I ordered a rebuild kit assuming my 2.5 had never been rebuilt, now I am not entirely sure. I pulled one of the main bearings out and it has an A stamped on it, as does the inner part of the main bearing cap. The A is stamped in a circle and not directly on the tang of the bearing, and has GM 4 stamped right after it. The manual says an A marking would be .0005 undersize, but I am not sure if that is what this mark is meant to indicate. With a pair of calipers the new bearings measure about a thousandth thicker than the old ones. Here are some pics, let me know what you think.
If the crank was cut, meaning the engine was rebuilt, the bearings would be OVERSIZE, not undersize. It was often done on the assembly line, mixing and matching bearings. The same was done for pistons, they were sorted by diameter and selected to match the bores to correct any machining variances. You really need a more accurate tool than a dial caliper to measure bearing diameter and journal diameters, even a good caliper does not have the resolution to measure under 0.001". You need to install the bearing and torque the main cap and measure the ID with a bore gauge or inside micrometer and measure the journal OD with a micrometer. Both mics should be capable of measuring to 0.0001" for best results. You subtract the OD from the ID to get the clearance.
Undersize is the correct term. It refers to the crank journal size, not the bearing itself.
When rebuilding any engine, it's best to wait until it's apart and fully inspected before you order a kit. You never know what you'll find in there. You'll probably end up having the crank ground to the next undersize, anyway.
The motor is still in the car and I'm not going to pull the crank. I wasn't even planning on replacing the mains, but the kit could only be bought with them so I just ordered them in standard size. According to the manual there is a .0005 undersize option. Can anyone confirm whether or not the A in a circle marking means this is a non-standard size bearing?
I can't provide you with a definitive answer, however I wanted to point out that the "A" on the bearing cap appears to have simply transferred from the bearing to the cap just as the part number did. You can tell the part number on the cap is just a stain because the numbers are mirror images of the ones on the bearing shell so the "A" is likely the same.
Out of curiosity, where did you find the information regarding the letter A representing an undersized bearing? I looked in the 22P and see there are three kits for 0.001" undersized bearings but there is no mention of any special markings other than different part numbers... none of which match the one on the bearing in your picture.
That's a good catch on the reverse lettering, I hadn't noticed. I've got an original service manual. You can see the part about different size bearings in the upper right corner.
[This message has been edited by smmilke (edited 11-07-2013).]
Hmmmm... I see... if I were a betting man, I would say that bearing is indeed 0.0005 undersized, especially given your direct measurement as a back up. I can understand why you would want to know for sure, but just bear in mind that the specifications for building the engine according to the '86 FSM indicate that the allowable tolerances for bearing to journal clearance is 0.0005" to 0.0022" on the mains. When (if) you get to the stage where you decide to replace the main bearings, the size of the original ones will be meaningless though since the journals will likely have to be reground to a smaller, known size anyways. And if not, then when you reassemble it with new bearings the plastigauge will tell you if you're outside the limits. But I suspect you know that already.
Remember that Many engine were replace under warranty/recall work. Many of those were rebuilt by GM or GM's contractors.
Making sure you order the right bearings is very good.
87 and up should have part of VIN on engine and trans. No match VIN then engine was replace. Not sure if 86 and older has VIN on block etc.
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