to save a crankshaft that may have been ground out of tolerance rather than scrapping it. No matter what engine, you want main and rod bearing clearences between .015 and .025. Some times you have to split a set of bearings to get the proper clearence,such as using a standard size in one half and a + or - .001 in the other half. This is common practice when blueprinting race engines. If you can't get the correct clearence by doing this the only option is to have the crank ground to the next undersize. You will probably have to go to GM to get the odd under and over size bearings. Most of the afermarket bearing are only available in .001, .010, .020 undersize.

 

[Don S]

Re: Different size main bearings ?!

Originally posted by Buttanic:<br /> No matter what engine, you want main and rod bearing clearences between .015 and .025.

I think you missed a 0 in there. I'm sure you meant .0015 to .0025 bearing clearance.....my bet is the .015 to .025 would make a bit of noise....

 

[Buttanic]

Re: Different size main bearings ?!

I sure did miss a zero, thanks for correcting it.

 

[CTD]

Re: Different size main bearings ?!

If you want to do a really nice job have the block line bored (honed) then all the bores will line up and be exactly the same size so the bearings will fit right. You can use a micrometer and get good measurements if you know what your doing and have good equipment. Plasti guage is faster and almost as accurate. If you were going to all that trouble you would probably want to resize the rods too. Its not necessary unless you'r building a high preformance engine. I just looked at my OMC book for a 4.3 and it shows .00085-.00135 clearance on the rod bearings and .003-.0029 on the mains. Anyone have the specs from a mercruiser book?

 

[rodbolt]

Re: Different size main bearings ?!

hello<br /> while its possible to measure the bearing shell thickness and the rod big end ID and the journal and do the math to come up with a clearence plastigauge just works to easy. and less chance for errors. if you measure the shell it takes a set of tubing micrometers. the fixed anvil on tubing micrometers is radiusd to give an accurate measurement on a curved surface. or you can assemble the rod with the bearing torque to specs and measure the ID and the crank od and do the math. plastigauge is infinatly easier and less error prone.<br /> the reason for production VS service specs is the manufacture has acess to bearing sizes that the service person does not.<br /> we are usually limited to .001,.010,.020,.030.<br /> but years ago we actually burnished bearings to finnished size and knurled pistons and shot peened skirts and file fitted rings, all that is a thing of the past now.<br /> as labor rates rose and parts prices dropped all of that went away <br /> so if you have one journal that is .006 under from the factory and the bearing is no servacable your back to grinding the crank.<br /> a good crank grinder will hold a tolerance of about .0005. I did it and was not among the best.<br /> if the crank is ground make sure the oil holes get chamfered.

 

[16Pilot]

Re: Different size main bearings ?!

If you want to do a really nice job have the block line bored (honed) then all the bores will line up and be exactly the same size so the bearings will fit right.

I have been through all of this on my rebuild. The caps are drilled in the factory as one big piece of metal, connected to the block. That is why they are all 4 diffrent, at least that is how I understand it. Just make sure you PUT THEM BACK wher you FOUND them. If not this could be ugly. If it does not turn very very easily with caps minstalled and torqued w/o pistons installed. Take them of and have them cleaned. i could not see the numbers stamped on mine until it went through the machine shop. this is extremely important because you lock everything up or snap the crank in half. correct me if I am wrong Don, but this is my understanding from my bad experience. It is very expensive and not nessecary to line bore as long as you use new bearings and put evrything back. A new crank kit with reconditioned crank and all rod and main bearings is only $158.00 the bearing alone will run you about $60 for all. This way you know it is all matched up.(Still platiguage though) Oh, and one more thing, do not nick the crank on reinstallation. there is my $.02

 

[cuzzi23]

Re: Different size main bearings ?!

Great advice guys, there's no place like this one!!<br />I just realized I made the same mistake that buttanic. In the original post I meant .0006"us<br />not .006 for #1 bearing ( those d#@& zeroes ). <br />I just ordered the mercruiser manual ( thanks <br />Don S for the pick ).<br />Bearing caps were not marked ( I did marked them before removal).<br />I'm good with micrometers ( that's what I do for living )<br />If I assume that #1 is at front and crankshaft ands pecs from Clymer are correct, then, for what i measured, crankshaft dimensions are between tolerances ( altough in the lower limits ).<br />If this turn out to be the case ( by looking at mercruiser book when I get it ), then do you think I should buy a set of standard bearings and then chek clearances with those installed in block ?<br />I mean, if crank and bearings are between spec then clearances should also be between spec. ( yeah, like this is a perfect world, right? <br />Hey 16 Pilot, just in case, where do they sell those crank kits. It sound like a good alternative considering the price.<br />Thanks guy, I've being taking some pictures from this project and I'm going to try to post them soon. I'm sure you'll enjoy them.

 

[Don S]

Re: Different size main bearings ?!

If you were building an engine that was going to be seeing a lot of 8 or 9,000 rpm operation, it might be more of a problem, but for a marine engine that has been running since 89, and assuming the crankshaft is in good shape, I think the tolerances are fine. Put it back together with a set of standard size bearings and use Plastigage to test every bearing. It's fast and more accurate than measuring. Then use a dial indicator to check the end play in the crankshaft. If something is out of spec with the new standard bearings, then change what is needed. But until then I think your doing a lot of measuring and worrying for nothing.