How to fix broken thread of spark plug hole?

[andreipou]

Hi,
I have 2 motors (25 HP 1976 Jonson).
They both have the same problem: One of the spark plugs hole (opening where you put spark plug) has thread broken\cut.
Is there any way to fix it without buying whole power head assembly?

Thanks.

 

[Rick.]

Re: How to fix broken thread of spark plug hole?

You could but a new head or many times folks have used a Helicoil that threads into the hole. I haven't used a helicoil yet but you can get them at Automotive stores. You might try a search on this site and get more information. Best of luck. Rick.

 

[F_R]

Re: How to fix broken thread of spark plug hole?

You could but a new head or many times folks have used a Helicoil that threads into the hole. I haven't used a helicoil yet but you can get them at Automotive stores. You might try a search on this site and get more information. Best of luck. Rick.

Heli-Coils are good, but it will be cheaper to remove the head and take it to a shop to have it done. The installation kits are expensive. Even if you do buy the kit and do it yourself, you really should remove the head and do it in a drill press to get it straight. It is near impossible to get it straight by doing it free-handed. Most automotive machine shops can do it for you. You will need a new gasket to reinstall the head.

 

[JB]

Re: How to fix broken thread of spark plug hole?

Instead of helicoils have them install Time-Serts. Helicoils are steel and change the heat transfer characteristics from the spark plug. Time-Serts are alloy sleeves that transfer heat as designed.

 

[kenmyfam]

Re: How to fix broken thread of spark plug hole?

Head off, new head gasket and a thread insert.

 

[andreipou]

Re: How to fix broken thread of spark plug hole?

This is great!
I have a hope now.
I looked at Time-Serts.
I have original manual for this engine. It recommends Champion J4J plug.
They do not make them anymore but I saved one for referrence.
It measures from the washer to last thread as 7 mm.
I found that I need:

1. Standard M14x1.25 kit (p/n 4412)
2. Aluminum inserts 44121A


Can anybody who done it before confirm that I am correct?

That would help me a lot.

Thank to all. This help is priceless.

 

[1946Zephyr]

Re: How to fix broken thread of spark plug hole?

The J4J was more or less relabeled J4C and myself, I would just go up to a
J6C. I think they run much better. Your thread size is correct. It is better to remove the head to insert the new thread, so you won't risk getting shavings into your cylinder, but occasionally, there are outboards where you can't remove the heads, such as the Mercs. In this case, I used a vacuum cleaner to suck the shavings out as I was cutting.