bearing clearance question

[boler76]

the book says running clearance with standard mains is 1.5-3 thou, plastigage indicates clearance is 5 thou, will 2 thou oversized mains bring running clearance down to what is in ball park or is the 5 thou acceptable? thanks

 

[wore out]

(quoted from post at 09:36:46 10/16/21) the book says running clearance with standard mains is 1.5-3 thou, plastigage indicates clearance is 5 thou, will 2 thou oversized mains bring running clearance down to what is in ball park or is the 5 thou acceptable? thanks

First of all, you are looking for UNDERsized bearings.

Your plan MAY work if the journals are not too "egg-shaped" and whether you took your plastigage measurement at the least worn or most worn area of the journal or somewhere in between.

 

[used red MN]

Might be nice to know what tractor or engine you are asking about. A Farmall M would be far more forgiving of an off tolerance than a DT414 in a 1066. Second question is did you have the crank out and check the roundness? Say this is a Farmall M and you have not had the crank out. In my opinion if you plastigauge it in 3 positions split by a 1/3 of a revolution each. And while doing it you have card board snugged under the bearing halves next to it and it gives you three 0.005 readings I will say you could go ahead and get 0.002 ..undersized.. bearings and be okay. But after installing them I would again repeat the 3 position plastigauge steps to check final clearance. Probably any IH diesel I would not be comfortable with this short cut. If a crank had that much wear on a diesel it would come out to be check to be in tolerance of all specs. The check of roundness of the journals being the most important.

 

[caterpillar guy]

First of all which size plastigage are you using? Then are you tightening the main cap up to torque spec? A main should not get out of round like a rod will. If the correct size plastigage is giving you that reading then you need to consider the use and loads you will be putting on it. Or pull it and mic it for accurate measurements.

 

[billfromwisconsin]

Unless you are planning to use it as an everyday workhorse(tillage, heavy work) you will be fine.

 

[redforlife]

Well, if I was going to rake hay and perhaps drive through a parade with it, I don't think I would be to concerned. But if I was going to plow all day, and load it up to take to the weekend tractor pulls, I don't know that I'd be happy with 5 thousands.
Basically, wear equals oil pressure loss. Hard use, and low oil pressure is not good combination. Was oil pressure an issue before tear down?? I'd hate to do an overhaul because of low oil pressure, and have low oil pressure after the overhaul. But if oil pressure was good, and burnt valves, or low piston compression, etc., was the reasoning for overhaul, then hey, oil pressure won't be any less than when you took it apart.

 

[boler76]

thanks for the input, it is a super c that had water in 2 cylinders from open exhaust and therefore was dismantled and I got the tractor with engine in parts. it will be a trailer queen when I am done

 

[boler76]

I must be dense as I thought if the crank was smaller than original it would be undersized and then
the required bearings would be oversized to take up the difference???

 

[used red MN]

You most likely are not dense, just not familiar with the terms. The bearings nomenclature in engines as far as undersize..oversize has always followed the change in shaft or journal size. At least in American terminology I have always found it to be true. An ..oversize.. bearing in engine terms would have a larger than stock outside diameter of the bearing shells. This would be to accommodate a bearing saddle bore that has been machined or honed larger due to damage like the bearing spinning or moving. These types of bearings are only for special engines like very large stationary generators as an example. In normal automotive or tractor application if the main bearing spun the the block would be ..line bored.. which is accomplished by facing of the surface of the main caps that mate to the block. This makes all the main bearing bores slightly egg-shaped. The boring process returns the bore IDs to original spec. This does however move the crankshaft up in the block slightly in terms of it position to say the cylinder head deck. Anyhow, back to your post subject. You have to plastigauge every bearing, because the underside bearings come as a full set, but can be ordered individually but usually at a premium price. Secondly, if you are using the original bearings and these are the readings you are getting you need to measure the bearings to see how much they are worn in the thickness of the shells. Saving all other discussion, if the crankshaft is out of the engine take it to a machine shop and have them measure it and tell you what you need. This is the right way to do it. Likely they will not charge you anything if you have them get the bearings for you and do the valve work needed on your cylinder head. Another two cents worth.

 

[560Dennis]

Take more measurements at different locations on crank shaft. I think you need to know if the crankshaft is round . A two thousand bearing could interfere if crank is not round . And you would being wasting time and money ,having to rework it. Like others suggested ,take to machine shop if you dont have micrometers to do .measurements.

Suggestion take head in also have it magnaflux for cracks. Might have more issues .
Hate have you do all this work and end up with many redoing that cost big money .