95 MerCruiser 350 spark plug thread repair

[PW]

Magnum Tournament Ski TBI Ser.#0F562552. Routine tune up.Changing plugs and one old one was easy-hung up-easy, etc. coming out. Threaded the new plug in and when it got to the seat, it wouldn't tighten. I'd get it hand tight and then when I put the wrench on, it broke loose. Also, two plugs I'd pulled had clean seats, and the one that won't tighten had what looks like blow-by on the seat.

I've spent two hours searching for what the heads are made of. I think they're cast iron. Can anyone confirm?

I'd like to hear from people who have actually done this repair. Should I try a thread chaser first? If drilling out the hole for a HeliCoil insert is the answer, and the head is cast iron, it's going to the shop. I'm not confident that I could drill it straight.

Many thanks for your help, PW

 

[Scott Danforth]

yes, your heads are cast iron (the only two choices are cast-iron or aluminum)

yes, you need a threaded insert (helicoil, thread-sert, time-sert, etc.)

at a minimum, the exhaust must come off to get clearance to work

 

[tank1949]

If you rethread, have a vacuum ready to suck up metal shavings/drillings so they don't get into fire chamber. Auto shops may have a special kit. I am sure that your are not alone. Lots of rusted heads/plugs out there. Scott is 100% correct. You should pull exhaust manifold in order the see what you are doing. I suspect that over their lifetime, GM has modified metallurgy in their block materials. Growing up in the 50/60s I was told that we could not weld a GM block (due to it being iron) that had cracked, perhaps due to freezing. However, I can tell you that I have welded cracks in GM blocks, not brazed. I pre-heated with a torch and then MIGed. I don't know if it would have held under a closed cooled pressure, but it didn't leak, or I didn't see it on a raw water cooled 350/OMC.