I'm rebuilding an engine and noticed that there are about three different kinds of main bearings, ranging from a "standard" set to "oversized" sets in various increments (.010, .020, etc.). Why would I need an oversized set, and how could I tell?

 

The crankshaft is sometimes ground down in order to resurface and smooth out the finish on the journals.

The machine shop grinds off as much as they need to in order for the surface to be as true as possible. They take the material off in .010" increments.

Since they grind material off, the bearings have to be just a little bigger to make up for lost space. That way the clearances stay closer to optimum.

.030" is the most you ever want taken of a SBC crank.

 

So, when it refers to oversized, it basically means the thickness of the bearing? If material is ground off the crank, then the bearing diameter actually has to be smaller than stock, correct?

The engine is actually an old Jeep 258 six-cylinder.

 

it's good that you ask, but since you had to do you think maybe you ought to get some outside assistance? it seems like you lack some basic engine building knowledge. it also would seem you never botherd to measure the crank, and most likely anything else, or you'd know and understand the undersize bearings better.

 

No, you have it opposite. Think of the crank journal, where the bearing halves are wrapped around it.

If you "turn" the crank (grind it to make it smaller) then if you reinsert the bearings, they'll be sloppy with too much clearance. Inorder to make up the clearance, the bearing shells need to be thicker.

There is a standard for stock bearings thickness...when you see .010, etc...that is added to the original thickness.

As ede mentioned, best to get a shop/rebuild manual and understand what you're getting into and what needs to be checked and how.

Good luck.

 

I expected someone to bring that to my attention . This will be my first engine rebuild, but I'm not doing it alone. A few of my friends will be helping. I haven't an opportunity to measure any clearances yet because I have yet to buy a micrometer. I do have all of the OEM sizings and clearances for comparison, however. I'm going to buy a book about rebuildng engines, too.

I'm really not worried about screwing up this Jeep motor because I got it for free. When it comes time to rebuild the engine in my Camaro, I'll definitely seek outside assistance.

 

I just learned about this myself - but I'm no expert. In Layman's Terms:

There is stuff called PlasticGuage - you can get it at any auto parts store. Basically, it's a very thin plastic wire looking stuff - you break off a piece, put in the bearing with this piece laying across it so that it gets mashed in there between bearing and journal. Tighten cap bolts to specified torque, and then remove the bearing, and measure with the guage on the package. The plastic gets mashed - the smaller the clearance between the bearing and journal, the more the plastic gets mashed, and thus the wider the mark that remains. This mark tells you oil clearance with that particular bearing size, and that is compared to specs - you need the right sized bearing to provide for the right amount of oil clearance. Too little clearance means not enough lubrication of the bearing. Think of it as a sandwich, you have a journal (bread), bearing (meat), oil clearance (mayo), and bearing cap (bread). Not enough mayo, and you choke (seize) - too much mayo, and the meat slides out (too much play, journal slides around).

Anyone hungry now?

 

Mmm, tasty... good explanation. I meant to buy some PlastiGauge but I couldn't remember what it was called. Thanks for reminding me!

 

Do it the right way with a micrometer and dial bore gauge. I didn't do it in my engine but wish I had. I don't have any problems, but I still wish I would have done it.

Go out and buy the newest edition of Chevy High Performance Magazine. There is an article in there this month that goes into great detail with blueprinting the bearing clearances on an engine. There is a great explanation and a good number of pictures. It would be a great education on the topic for $3.

 

Thats kinda false statement, you have to take .200 off the mains to make a crank for a 383 and they make bearings for upto .060 for sbc rods, but because most old timers think that they are weakened they dont sell many! Most older people think that you go thru the hardness but that just not true!

 

I had my 327 crank at .040 on the Rod bearings...But that is a nice steel crank.. The Machinest told me it wouldn't be a problem.

 

You're right. I'll rephrase that.

You don't want a bearing thicker than .030" Or at least I don't. :nono:

Of course bearing "spacers" are different.

 

Aron213-

i know your a machinist by trade, so i will ask you this statement i have been pondering


somebody once told me not to have a cast SBC crank cut down, because the hardened layer would be cut off and you would wind up with a "soft" crank, he told me instead to spend $200 and get a new crank from summit


the crank from summit is just extra info really, but i would like to know if there is any truth to making a soft crank like that and if so how much would need to be cut off to render a crank useless (soft)

thanx