Color Coded Bearings?

I just received my factory service manual for the I-6 in my 88XJ. The manual indicates that the rod and main bearings were color coded as to what size bearings were used during assembly (std or undersized). For those of you who have replaced these bearings, did you see any color coding on the old bearings when you opened up the engine? I was about to order a set of std sized bearings, but after reading the manual, I'm thinking I better wait and see if I may need a variety of sizes. I just didn't want to have the Jeep up on blocks so long.

Has any one run across color coding during bearing replacement, or have undersized bearing halves on an original engine?

The colour isn't on the bearings themselves but rather the bearing caps. There is a little dab of paint on them.

Now, I've taken apart two 4.0L's one of which used standard bearings and the other used std on top and .001 undersized on the bottom...yes,that is right, it came with non-matching bearing halves from the factory. Since they don't sell non-matching bearings if you have this scenerio you'll have to choose between the two sizes or buy two sets if you want. I know various part stores only sell std, 0.01", .02" and .03" sizes and don't have the .001, .002, .003 that the FSM says can come from the factory. Most people I've talked to say to just use the std even if your engine had the .001" undersized. Apparently a little too big is better than a little too small when it comes to oil clearances.

Thanks Bender, That helps out a lot. I did find one auto parts store that sells .001" undersized rod bearings, but replacement main bearings are as you described. I'll most likely pick up standard sized bearings and may then buy undersized rod bearings as needed to replace the original ones.

Well, I've opened up the engine to replace the rod bearings and, as luck would have it, 4 of the six rods were manufactured with .01" undersized inserts. The counter weights were painted red. That opens up the next question, what is the maximum size insert that can be placed into the connecting rod? I have found one auto parts web site that has .02" undersized rod bearings avaialble, although that is quite a jump over .01" inserts.

desertred said:

Well, I've opened up the engine to replace the rod bearings and, as luck would have it, 4 of the six rods were manufactured with .01" undersized inserts. The counter weights were painted red. That opens up the next question, what is the maximum size insert that can be placed into the connecting rod? I have found one auto parts web site that has .02" undersized rod bearings avaialble, although that is quite a jump over .01" inserts.

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Don't go by color alone. I don't think the factory mixed sets within one engine. There's an engine build code stamped in a boss on the passenger side of the block, next to the distributor. Those codes will tell you what you have for bearings.

Whatever you should have and whatever you use, plastigage everything when you reassemble. Too much clearance is just as bad as not enough.

Thanks Eagle, I do have plastigage to measure the tolerance. Looking at the build code for the engine, it appears to be 803MX2I. The FSM states that this would indicate a 4.0 manufactured in March of 1988, but does not indicate what the X or I may indicate (unless the X is a check digit, like the VIN). Anyone have a clue as to what the X and/or I may indicate?