The Stovebolt Site Forums The Shop Area Engine Shop Clanky noise on gmc '56 engine

My GMC'56 truck engine developed a clanky noise after 250 miles ride. Sound started at the beginning of thet trip but It was not any loud, more like a misadjusted lifter sound.
About mile 250 the engine started making oily smoke and ended with a very loud clanking noice.
I do not know where to start, have no idea what happened and what can I do. Help will be very much appreciated

You might want to take off the rocker cover and look for a broken valve spring. That might explain the loose rocker sound, it might have put a valve through a piston. I hope what ever happened to it is not too serious. Jeff

Do you have a foot starter....without a return spring?

Or maybe loose regular starter

I once lost a chunk of the inside of the fan belt
sounder like a busted piston....vibrated TOO!!

Another thing to check is that your damper pulley is is still attached to the crank & hasn't started to walk off. It's a press fit on smaller GMC's and they get loose over time.

That wouldn't explain "oily smoke" though. Did the motor expire, or did you shut it off? Have you tried re-starting it?

Jeff, I will take off the rocker cover and see inside. If any damage has been made, could it be to the actual piston or to the valve?.It will take me about two weeks to start the job.
J.H. No, My starter is a newer model (Automatic they say) I have checked it and is still on its place. Once in a while does not operate (That's another issue).
Sixpack, I did stop the engine. I have started it again and still works, other than the noise that makes everybody to look at me, the engine starts very quick and behaves smooth.

Check all the rocker arms too,I had one brake in a 50 that I had years ago. It had a miss when it did it and check all pushrods.

Which engine do you have in this truck?

If it's a 228 or 248 GMC you can touch valves on the piston top.

How many miles on the engine?

Jeff
I will check it and let you know.
J.H. It is a 248 GMC. Have no idea on the mileage. Odometer stopped on mile 30,000, but who knows!. You mean that valves may touch the piston in a situation of a troubled rocker arm?

Yes 228 & 248 GMC motors have pistons that 'popup'
and come closer to the valves than 270-302

If you broke the cam thrust washer and the gear walks it will eventually machine the teeth off itself and valves can hit pistons.

In the GMC army manual they give a procedure for checking the cam timming in place but if your valves have touched you will not have any compression in the clys affected.