big hole in 1 piston??

Best first guess, pre-ignition

There are other possibilities, but detonation causes most piston failure.

What was your initial timing and what is the total timing?
You say you had super unleaded but what was the octane rating?
Your carburetion could be causing a severe lean-misfire. Do you have access to a gas analyzer? Does the engine spit back on hard acceleration?

There are a lot of possibilities, but I would focus on timing. Both ignition and cam timing should be double checked.

Sorry about your predicament. Let us know what you find.

good luck

 

I agree with the above conclusion that pre-ignition detonation is likely the problem and suggest that you check the type and gap of spark plug that you were using.

You may find destructive detonation within the cylinder without the total "meltdown" of the modern sparkplug. You might see some discoloration of the ceramic and still have the electrodes intact. (The operating temperature of the plug should not exceed 850 degrees C., if it has exceeded this temperature there should be some discoloration and/or blistering of the ceramic.)

If a modern platinum or iridium plug of a heat range greater than that originally recommended was used (out of the box) without adjusting the gap to the recommended range for you particular engine, there is a possibility that a problem with timing (cam or ignition) could have been amplified by the plug causing the destruction of the piston.

You did not mention that your particular car might be a 208 GT/4 Turbo, so one assumes that your vehicle is the standard 208 GT/4.

Best of luck in locating the cause of your problem!

Best wishes

 

HJ, look inside the cylinder again. Try to look inside while someone turns the engine slowly by hand. You will be looking to see if the intake and exhaust valves open and close normal. I have seen the head of exhaust valves break off and fall into the cylinder . The piece of valve breaks the top of the piston and then is forced back through the exhaust valve seat.

 

I once had a hole in a piston on a motorbike (thankfully!).

It was due to overheating because it was running too lean.

Agree with the suggestions so far, but would also be worth checking there isn't an air leak causing #1 cyl to run lean?

Dan

 

I stumbled across the following site that shows several pistons which were destroyed by pre-ignition and detonation which may be useful in comparing with your piston for reference toward a solution to your present situation

http://www.hastingsmfg.com/Service%20Tips/detonation_and_preignition.htm


Good luck with your rebuild...

Best wishes

 

Just a thought - you mentioned that the spark plug was destroyed because something hit it. I wonder if maybe you lost a chunk of ceramic off the plug and that might have caused the whole shamoozal. You said all the other plugs looked good. I understand that detonation may not be noticable on frequent plug changes, but to cause this kind of damage you would think there would be other evidence??? What do the other cylinders look like? That is why I am thinking maybe plug fragments. A buddy of mine lost a piston to a chunk of ceramic on his Lotus. It got plenty hot in there before he shut it down - pretty little melted hole. All the other pistons were fine.

Good luck! Let us know what the medical examiner finds!

Jim

 

Wrong spark plugs can cause sparks from plug to piston.
Result is a hole in the piston.
I would consider an upgrade to 3.0 liter in the old block.

good luck

cinquevalvole