C-123 Engine Rebuild. Too Tight?

Kevinez

Member
Hello. Working on assembling my C-123 after it's come back from the machine shop. First time for me to attempt a rebuild. Mains and rods are 10 under and clearance is .0015 to .0020 measured with Plastiguage on the mains and rods. New Sleeves, pistons, and rings. With the mains it seems smooth enough but when I have all the pistons installed I'm unable to rotate the crankshaft by hand. Should I be able to do this or is this normal.

Think it's that the new rings are very tight. Someone boogered up the threads on the front of the crank in the past so I haven't been able to get a bolt installed and try it that way yet?

Thanks in advance!
 
When my W-4 engine was rebuilt with new pistons and sleeeves and re-ground crankshaft I could only turn the engine 180 degrees bu crank (half turn) and the starter motor gave up at 3/4 turn. Tried towing, first attempt broke the tow rope. But after first start by towing no trouble with crank or starter (as long as there was fuel in the tank, learnt the hard way). No problems any longer.
 
A freshly rebuilt engine can require a decent amount of force to rotate at first.
The cylinder walls are honed to allow the rings to seat and this provides a good bit of resistance until the rings seat and the cylinder walls glaze a little.

I was just asking about ring end gap because you didn't mention that you checked or set them and a lot of people seem to miss this step when rebuilding an engine.
I just rebuilt a cub engine over the weekend, that was freshly bored to .040" over, and I was installing new pistons and rings. I had to file all the rings to get the proper clearance. If I would have just installed the rings without doing this, they would have been butted together at the end of the rings and caused some major problems.
 
Definitely forgot to check the ring end gap. I've already got the pistons back out so no big deal to do this. Gives me an excuse to spend the afternoon in the garage!
 
what do you mean by rotating by hand ? with bare hands or using a wrench? you definitly wont rotate a rebuilt engine to spec. with bare hands.
 

Umm... Perhaps you experience a rod cap on backwards yourself? Don't think that's the issue with mine. All of the numbers were on the same side. It's a good thought, though!
 
With just the crankshaft installed it should turn with just a little drag. As you install each piston assembly it should be a little harder to rotate. As you install each one the force required should increase equally. With a couple bolts in the flywheel end of the crankshaft you should be able to rotate the crankshaft with a pry bar, turning easiest with the pistons at the top and bottom of the cylinders. once the head is installed and the valves are adjusted you will notice additional force required to turn the engine due to opening the valves. If you cannot turn it 360° with a 18" pry bar you have something wrong.
 

We sell tractor parts! We have the parts you need to repair your tractor - the right parts. Our low prices and years of research make us your best choice when you need parts. Shop Online Today.

Back
Top