I blew out a spark plug. Yup. Blew it out.

I have a 79 Shovelhead and I was going to change plugs. The threads on the rear cylinder are just toast. Nothing left.

I bought a spark plug re-threader, not the helicoil type. It helped re-thread the head threads a little so I tried it and the plug threaded back in but I was not sure how well. I went easy on the torque.

Time passes..... Today I went out for a ride and she blew. You would not believe how much pressure a head makes. I rode it on one cylinder three miles home at a very low speed.

Here is my question, They make spark plugs with longer threads. I did some reading on spark plugs but didn’t find the answer I was looking for. If I had a plug with longer threads, I could utilize the full length of threads (half of them are still good deeper in the head). Otherwise I have to go with the re-thread kit. And finally a new head.

Any insight in this would be helpful. The plug I have now is a Champion RJ12YC



I would like a plug with deep threads.




Thank you for understanding in dealing with my ignorance to spark plugs.
Tom

 

AS far I know the threads of the plug go all the way into the head..........there shouldn't be any excess thread there if you know what I mean........you should be using the full length of the threads NOW!

It sounds to me like you're plugs are too short for the end of the thread to reach the right depth of the head, that would put the electrode INSIDE the threaded hole and not out the other end like it's supposed to......

I could be wrong...........I've never been wrong before...........but I have been mistaken a few times.............

 

The 79 if the heads are 79 dated you were using the incorrect plugs as they are supposed to be the long 14mm threads

2 - champing plugs in my 50 plus years of doing this is never a go to for any aluminum headed engine dont care what others think or use

every head we see that is stripped out was using those plugs or the champion Harley plugs they use from them they install the threads differently then any other plug manufacturer - is the reason —— and i dont care what one guy with one bike did or uses

 

Pull the head. Install either a timesert or helicoil. If there ain't enough meat left you'll have to get it welded and machined. Do it right. Anyone that's owned a shovel long enough has had to do this.

 

Knock yourself out OP
http://www.timesert.com/html/sparkplug.html

 

Ok, I've learned a bit about plugs and more about my bike. I don't know why the previous owner had those stubby plugs in this bike. As stated, the spark was occurring in the threaded channel before the cylinder head since the plug was the wrong one. Totally bizarre.

I bought a new plug, the correct one with the longer threads. Since half or more of the threads were still good was able to just screw in the new plug. Fits nice. The bike is running sweetly again.

Oddly, the bike ran nice even with those stubby spark plugs.

With the gap, I gapped it a .036. I have an S&S carb and the instructions from S&S say the gap should be .021 but Harley says .038 to .042

Speaking of gaps. On my points, it seems people generally like .014 to .018. My moco manual says .006. I just don't know who to believe.
I have my points at .016

Tom

 

When it comes to points/plug gap use the factory specs, if that doesn't work out adjust accordingly

 

That could be a misprint in the manual. I've never seen a point gap that small on any engine. Seems to me that the correct gap on harley ponts was .018.

 

In a pinch a match book comes in handy .012

 

Set the points at .018 dont believe the service Manuel it has 50 mistakes in it ask on this site when you really want to know

with a 5 ohm coil and points they like .035 like all the old cars did - auto lite AP4265 is a go to for most machines with long plugs and sometimes wall mart has them cheap or amazon box of 4