Getting ready to rebuild my 4.0l due to broken piston skit on #6 piston. Looked at the rod bearings they're stamped std, so I know to order standard. The main crank bearings are stamped .001. This has me confused as when I look up main bearings there is no such size listed, only std, .010 .020 .030 over sized and .010 and .020 undersized. Am I right in thinking I need to order standard size. I'm not the first owner of this Jeep, but it doesn't appear that the engine has been rebuilt before.
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4.0L main bearing size
- Thread starter oldbill
- Start date
old_man
NAXJA Forum User
- Location
- Loveland, Colorado
Before you order anything, you should do a good visual inspection and mike the crank to see if you need to turn it or not. Better yet, plastigage all the journals to find out where things stand. The one thousandth bearings were available some years ago and some manufacturers would use them if a crank wasn't perfect when they built the engine. If you have .001's, I would not even think of going with a standard set. You would probably be better to put the original bearings back in if they aren't totally hosed.
Digger87xj
NAXJA Forum User
- Location
- Buffalo Creek/Lakewood CO
OEM bearings were select fit. you wont find them in the aftermarket. same for the OEM pistons
8Mud
NAXJA Forum User
- Location
- Central Germany
Like Old Man said, plasti gauge the bearing fit. Look up the tolerances in the book. They likely used .001 over to get it into tolerance. Your wear has likely opened the clearance, maybe a lot, downsizing to stndard may put it way out of tolerance.
Same with the connecting rod bearings, plasti gauge (top and bottom) they tend to wear more on the top than the bottom. You can mix one standard and one .010 over to reach the tolerance. But it is likely better to mix smaller increments than 0.01. You may have to phone around to find bearings smaller than 0.01 oversize, I repeat, too tight is worse than too loose. Too tight can destroy your crankshaft, too loose is unlikely to cause any lasting issues.
Mains are a lot harder, they rarely tend to wear evenly across the face and if you put in flat new bearings, on a cone shaped crank, they get hot spots while they are trying to wear in. Sometimes you get away with it, sometimes you don't. When you don't get away with putting new main bearings in an old crank it is usually catastrophic, though it may take awhile.
Whatever you do, don't ignore the minimum clearance. Too tight is often worse than a couple of thousandths loose.
The only real problems with plasti gauge is the metal has to be completely oil free, oil melts plasti gauge. The other problem is it is time intensive and takes awhile fitting and testing your new bearings. But the extra time spent in setup is usually saved in the tear down, when things go wrong.
A little loose may hurt your oil pressure a little, but is likely to last a lot longer. Get some assembly lube for final assembly, the first few minutes after start up are critical and a name brand assembly lube may save you some problems.
If you decide to do the mains anyway and hope for luck, Turn it over by hand before you start, with and without the connecting rods connected. Get a feel for things. If you can't tun it over after installing the new bearings, your crank may be warped and you may never get it right.
After new bearings, main or connecting rod, you should break in the motor just like a new one. Keep the rpm's down for the first 500 miles.
Most times if my mains are way out of spec. I send the crank off to have it cut and new bearings fitted. Hopefully the shop will pick it up if the crank is warped.
Last tip. make sure things go back together like they came out (direction and cylinder). I may be the only one on the planet to get two connecting rod caps, both marked "3" but I doubt it.
Your cousin Jose' may say just slap some new ones in there and they will wear in. They may or may not overheat and destroy themselves. I learned the hard way, I actually welded a crank to a block once, the bearings can get way hot really quick if you screw it up.
Same with the connecting rod bearings, plasti gauge (top and bottom) they tend to wear more on the top than the bottom. You can mix one standard and one .010 over to reach the tolerance. But it is likely better to mix smaller increments than 0.01. You may have to phone around to find bearings smaller than 0.01 oversize, I repeat, too tight is worse than too loose. Too tight can destroy your crankshaft, too loose is unlikely to cause any lasting issues.
Mains are a lot harder, they rarely tend to wear evenly across the face and if you put in flat new bearings, on a cone shaped crank, they get hot spots while they are trying to wear in. Sometimes you get away with it, sometimes you don't. When you don't get away with putting new main bearings in an old crank it is usually catastrophic, though it may take awhile.
Whatever you do, don't ignore the minimum clearance. Too tight is often worse than a couple of thousandths loose.
The only real problems with plasti gauge is the metal has to be completely oil free, oil melts plasti gauge. The other problem is it is time intensive and takes awhile fitting and testing your new bearings. But the extra time spent in setup is usually saved in the tear down, when things go wrong.
A little loose may hurt your oil pressure a little, but is likely to last a lot longer. Get some assembly lube for final assembly, the first few minutes after start up are critical and a name brand assembly lube may save you some problems.
If you decide to do the mains anyway and hope for luck, Turn it over by hand before you start, with and without the connecting rods connected. Get a feel for things. If you can't tun it over after installing the new bearings, your crank may be warped and you may never get it right.
After new bearings, main or connecting rod, you should break in the motor just like a new one. Keep the rpm's down for the first 500 miles.
Most times if my mains are way out of spec. I send the crank off to have it cut and new bearings fitted. Hopefully the shop will pick it up if the crank is warped.
Last tip. make sure things go back together like they came out (direction and cylinder). I may be the only one on the planet to get two connecting rod caps, both marked "3" but I doubt it.
Your cousin Jose' may say just slap some new ones in there and they will wear in. They may or may not overheat and destroy themselves. I learned the hard way, I actually welded a crank to a block once, the bearings can get way hot really quick if you screw it up.
