Well to make a long story short I am rebuilding a 400m that was rebuilt not too long ago (supposedly according to PO). The crank and cam were totaled in it, So I got a new crank, cam and bearings.

Anyways I have ran into a few issues. When I tore the motor down I looked at the pistons and they looked alright, so being stupid I assumed that the pistons would all be the same size, because "Who would ever build a motor and use different size pistons". Apparently the PO of my 79 would. The motor has 7 pistons that are .030 over and 1 piston that is .040 over. My question is what should I do with it? I imagine the crank will be imbalanced? Can I just run it or does it need "fixed"?

Also I got my new bearings and put the crank in. I then plastic-guaged them and found out my bearings have .0019-.0023 of clearance. I would almost think that`s too tight and I might end up spinning a bearing?

Thanks for any info.
Just looking for some advice on what to do.

 

Are your bearings under-sized? As far as pistons, seems like a pretty half assed deal to me, but probably the cheapest way to get the engine going again, i assume that one cylinder was chewed up and they didnt want to spend the coin on boring 7 more cylinders and 7 more slugs. Obviously the right way would be to bore all of em to .040, however, i think it would probably be fine as run around rig. We have a 351m on the ranch with a few shady repairs, but works just fine for running pipelines.

Personally, i would go find a junk yard engine, have it bored to .030 and buy a piston.

 

Not the best way to rebuild but I don't think the +.040" piston will cause you any problems.
As for the bearing clearances, you're okay there too (just barely). You might consider a high volume (not high pressure) oil pump though.
From my Mitchell Manual:
Rods: .0008"-.0015", max allowable .0008"-.0026"
Mains: .0008"-.0015", max allowable .0008"-.0025"

 

Well thanks for the information. At least now I have a little peace of mind about putting this thing back together.

FTE is #1