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A problem has developed with my 359 aluminum cylinder head. When removing #3 spark plug, it came out with a sleeved insert attached to it that had a fracture in the sleeve's bottom two threads. This assembly will not screw back on, I'm sure the fractured thread section did a number on the head's threads. Any thoughts on how to deal with this? I'm not sure a new sleeved insert would hold in the head and likewise with a helicoil. It looks like the hole in #3 is at least 16 MM in diameter. I'm wondering how advisable it would be to retap #3 to accept an 18 MM spark plug. Or are there any other repair options available? Comments and suggestions, please.

Cracks thru spark plug holes in the aluminum 359 head are not at all uncommon and the "sleeve" you found was a prior insert repair. I doubt you'll find an oversized thread repair insert so I'm thinking that welding the aluminum in the hole and redrilling and tapping for 14mm is really the best option. I think the length of the threaded section of an 18mm plug precludes that idea.

IN PRODUCTION Indian used a BRASS 1/2" PIPE thread insert into a Al cylinder head. I bought somee succh similar braass inserts at hardware store many years ago. But i had to thread the hole for 14mm plug.

Also used a staandard GALVANIZED IRON pipe fitting collar of some sort, i don't remeber the trade name for it, that i tapped for 14mm plug. It had a very thin hex on it.

Try going to a hardware store or plumbing supply house to see what they have.

Of course that means u'll have to thread the head with a tap. Just grease up the tap real good so the grease will catch the chips.

Thank you for the quick responses. I was relieved that no one said to replace the head! After posting, I found that I could break off those two errant threads in the insert. The rest of the insert then threaded back into the head, so the threads weren't as bad as I thought they were. I then bought an insert kit because I wasn't comfortable with the idea of just 3 or 4 threads holding that plug in place. The new insert initially wouldn't thread into the head because the new insert's threads were a finer pitch than the old one was. The kit contained a tap, and since fine threads are slightly larger in diameter then coarse ones are, I was able to retap the hole. The new insert went in smoothly, so the problem seems to be solved for now. Thanks again for all those suggestions.

Good idea to use an anti-szie on the spark plug threads when installing spark plugs into an aluminum head to prevent future problems. Steel and aluminum do not play well together.

(o{}o)

JW, I wonder if that's a good idea or not; I used Never-Sieze on spark plugs once and one of them backed out while on the road, flew upwards and put an ugly dent in the hood.

O_D, I have no direct experience with anti sieze (thanks for the correct spelling) on spark plugs in aluminum cylinder heads. I based my recommendation on advice from others. Was your bad experience with an iron or aluminum head?

(o{}o)

14mm plugs in an aluminum head.

Okay that helps. Maybe others with experience in this matter will add their comments. Torquing spark plugs in aluminum heads is critical, more so than with iron heads. Easy to over tighten and ruin threads.

(o{}o)

Indeed it is. I have an old spark plug chart with torque specs for many applications, if I can find it easily I'll add it to this post.

EDIT: From AC catalog, January 1962

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