Finally got a junk cylinder head that I can use for research.

Using an OEM spark plug, I started at 25ft-lbs and increased it by 5ft-lb increments until something gave.

Anybody want to take a stab at how high a torque I reached, or what happened at that number?

Will post results in a day or so.

 

suscribed...

 

It seems like you often hear of people stripping the threads cranking down on spark plugs without reference to the proper torque. It would therefore seem to be a lower number, maybe around 40 or 50. However, Honda makes high quality components so it is very possible the number could be much higher, closer to 100 or over.

The fact you are starting this thread makes me believe that the spark plug failed before the head threads stripped. I bet you got over 100ft- lbs.

 

Eeenteresting.

I'd guess a very high number, given the diameter, big size and pitch of threading. Maybe 60-80ftlbs, maybe more?

 

This is a very cool test Bill, my guess is 60 ft lbs.

 

90 lbs

 

35lbs?
Ground electrode hit the centre electrode n bang? (got no idea if that would happen but sounds alright).

 

45 ft lb. I want to say the plug failed before the head, but that does't mesh with all of the "stripped threads" threads I've seen over the years

 

Correct.

Was able to click off 75 ft-lbs.

When I stepped up to 80, the threads broke off the spark plug in the head.

Was able to spin the broken threaded piece out with pliers from the cylinder side.

Afterwards, I put a new plug in the hole. It went in with slight resistance as the threads were barely deformed.

Was able to torque to spec no problem. It could likely take another 70 ft-lbs.

Good insurance that 24-26 ft-lbs is super safe.

A lose plug will allow combustion gas to torch the threads resulting in stripping. Very safe to say if the threads are stripped, it was followed by torching.

 

Good info. From my experience it is not the torque going in that always causes the damage but sometime it is removing the plug after it has been over-tightened.