What spark plug should I use?

[gmarshall43]

I have a 1971 Johnson, 9.5hp. I thought I had the right spark plug in it but was told it wasn't the right one for this motor. The manual I have list Champion J4J but I was told it is no longer available and J4C is a suitable replacement as is L77JC4. Which is the right one?

Thanks,
Gary

 

[F_R]

Re: What spark plug should I use?

J4C is the correct one.

 

[fast_eddie]

Re: What spark plug should I use?

Don't use L77JC4 in an engine that calls for j4c as it will very likely cause piston damage. My grandpa did this on our pontoon boat motor and now thier is a pit in the bottom piston where the plug was actually hitting it.
L77JC4 threads into the the head deeper than a j4c and thierfore thier is less clearance between the electrode of the spark plug and the piston when it is at top dead center.