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Confirming main and con bearing size #1338156 Tue Dec 17 2019 11:24 PM
Joined: May 2006
Posts: 739
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tom moore Offline OP
Shop Shark
Just want to make sure that I am reading the stamp correctly on the back side of the main and connecting rod bearings on a GMC 270 I am working on. They are stamped 4M.

These need replacement due to scratching. Outside of measuring the crank...

Would these be .004 oversized bearings, correct? (or 4/1000...)

An EBay set at labeled .040 are really 4/100”.

Need to know if I am understanding how bearings are sized and your confirmation

Thanks.

Last edited by tom moore; Wed Dec 18 2019 06:28 PM.
Re: Confirming main and con bearing size [Re: tom moore] #1338199 Wed Dec 18 2019 03:17 AM
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Posts: 520
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showkey Offline
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No matter how the crank is “marked” or “labeled”............measuring the journals is the only way to confirm the size.

Re: Confirming main and con bearing size [Re: tom moore] #1338232 Wed Dec 18 2019 02:48 PM
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Posts: 2,423
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coilover Offline
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4/100 is 0.040 in thousands which would be very unusual as most grinds and bearings stopped at 0.030. Some emergency uses or sizing to a completely different app (like Hot Rod Lincoln's 216 shell bearing set up) are exceptions but for years 0.030 was the standard max. As the man said: measure.


Evan
Re: Confirming main and con bearing size [Re: tom moore] #1338240 Wed Dec 18 2019 03:53 PM
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tom moore Offline OP
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I am planning to reference this spec page (below) when I measure the crank. To be honest, I don't have a clue what some of the terms mean.

I will definitely need the experts to walk me through the terms and what this means once I post the crank measurements. I will also post pics of the bearings to get opinions about their condition.


http://www.oldgmctrucks.com/photos/EngineSpecs_Pg1G.gif

Re: Confirming main and con bearing size [Re: tom moore] #1338247 Wed Dec 18 2019 04:58 PM
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Joe H Offline
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To measure the crank, any half decent dial caliper or digital caliper will get you close enough. You want to look at the crankshaft section of the page you linked, main bearing journal size and rod bearing journal size, these two will tell you how much if any your crank has been ground. The other diameters listed are bearing inner and outer diameters, these are set by the bearing manufacturer. Most aftermarket bearings are marked with a date and size, example 2/87 STD, February 1987 , standard size. You likely will find a manufactures name also.

Most cranks are only ground to .020" undersize, .030" is pushing it, the hardness applied from manufacturing only goes so deep into the metal and most stop at .020" to stay in the hard surface. Not sure all that is true, but it has been told thousands of times over the years.

Re: Confirming main and con bearing size [Re: tom moore] #1338254 Wed Dec 18 2019 05:40 PM
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Dragsix Offline
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These 235 cranks are pretty rugged, cast steel. Its not forged steel, but just the same very durable. I have taken these cranks as far out as .060 (yes I have bearings in my stash of stuff for .040 and .060). The crank in my 261 is .030 and I am pretty hard on it. I prefer .030 as my outside number for my modified motors, but for a stock rebuild, even a mildly modified motor, I would not hesitate to go .040 or .060.


Mike

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