Engine Building 101

[thekyguy11]

This may or may not be a newbie question, but I don't want to risk deletion or being banned in cylinder head and short block forum.

I'm assembling a shortblock. All of the shortblocks I have assembled in the past, I had the machine shop do the bearing sizing. I decided to do it myself this time.

Im using a brand new OEM 7bolt crankshaft. All OEM bearings. Eagle rods. I got the crank balanced and the journals polished. I figured new crank should call for standard bearings. So I bought all standard OEM bearings (which were expensive!!!). When I measured the main bearing clearences, they were all at about .0015-.0017ish. This was after torquing the main studs (arp) to factory spec. When I checked the rods, they were close to .0025 which is out of spec.

Is it common to have to use undersize bearings on a brand new crank? I just ordered bearing #2 assuming they are .001 tighter because the people at the dealer dont know, all they can tell me is that there is a #1, 2, and 3.

Thanks.

 

[spoold]

Is it common? Yes.
Should you address and fix the tolerance issues? Yes

 

[natedogg814]

I've never heard of that. You learn somethin new everyday. I know when I did my main bearings they seemed to fit nice. It wasnt a brand new crank though.

 

[GVR4592]

Where did you get your crank?

 

[thekyguy11]

The crank was purchased from ebay. The auction clearly stated that it was Brand New Genuine OEM Mitsubishi and the seller had very good feedback (over 600 or so). I've got the number 2 bearings, I'll post how it measures up, compared to the #1's.

 

[groundPork]

It's less likely the bearing or crank... more likely the rod bore. Aftermarket rods tend to hone the bores to a loose standard tolerance. This ensures, they have less of a come back issue with too little clearance, which is almost always worse. I like to always set my bearings at .002-.0025 on the mains, and .002 on the rods, but sometimes it can't be helped, or it's more trouble than it's worth to chase down knats ass hairs... know what I mean? You won't have any problems because of that clearance. run it... you'll pull all of your hair out trying to chase 3-5 tenths. If you just can't stand it, go one smaller on each half shell, and double check your cleaarance.

 

[FORMONTOYA]

The problem with these motors, the 7 bolt, is that you need to look at the casting numbers/dots on the block and crank in order to get the proper bearings. You can't just say I have XX year 4G63, give me crank/rod bearings. Even though I pulled up the mains, look in your manual and you will see the same scenario for the rod bearings as well. You have to be careful in your interpretation of these marks as it's easy to mess up. Even guys that do this for a living screw up every once in awhile.