Long story short I moved house after I got all the parts to build a new motor. And I thought I put everything safely in the new garage..

Forged internals.
Honed block with matching pistons / rings.
Milled crankshaft with matching bearings.

And I just found my main bearing caps in the back of the Ute stuck behind the air compressor, with a few months of rain on them 😭

There's plenty of main bearing caps on eBay etc.
However I feel that's a bit iffy.

So is throwing in random bearing caps all good??
Or
A new block and matching it to everything else is the best way to go??

 

I'm no machinist, but my understanding is that the main caps need to be matched to the block. You *can* replace them, but you'll need to do a ton of machine work to make the replacements fit properly. I expect you're better off buying another block.

--Ian

 

Yeah I figured as much.
Cheers.

 

Isn't there a company selling billet mains now? That would indicate to me that main bearing caps don't necessarily have to be matched to the block. This is way out of my area of expertise though.

Edit:
Just did a quick google. Mazworx apparently makes a set. The say that a "line bore/hone" is required after install. Looks like Ian was right, replaceable, but machine work required.

 

Yeah, but you need to skim the surface, bolt the new caps down and torque to spec, then line bore and line hone them to size.

 

I'd have the caps blasted to see what I have - lightest media that does the job - soda, glass/vapour, sand. A few months is not a long time for deep rot so may look a lot worse than they are. If they come up clean - then check bearing clearances and take it from there.

 

I wondered about something like this when I first read the post. Surface rust not the issue, the caps need to seat the bearing shell, is all. I would be willing to take the risk on that surface if it was largely intact - how 'largely' depends on your appetite for risk/reward considering your investment in that block. Cleam 'em up (carefully!), put the crank in with some bearings and use some plastigauge on both shells then compare results top and bottom. Somebody with more experience with bottom ends than me can then tell you if it is totally knackered, or not.

 

I might even go for a chemical rust removal and see how they look. ****, even a soak in household vinegar, which is a very mild acid, for about a week and the rust will melt away. Avoid anything too harsh that might eat the metal if you go the chemical route.

 

To expand on my post, treat the bearing shell surfaces with kid gloves - that is the critical dimension, you don't want to take off anything more of that than absolutely necessary. I would go so far as to say, as long as there is nothing loose, moveable, compressible there, it's good to go! OK, maybe not, but all you are looking for is a good surface to seat the bearing shell, so do the minimum to achieve that.

 

AIUI, chemical rust removers convert the rust to a different type of iron oxide. That's fine for something like a frame rail where it's the strength you're worried about, but a main cap needs to be dimensionally accurate to within thousandths of an inch. I know rust takes up more volume than normal iron (that's why it flakes off), and I'm pretty dubious that you're going to get something the right size out the other end.

Assuming that's the case, you're still going to need to machine it after you're done, in which case you might as well go with the billet main caps (stronger) or just get another block and machine it for the pistons (probably the cheapest option).

--Ian

 

It's not just the bearing shell surface (the semi-circular one that the crank rides in) that is critical, it's also the flat surface that mates with the block.

--Ian