Ok, let me start off by saying I'm not an engine guy. I know that, and when you get through reading this, you will know it too. I just wanted everyone to know that I already know it, I just like to remind myself of that once in awhile . So here goes:
My '71 F250 4x4 will be getting a 460 next year (and no, I will not be building in myself ). In the mean time, I wanted a reliable motor to get around in and so I took the 360 from another truck I am parting out. It has always been a good motor, but the rear main seal leaked. So during the transplant I decided to change the rear main seal. In the process of doing that I marred an already worn main bearing cap getting the cap off. So, I pirated a cap from a 390 I have. Just the one cap (see, I told you I wasn't an engine guy). I got it all back together and now I have a strange noise that sounds like rocker arms clicking. The noise is on the deceleration side. I took off the valve covers to check the torque specs and they are all ok. It's getting oil to the upper half, but I am worried. I need to take the truck camping at the end of the month and don't want it to die on me in the woods. I need to get a year out of this motor, will it be ok? It's not overheating or anything. I don't want to dump a lot of $ into this as it is a temporary solution, but I do need to drive it (recreationally only). Am I screwed?

 

Whoa. You're not supposed to transplant main caps from one motor to another. They wont be lined up right, and you'll screw up your crank, or if you're lucky wear down the bearings quick. What part of the cap got screwed up when you removed it? If it's not a part that touches anything, like a bearing, you're probably ok.

Here's what I would do. Go get yourself a new .010 undersize bearing set, and have your crank turned .010" undersize. Use your 360 cap, having it welded and reconditioned if necessary. Then install the new .010" main and rod bearings. I mean, you're in there, why not? It'll make the engine last longer, as well as fixing your leak.

Swapping main or rod caps from motor to motor is NOT the thing to do.

 

The wear surface was marred and also galled from wear down to the "copper." So if I remove the crank and measure it to spec, can't I just gets caps to match? That's assuming the crank is ok and at 0.10, or .020 over.
By the way, when I said bearing cap, I meant the bearing "shells" themselves, not the cap.

 

Just replace the bearings with the same size. It will last you more than a year with any luck. Make sure to seat the thrust bearing before buttoning everything up. The old bearings should say .010 or whatever undersize they are. A bearing set should run you $50. Make sure to replace the bearing on both sides of the crank and coat them with assembly lube before installing them.

You could keep running it but who knows what would happen. Likely it would knock its way into next year but it could leave you stranded. While you are in there what is you oil pressure? High mileage FEs suffer from worn pumps a lot. For $35 you can replace this as well and get some better pressure.

Edit: One thing to think of though now that I re-read your post again... Bearings and rocker arms make two totally different noises. A bearing will sound much deeper and hollow like. Rocker arms sound like ticking. If you have the engine back out of the truck then fixd the bearings. If its still in there then I would keep it going a little longer and put some marvel mystery oil in to help clean it up.

 

Oh yeah, if you just put an old used bearing in you're probably ok. Get a new set of bearings and go. Like rat said, dont forget to seat the thrust bearing.

I was under the impression you switched a cap; good thing we got that cleared up!

 

Yeah, the sound is more like a clicking (high pitch) and it's on the decel side when I let off the gas. Oil pressure seems good, but that's based on the crappy OEM gauge. Maybe I'll throw a cheap aftermarket gauge in and see what it reads and keep an eye on it. The motor is back in, so I'll try Marvel's and see what that gets me. By the way, I hadn't run this motor in maybe 6 months and I just remembered it started to click a little last time I shut it off, but this is much worse and on the decel side. It sounds different too, maybe a little higher pitch. It almost goes away at idle and on acceleration.