I'm up for any suggestions as to what could be causing it as i've alot of factors to consider!.

We've got a newly rebuilt Pinto running on R1 Throttle Bodies and MegaSquirtNSpark.

When we first go the engine to run on the Tb's it sounded like a Subaru, it turend out after about 1 hours use from a complete new set, No3 plug was failing under load, sparked like a good'en on the manifold. It appeared that the nipple on the end could be twisted even though it wasn't loose, as if there was a breakage in the inner core. This was replaced with a new single plug from a differnt supplier.

About 100 miles after this (tonight) it appears No3 has gone down again and also atleast another one has the twisty nipple problem.

Its quite likey that the engine has been running timing thats too retarded on the whole, but would that cause the plug to physically break in this way?. They look quite normal, not overly sandy, white or wet.

Think i'll be buying a new set anyway after this, but does anybody have any reasoning why?

this may be helpfull?

marc.

Thankyou, but I wouldn't say its any of those

Probably the closest is Oil Fouled, but a quick wipe and its back to normal.
The engine is having a little trouble breathing, but nothing too major I've had worse on previous engines and it hasn't destroyed the plugs in such a short space of time.

hmmm, tricky one!

I'm not sure whether the "breaking" bit is a red herring, how hard do you have to twist em before they turn??

Certainly only really a faulty plug or physical damage causes that.

Are you sure that the injector for that cylinder is ok and delivering the same amount of fuel as the others, not unusual for one to stick open, or not atomise properly, pull the rail off and squirt all four into glasses or something to see.

I have had plugs break down on a pinto, but only due to them being too soft (hot) for the compression and cam fitted and causing pinking/detonation before they failed.

Were running a wideband and had have had trouble tuning it so its been running pretty rich for safetys sake.

We've had No pinking at anystage, if anything it was over 5 deg too retarded!

The engines just been rebuilt by us with all new seals, althought we have been having breathing problems (seems to be coming out of the dipstick now!)

I found a number whilst at work for NGK and rang to speak to technical.
I explained that the Nipple on the end could be turned about 5 Deg either with your fingertips (whist still being tight on its thread). He pretty much confirmed as I expected that the central core must be broken inside the pot.

The only things he could suggest that could cause this (as the 2 No3 plugs that have gone down were different batches) was stress on the plug, from HT lead being at a tight angle or maybe from vibration if the plug wasn't tight.

I've just bought a new set from Pages on the way home and he asked were they for the injection model.. Hmm whats the difference? There coded a 7 which is a Hotter plug (although oily plugs should indicate the plug isisn't getting hot enough, I think its too over fueld and thasts whats keeping it cool).

Looks like i'll have to try again an air less on the rich side of caution with the fuel maps!