save a thread on cyl heads

[crosbyman]

Just read in the old OMC Millwakee maintenance manual not to use thread inserts on cyl heads to poor heat transfer from the plug to the engine body

Would using a cooler plug help or is the concern still applicable today ?? I am using J6c

I just rethreaded my 1957….7.5 cyl head’s upper plug with a save a thread . coud this overheat concern damage the piston face ??

thanks for opinions and your experience

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[Casey Lynn]

I have used that system on quite a few stripped spark plug threads over the years. Although I cannot be 100% sure, no one has ever brought back nor mentioned a piston issue after having had this done.
Honestly, these old engines we work on run fairly rich by todays standards anyway, there is a cooling jacket on the cylinder head (where applicable) that regulates the head temp pretty well plus the amount of oil we typically use pulls heat like crazy from the pistons. I really wouldn’t give it a second thought.

[r-c]

For your application you might want to consider the J4C plug. In general any of the old engines that used a J6J now J6C can use a J4C, simply because we have better oil. If you idle a lot the J6C is better.
With inserts, interestedly, OMC had a heil coil part number that was brass with chrome plating as they thought it transfer heat fairly equal to the stock thread. While a stock heil coil is stainless steel. I never saw much difference.
The bigger steel inserts do hold heat in and change the plug heat range in some applications.
Run the engine and read the plugs, go from there.

[joecb]

One of the four cylinders on my 1961 75HP V-4 Evinrude has a helicoil and I have not seen any indication of that one plug (J4C) running hotter than the others. This over several seasons of operation.

Joe B

[david-bartlett]

On a full blown race engine it "might" make a difference. However, I have used several of these on older outboards with no apparent negative effects.

The engines are much more likely to die from improper fuel mix than anything else!

[crosbyman]

phew….. 😎

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[Mumbles]

They are probably referring to a standard wire type thread repair insert which are not recommended for spark plug thread repairs. Heli Coil has the proper solid inserts designed for this application.

[joecb]

Yes, the wire type Helicoil on my 75hp , no problem

Joe B

[crosbyman]

Save a thread is a solid insert not the curly spring type

and that OMC book was from….. WAUKEGAN not… Milwakee 🙂

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