Main bearings and rings

[Burns]

Anyone know the basic important numbers on a 1990 302's piston rings and main bearings? Sooner or later I have to replace the main bearings, and I figure if I got them off, might as well throw some new rings in there. I just don't know what size my main bearings are, undersize in in and mm and what kind of groove, and for the rings the bore in in, mm, and top/second ring thickness. If anyone's got the numbers or any info on where to get them short of tearing the engine apart now, that would be awesome.

 

[Justshootme84]

If your engine has never been overhauled, it will have standard-size rings, bearings, pistons, etc. When you go to re-build it due to bad rings or bad bearings, you often have to get new parts according to how much machining is needed. A typical example would be to bore the cylinders out 0.030", so you would then need "30-over" pistons and rings. This term is basically thirty thousandths of an inch larger than stock. For main bearings, you ca neither have the stock crank machined 0.010" to remove ny grooves or scoring on the crank journals where the bearings ride. That would be refered to as a " 10 under". You could also replace the crankshaft with a new one with the stock bearing diameter. SO, the answer to your question will be determined by the machine shop and you when you have the block worked on. That would include boring out the cylinders, re-surfacing the deck for the heads and replacing the camshaft bearings. JSM84

 

[Burns]

If your engine has never been overhauled, it will have standard-size rings, bearings, pistons, etc. When you go to re-build it due to bad rings or bad bearings, you often have to get new parts according to how much machining is needed. A typical example would be to bore the cylinders out 0.030", so you would then need "30-over" pistons and rings. This term is basically thirty thousandths of an inch larger than stock. For main bearings, you ca neither have the stock crank machined 0.010" to remove ny grooves or scoring on the crank journals where the bearings ride. That would be refered to as a " 10 under". You could also replace the crankshaft with a new one with the stock bearing diameter. SO, the answer to your question will be determined by the machine shop and you when you have the block worked on. That would include boring out the cylinders, re-surfacing the deck for the heads and replacing the camshaft bearings. JSM84

Man, that was quick. /emoticons/[email protected] 2x" width="20" height="20" />

Thanks. Looks like I got one step ahead of myself.

 

[Justshootme84]

Just looking sump'n up in the Haynes manual, may give you more than you need to know. The 302 cid engine has a 4.00 inch bore with a 3.5 inch stroke. The crank journal main bearing diameter is 2.2482 to 2.2490 inches, while the rod bearing journals are 2.1228 to 2.1236 inches. you can find all of the info in the manual, my point is that the toerances are very close, to thousands of an inch. So if you pull off the old beaings and put new standard sized ones on the old crank, you exceed the tolerance enough so that you don't have good lubrication to the bearings and burn them up right away. you get just enough slack or play that the crank will wobble around, eating them up, too. I once found a thin metal shim under the rear main cap on the motor in a truck I bought. The shim was used to reduce the slop between the cap, bearing and crank journal. When I removed it, the shim had lettering printed on it that was from a Coors can. PLease, don't do stuff like that when you rebuild a motor...Do it the proper way.

 

[Broncobill78]

Whoa there Hoss. Make sure you do this right. .0010" under bearings work just dandy with a crankshaft that's ALSO been turned .0010" under but just slapping a set of oversize bearings or rings into an unmachined block/crank is just asking for trouble. Plastigage that $hit before you do ANYTHING. The oversize rings .0010", .0030", .0060" all work great once the cylinders have been bored (so that they're *actually* clyindrical top to bottom) and you have matching oversize pistons (granted with .0010" over you can maybe get away with it but over that the side pressure bends the rings up & down and you're just *asking* for a broke ring or spun bearing). I would NOT just slap a set of oversize bearings or rings into an unmachined assembly, some guys get away with it for awhile but having tried it twice & wound up picking shrapnel out of my oilpan I would advise against it. If you're going to go thru the trouble to change bearings then *at least* bring the crank in and have it turned. Just my $0.02.

 

[bidibronco]

I agree with BroncoBill. If you're going to be that deep into the engine anyways, you may as well have the crank turned to make sure it's good and also you'll have to have the cylenders honed so the new rings will "seat" properly and never ever ever ever rebuild an engine and put synthetic in it right away. That'll make the rings not seat and it's not good for the crank or the lifters or anything else for that matter.