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yesterday I had my first spark plug failure . cruised around the yard, loaded up the truck with some scrap, everything was fine. made it out the gate and a block down the road, then as I changed up into 4th i felt a miss. not wanting to get stuck on a bridge I turned around.with the hood up, I started by pulling the plugs. they all looked the same , next I pulled the cap, and checked the points. gap was good, inspected the cap, all looked good, went to replace the rotor and found a crack. tossed it and went digging for a replacement. new rotor installed cap back on and still the miss persisted..... started pulling the leads one by one and fond a no drop in rpms when no6 was pulled. replaced and everything was smooth.

thats pretty good 15 years and 30,000miles from a set of plugs.

do I replace them all or just carry a few spares ?

I've always replaced them in sets-if you've already gapped them you can change the set in 15 minutes. Mine are civvy E3s (no gapping option) and holding up well-1500 miles or so on them.
Gary

RMS,
I'd swap 'em.

The reason I say this is I had a series of ignition issues a while back that were started with a faulty wire. When the wire failed it progressed from intermittent to nearly constant under WOT. It's my belief that when the wire didn't provide the path for the voltage it found another path through the cap. Once the wire was replaced the miss was fixed, for a few thousand miles, then the cap failed when hole was burned through one of the towers.

My concern is your remaining plugs may be on the way out and if they start misfiring you may stress the remaining ignition components and have issues similar to mine. If you are running civilian stuff at least the swap is cheap !

Andy

I feared that was the response I was to get , and those caps are not cheep.... mill spark plugs are not cheep.... I see a screw can they be rebuilt ? cut up some civi plugs for new carbon ?

RMS,
I subscribe to some machining mags where the guys build plugs for their small engines. It can be done, but you'll be busy! At the end of the day purchasing them makes sense, but if you want I can try to scan the articles and send them your way.

I'd like to have a civilian set up on hand just due cost and ease of maintenance, but the waterproof stuff is pretty neat!

Andy

Home Shop Machinist? I've read some of those home-made spark plug articles, and for an M37, that would not be a beginner project.

With the increasing prices on these plugs, I'm wondering at what point does some manufacturer start to re-think ending production?

John,
That's the one .

Andy

They still make shielded plugs for airplanes so why can't they make the heat range we need? 2245 covers all of the M-37 and M-151. I don't know about the 38 and 38A1.

I suspect someone will. Especially at the current market pricing.

The brake drums for our trucks have just been reproduced. If that can be done, spark plugs can't be impossible.

are you running champion plugs? I have had those fail (centers literally melt away) so never run them. Autolites only for me. But those are now made in china so who knows if they will last either