TR4/4A "Engine seized after engaging overdrive at low speed.

[malbaby]

Try another mechanic first.

 

[malbaby]

Try another mechanic for correct advice.

 

[bobhustead]

I would second, only louder, the skeptical comments about the mech you are consulting. Don't let him touch your car.

Bob

 

[Sarastro]

For the sake of argument, could a piston ring or rings broken on one side cause a piston to go off center and jam in the cylinder? I have no idea.

Yes and no. I suppose it's possible, but in fact, cars often run for a long time with broken rings, and they aren't discovered until someone tears down the engine. There just isn't enough space between the piston and cylinder for a piece of the ring to get trapped in between. When I tore down my engine, I found broken rings on two pistons, and the compression was still decent. A similar thing happened to a friend when his Mercedes engine was disassembled for another problem. I suspect it's more likely that a piston-pin clip ring might come loose (maybe not installed right) and then it could get scrunched up and gouge the cylinder wall. But that would not be something you could hone out.

 

[DavidApp]

Where is he going to get the new crank?

David

 

[Andrew Mace]

For whatever little it's worth, I once started overhauling an early Spitfire 1147cc, due to some smoke/compression issues. On at least one cylinder, after unbolting the rod bearing cap and pushing the assembly up through the top of the block, about seven pieces of compression ring neatly presented themselves around the circumference of the bore! (Not even a problem of forcing the piston past any ridge at the top of the cylinder, as I'd already taken care of that issue.)

 

[TomMull]

I also would suggest a different mechanic.
Here is another shot in the dark:
Check for a slipped or broken timing chain. That would account for your symptoms and those often break or slip with a sudden RPM change (as from a overdrive switch).
Tom