Knock after rebuild

Looking for a little advice, just finished a engine out rebuild on the 134l. Immediately after start up I hear a little knock low on the passenger side on the #3/ maybe #4 cylinder.  Don’t hear it on the front two. Used a stethoscope, knock increases with Rpm but can’t hear it by ear at higher rpm. Rebuild included new crank and bearings.  All bearings plastic gaged and were in spec. One of the cylinders has about 2 thousands more side play than it should had but it was not number three.  New oil pump with the rebuild and I have 40 psi at start up and didn’t seam to vary to much with different rpms. 10 miles into test drive pressure would go up to 45 when I would let off the gas.  Oil pressure really never went below 35 in any situation.  Engine ran great other than this issue. Opinions on what the next step should be?

Try pulling the dip stick and running it.

The odd sized piston should be a clue. Unless you gauged  them all? They sometimes get stamped as the wrong size.

I still have standard size pistons with new rings 500 miles ago. Everything check out at that time.  I had to do the crank because I did a valve job and re-ring 500 miles ago and screwed up and use standard rod bearings when I shouldn’t have and chewed up the rod journals on the crank. So when I just did the rebuild I reused the pistons and rings as everything still checked out. I did have to change a piston rod as the one that chewed up the bearing looked like it had been pretty hot and was hard to get the cap on and off without force.  I replaced it with a like rod. My motor only has 20,000 original miles. 

If my aged brain still works, the rods are the same but you turn them 180 degrees to change the offset.

You seem to be heading down the right road.

I did not, probably should have. It seemed to feel like the other assembled pistons. Does the wrist pin turn in the rod? I assumed the piston turns on the pin but doesn’t on the rod because of the bolt. 

I would be interested in hearing what you find. I have a very similar knock in my M38. It developed about 9 months after I got it running. I have been using it as my daily chore jeep for a little over a year since the knock started and nothing has changed. I have pulled the pan and checked the rod bearings, pulled the head and checked the top side, ran the valves twice with out finding anything. Good oil pressure, 40 at idle and around 45 running, normal vacuum and compression. I feel it’s a lose wrist pin but really felt it would worsen over time. I drive it around 10 miles a day 6 to 7 days a week and have for over a year with no change. Funny how it’s easy to hear at idle but once you get your rpms up you don’t hear it. Don’t always hear it on a warm startup but most of the time you do.