Strange engine problem (not Toyota)

[danf]

So a friend has an old Ford truck ('78 I think) and went to do a swap from the M series engine to a Windsor engine that's going to be on propane.

He bought an engine that had been recently rebuilt. It sat for quite a while and finally got around to installing it a little while ago.

The truck is up and running on propane but has a weird problem (note, I haven't seen the truck yet, as far as I know it's at the propane conversion shop.)

When cold, it cranks and starts fine. It will run until you shut it off. You can bomb around town, etc no problems.

However, when the truck is warm and you shut the engine off, it will not crank (as in: you can put a socket on the crank pulley and it refuses to budge.) If you leave it overnight it fires right up the next morning. 

Can anyone think of anything that could cause this? I suspect whoever rebuilt the engine made a mistake somewhere. Given it's a conversion, I can't think of anything that can cause this. Starter shims? It's pretty obvious that something is expanding causing the problem.

I told him my best guess would be the bearing were not aligned with the oil supply holes. But even I'm not so sure about that. 

[ozzie]

Wow, that is a weird one. How's the coolant temp? It sounds like either the rings or the pistons are expanding and seizing in the cylinders. It would be really hard to have the main or rod bearings not aligned with the oil holes, they should be notched so they'll only go in one way. It is possible they screwed up the cam bearings and the cam is seizing. In any case, I think the only solution is a teardown and check of all clearances.

[danf]

I forgot to mention they did check the temperature and confirmed it with an infrared temp gun, no overheating. They even pulled the spark plugs when hot, no difference.

My suggestion about the bearings were a longshot.

However, I was just thinking if they didn't plastigauge the bearing properly they could bind up but you'd think that after running for a while like that it'd stall on its own. I doubt a piston ring end gap could cause that kind of behaviour either... but it really has to be something attached to the crankshaft (you'd think, anyway...) 

[ozzie]

I dunno, I've seen lawn mower engines that overheated, the rings expanded and seized, and it broke the top off the piston - while the engine was running. Or, if the engine was bored without enough piston to wall clearance, it would also be a problem. It should be pretty obvious in a teardown what the cause was, some part is going to show excessive wear, hopefully it's something cheap.

[danf]

Well he has the truck back now.

I don't know what he's decided to do with it. He plans to put a permit on it to see how it drives. Mechanic warned about the engine and a possible shake. So he's going to drive it a bit on the road to see what the mechanic is talking about and go from there. Maybe the engine is not balanced correctly after the swap?

[ozzie]

Possible. IIRC SBF had external balancers that were different weights, you have to match the correct flywheel to the crankshaft.