Physical damage to a plug electrode is usually detonation, or a piece of piston flying around inside the cylinder. You are pretty conservative with your timing so that doesn't seem likely. Plug looks pretty wet (fuel)
engine type and size, compression ratio, camshaft, trans type and vehicle useage? What fuel are you running? JW
Plug is wet from unloading car in 40deg weather after last race....run for 45 seconds into garage. I have alum rods so maybe some stretch...doesn't look like it was hit tho...
Got a decent borescope handy? It'd be interesting to have a look at the pistons. Of course your next question's gonna be "what am I looking for?" and I'm not sure. Maybe signs of detonation on the pistons. Have a look at the cyl walls w/piston down toward BDC, any scuffing, maybe. Devon
https://www.denso.com/global/en/pro...e-parts-and-accessories/plug/basic/diagnosis/ Ground electrode is broken in the middle or at the base; much of the cross section shows fatigue fracturing. Results Ignition failure, damage to the engine or auxiliaries from the grounding terminal fragment. Causes Increased stress on the ground electrode due to high-output, high-torque engine or tuning, modification, etc.; excessive vibration due to poor maintenance, severe operating environment, etc.; insufficient fastening ⇒ excessive vibration due to loose plug Countermeasures (1) Spark plug selection ・ As a countermeasure to increased stress on the ground electrode, changing to a spark plug with a small ground electrode is an effective way to handle the problem from the plug side. Examples: 2-ground electrode plug, small ground electrode plug (taper cut), racing plug, surface discharge plug, etc. However, fundamentally, it is necessary to select a spark plug that matches the vehicle’s level of tuning. (2) Engine and vehicle body maintenance ・Reduction of excessive vibration through engine and vehicle body maintenance ・Tightening plugs with the recommended torque
What plugs are you using? Looks like anti seize on the threads? That can interfere with heat transfer.
Thanks Larry....I think vibration..along with a little detonation....that was the only not so tight plug. I have a pack of plug washers and from now on when I pull a plug I will use a new washer. Yes I use a very little anti seize...maybe none next time.
Bruce, why not try some NGK plugs. They have plated threads, so no anti seize is required. Don't the XLS plugs have a longer reach? Maybe the threads are extending into the combustion chamber. That can create hot spots on the sharp edges of those threads. That can cause abnormal combustion. If it has happened to 5 other cylinders, it sounds like a sparkplug problem. Looks like an NGK 5 or 6 heat range. BP5ES, BP6ES.
I looked at the interchange for an R42XLS to NGK, and the plugs listed were 5 and 6 heat range. 7 or 8 would be a colder plug in NGK. Could be too hot a plug, combined with the anti seize on the threads causing plug over heating, and detonation? https://www.sparkplug-crossreference.com/convert/AC_DELCO/R42XLS
