Main bearings making me batty!

[Maverick H1L]

I've been working on getting my base 3.1 block ready for installing my 3100 heads. I pulled the engine in March, when I finally got the hoist, and promptly tore it down to only the block and cam (I'm not touching the cam in any way, shape, or form, yet). I noticed that all of the bearings were factory GM (nice considering I have no idea on the engine's history except that the VIN derivative on the side of the block says it's from a 1991 Camaro). I also noticed that the mains were marked as being .016mm oversize. Now, I've tried standard bearings, which I've had in their package since 2006 along with the rod bearings I'll be using, and Plastigage says that the oil clearance is too large (but I was using the wrong Plastigage, blue instead of green). I ordered a set of .010" oversize bearings last week (I've been fighting with Northern Auto about a set for over a month and finally gave up) and got them in today. My crank won't turn with the .010" mains in. Should I get another set of standard bearings and the right Plastigage and hope for the best? GM wants $25 A PIECE for the mains, and I hope I don't have to resort to that since I've already spent over $100 on just the mains alone. I have everything I need to get the base engine assembled with the exception of the oil pump drive and the blasted main bearings.

[2.8-350TBI]

.016 oversize was probably done for oil pressure from the factory but that was in Millimeters, not inches.

You got .010 over bearings, but in Inches, so you have two options.
Get stock bearings or cut your crank .010, which is what I would recommend, since you start off with fresh surfaces

[geoffinbc]

Agreed. If stock bearings wont work then you need to do some measurement. If standard over sized bearings wont work on the stock journal then you need to cut them to match the bearing. But you need to get the bearings first. Measure the bearing bore and then have the crank cut to get the proper oil clearance.

The easy way and cheaper way is to take the engine to a machine shop.

[Maverick H1L]

But a machine shop is the least cost-effective way... I already need to have my flywheel resurfaced (heat checked BAD) and I don't know how I'm going to come up with the money for that, even. It's not just the engine work, my car is completely torn apart (I mean literally, it's just the frame with doors and front and rear glass on jack stands) due to a bad quarter panel (among other rust damage) and finally getting around to replacing 80% of the suspension components that have been bad for years.

I'll get the bearings and green Plastigage first and then see where I need to go. I was hoping to get the running gear done and set aside while the rust-killing is going on, but now it looks like that isn't going to happen...

[geoffinbc]

I mean it is cost effective to have them measure and machine. If required. Rather than buy the measuring instruments, figure out the math, drive to the shop to drop the crank off, wait for the work to be done, go pick it up, install it and then measure again to make sure the job is done right. If it is not done right you may be out a crank or have to get it cut again to take more off.

[trotterlg]

We should look at your numbers. Your crank is .16mm under sized (you said .016 but I think it is .16) so in inches that is .006 under. Bolting .010 under bearings on it will lock up the crank because they are .004 smaller than they should be. It is doing just what the numbers say. Standard size bearigs are probably going to leave you with about .008 inches of clearance or so with is too much. Did I miss something with your numbers you provided? Larry

[Joseph Upson]

I personally wouldn't turn a perfectly good crank down without it being absolutely necessary. I've always had them polished for a fresh surface unless the machinist suggested otherwise, but that's me.

It's probably best for you to take the crank to a machine shop and tell them what your situation is and ask if they can set you up with the right bearing combination. It is okay to use two different bearing halfs to get the clearance you need. They'll measure it and make a recommendation based on the results.

[trotterlg]

It is okay to use two different bearing halfs to get the clearance you need

Do you put the tight bearing half on the top or the bottom ?

Larry

[Joseph Upson]

Do an internet search, it shouldn't be hard to find the recommended arrangement. This is the method I used to get my connecting rod bearing clearance tighter and I just don't recall which bearing went where.

[trotterlg]

Sorry, I am just the new guy trying to make a little joke. If you think about it, the tight bearing will have very tight clearance on each end and not so much at the bottom (or top). I am kind of thinking this is a bad plan. Larry

[geoffinbc]

Bearings are not round they are oval shape. The vertical clearance is all you should measure. Horizontal clearance will always be more. That is why it is ok to use 2 different bearings. Also cutting a crank 0.010" will not affect anything. In your case you may be able to just get a tiny cut and polish is order to fit standard over-sized bearings.

[SappySE107]

It is okay to use two different bearing halfs to get the clearance you need.

2 different size crank bearings?

I understand they are oval, but .010 is ~.005 at the corners and for the life of me, I can't find the logic in this other than to not spend money. I would never even consider such a method to make it work.

[geoffinbc]

Builders do it all the time to fine tune oil clearance. You could taper the edges if you wanted

[trotterlg]

If the crank is really .006" under sized, the only "odd" fix I would consider is to shim new standard bearings using .003 shim stock. You would have to file a little off the ends of the inserts so they would have the proper crush fit, but you would then end up with bearings the proper shape and size. Just remember to punch a hole in the upper shim for the oil hole. In the old days this was common and was also considered proper. Larry