When the rods and pistons were installed, the pistons easily moved on the pins. After just a couple of hours of run time, all eight of them are tight. They can be moved, but they are tight. What are some possible causes?

Detonation bent the pins, distorted the pin boss or lack of oiling caused them to gall

Perhaps they were not fitted correctly in the first place. They need to be measured just like rod big ends and crank throws. .0012-.0014 clearence for an endurance motor from what I've heard.

or there was not enough clearance to start with. You need at least .009

WTF!

maybe missing a 0?

Sorry, MY BAD add a 0 in there.

Why are you looking at them after a couple hours of run time?

Sounds like a possible oil problem. Were the rods installed in the right direction?
Piston pins are usually held within a total tolerance of 0.00015 so sounds like if all the pins are tight some scuffing has occured. Pull the pins and take a look to see if they got hot or have scuffing marks on the pins or piston bore.

The engine developed a knock rather quickly. We were moving along at about 3300 rpms. Trailered it and then pulled the motor to inspect. Spun a rod bearing on number 3. It was pretty bad and there was LOTS of metal in the oil and some pretty major chunks of metal in the pan. The engine guy totally tore it down today and it was noticed that the wrist pins were tight. Was it from the metal tainted oil, or did the pins tighten up due to tight tolerances and then take out the rod bearing. Hard to say what happened first. New crank, rod bearings, main bearings, oil pump will be here in the morning. It got a hot bath today and cleaned up very well. There was no block damage.

The engine guy could see no signs of any heat buildup at all in any of the parts, and he said that some of the areas that would be a telltale sign of an oil problem looked good. I am betting on the pin tolerances being too tight for a marine engine, but of course it is too late to be able to check them now. The cam and the cam bearings looked good, but we will be replacing the cam, bearings also. Of course we are also flushing the oil cooler and the remote oil filter lines.