Ford Lehman engine mystery

[Tim1945]

Good day all
This is a post from Tim moored in Greece

Would appreciate your thoughts on a mystery I have on a 6cylinder Ford Lehman engine I replaced 10 years ago.
I live in South Africa and spend the summers in Greece.With the recent Covit nonsense the last time 3 years ago that I tried to start the engine it jammed and I could only turn it by hand with a large shifting spanner between 10 am and 2 pm backwards and forwards with a solid sounding clunk at each end of this short stroke.
Well, this year I have traveled to Greece with a pal of mine to do whatever it takes to solve this problem.
Took everything off the engine including the ,gearbox,flywheel, starter ,alternator,and injectors.
Each time I took something off I tried turning the engine by hand on the front crankshaft nut to see if it would turn completely and each time the same 10 am to 2 pm solid sounding clunk.
Then we took off the head and voila !! bring out the champagne !!! the engine now turns freely.
Now the BIG mystery:
There appears there is nothing wrong with the block and head
Took all the valves out one by one to see if one of them was sticking and ALL move freely up and down.
I am loathe to reassemble the engine as I cannot for the life of me understand why this jamming happened in the first place.
There was nothing we could see that could have caused this while we took everything off.
The only thing still connected to the crank are the timing gears,diesel injector pump and camshaft.
We are going to measure the total movement of each piston down and ensure they all come up exactly to the top of the block.
We are going to replace the 2 no 6 cylinder push rods and physically try to hold them back so as to put them under load in case the cam turns easily under no load but under load might indicate it is sheared in two pieces
Have been in contact with Brian from Lehman and he has been most helpful but even he can’t seem to have a solution.said whatever jammed the engine must have now come loose.But what?This all has taken us 2 weeks in difficult conditions as the original owner should have used the 4 cylinder engine and he shoe -horned the 6 in with no clearance around to work with
The head is out on the deck with all the other stuff and the block is still in position.
To drop the sump is a major job as the cockpit floor has to be cut out and then the engine lifted.
All this has been done with the boat in the water as I would have to be towed to a marina 30 klms away.
Any suggestions would be most welcome
This is a Transpacific 49 ketch
The engine has done very little work and always well looked after.

Regards
Tim

[Emmalina]

Once had a pushrod fall of as the valve was sticking. Made a hell of a noise but ran. If you stopped the engine in the wrong place it wouldn't start again till I took the rocker cover off and saw the problem. Some wd40 on the valve guide to free it off and its been great since. Think it was due to an oil change as my supply of usual oil dried up and had to use some modern synthetic stuff.

[Lepke]

A timing gear lost tooth can cause your engine from rotating if the broken tooth is riding on the gear.

[Tim1945]

Thank you Emmalina

[Nicholson58]

Check all pushrods to make sure they are straight, in place, etc.

Fully operate all valves to see that they can make stroke.

Check the timing gears. Can you get a remote LED camera probe?

Since you have made the crank rotate fully it’s probably not in the bottom end.

Good opportunity now to have the head and injectors serviced.

[skipperpete]

When you lifted off the head, how much carbon and corrosion was there on the piston crown and the valve heads?
A couple of months ago there was a thread on the forum about a similar lockup and it was finally discovered to be caused by a layer of carbon on the top of a piston crown ( this actually did happen) so possibly you have the same thing going on considering how close the Ford piston comes to the valves ( the piston has a cutout to allow for the valves). .a 030” buildup would probably cause interference and likely a lockup. This would also explain the sudden ability to rotate the engine manually when the head was lifted off and the 120° rotation limit you mention coincides nicely with one piston up one down then one down and the adjacent one up.
Can you post a pic of the piston crowns and valve heads?
Here’s a photo of Ford piston. There’s also a chance that corrosion or carbon buildup in the port above the valve is holding it open ( or the carbon fell out of the port when the[ATTACH]265413 valve opened
Pete