As the title says. Im kind of at a loss for words on this as I have never had this happen on any car i have worked on, or owned. I just picked up my 71 with an injected 1.7 and did a few normal tune up things. New Plugs, oil, filter, ect... and i was headed up north to pick up the rest of the stuff like a cap, rotor and wires. The car was running great after each of the changes and I even had a chance to get on it a bit the day prior. So about 30 miles in on these spark plugs cruising down the highway and i start to smell oil burning, then 5-10 seconds later a good size "pop" and white smoke. pulled over and let the car cool down. I noticed a lot of smoke coming from around the passenger side valve cover. My plug leads were scorched on the same side as well, to the point of melting the rubber.

Upon returning from the tow I immediately removed the plugs on that side to find my problem. A hole melted in my head, running up along the spark plug. I have been told these heads are infamous for having hair line cracks. this would theoretically lead to a lean condition, resulting in detonation. However, one problem. The plugs color on the previous plugs, and these all look good. No signs of lean mixture.

Also, the seat of the plug, which is steel is melted. This would be fine, but the threads, which are a thinner metal are not melted...
If this did happen from the inside due to detonation, why are the threads (which form the pictures were obviously exposed to the same exhaust gas as the plug seat), or the tip it self in perfect condition?

I have had other theories such as the plug it self was not seated right, but the crush ring is compressed. If there was a leak of that sorts through the plug it self, it would not be running that good to begin with. someone also mentioned a default in the plugs insulation it self, resulting in an arc in that one spot of the head.

has anyone else ever dealt with a situation similar to this? I would like to get some brain storming and answers, as I am now down an engine. Thanks.

Seems like it would take a while to do that damage.

thats what i was thinking. it all looked good when i changed the plugs. 30 miles down the road and boom. and thanks for the welcome. im not a stranger to having a car thats on stands all the time, but this is a first for this reason.

sloppy heli coil repair?

From that view it does look like a repair had taken place. When i look at it up close i see no signs of a repair. it all looks like the stock threads. Ill give it a one over in the morning however.

It'll be interesting to see the combustion chamber when you get that head off.

Could it have been mis timed?

I am far from an expert on this subject but I did have this happen to my 71. This shows the classic signs of combustion gas blow by which melted the head. Mine was caused by poor spark plug thread repair. But I have also seen this with cracks in the head that spread up the spark plug threads.

Either way both heads have to come off and be inspected. I bet you find that there are more cracks or the spark plug holes were repaired at some point.

This is a joke, right? You didn't REALLY put Autolite spark plugs in your engine? Next you're gonna tell us you used a Fram oil filter.

Seriously, the torched out plug hole is a result of a cracked head. The flame makes its way out the easiest path.

The Cap'n

Hmmm, yup. Hot gases and high pressure. Think burned exhaust valves or Challenger solid rocket boosters.

Timing was fine. i had a light on it and everything.

The fact that you had this happen to you as well, while unfortunate, adds a bit more clarity to my situation. thank you. I still have not dropped the engine yet, but i see no signs of thread repair, it just looks pretty trashed from backing the plug out. the only problem I still am unable to answer is why the seat of the plug is melted rather than the threads which were exposed to the blow by gasses for a longer amount of time.

No I didn't use a Fram filter Krusty, but yes, autolite was a bad move I knew I was taking. I can see the name fits you well, haha. I am all for that explanation, but as before, I'm wondering why thinner metal of the threads would not melt (which were obviously exposed to the gasses, and for a longer time) but a thick plug seat of the same metal would. they are both exposed to the same heat and velocity.

Thanks for all the input guys, i appreciate it.

If there's a Helicoil in there, the aluminum would melt first. The most common place to see the effects of "flame cutting" is with a blown head gasket. Left alone, it'll torch both the head and the cast iron cylinder. Frankly, a crack would most likely be burned through before the plug hole IF the crack hadn't reached the threads yet. Odd, but a lot of things are .........

The Cap'n

Sounds good. I'm sending the plug in to get inspected, but I really doubt that was the problem after learning more about these things. I've already located another 1.7 as replacement. I'm wondering if I should just throw a 2.0 in there while i have it all open, but i want to keep injection, and make it all as painless as possible right now.

A good bunch of people on here. Thanks for all the input!

last thing and you can close this if you see fit. Whats a fair price on a running 1.7 long block? im going to do some homework and thread searching, just thought i would ask for an up to date figure.

Put some 96mm P&Cs on a refreshed 1.7 lower for a 1911cc short-stroke motor

Not nearly as much as you might think. Once the hot combustion gas gets a decent path... POOF! Think plasma cutter or oxy-acetelyne cutting.

High compression De-stroked 1.7? sounds like a fun time, and probably pretty rev happy too. I need to make an intro thread with the plans I have engine wise. money to power gains seem minimal in that case.

Mark, that's pretty much what it looked like. Like a plasma cutter was just held to the the head, all I could do was laugh in amazement. I still cant say I'm really even mad about it...

Anyways enough with this thread. On to an intro!...... actually ill probably just do a build thread since ill be dropping the engine tomorrow.