Hello to all...
I inherited my dad's 75 FXE Super glide and I'm going to try to get it started. (sadly he let it sit for 30 years)
So I'm gonna replace the plugs, was looking up the part numbers, and for reference sake the listed Champion number is RN12YC.
I removed the existing plugs from the engine and it has J12YC previously installed in it.
Normally, I would just chuck the old plugs, buy what's recommended, and be done with it.
After doing some homework by checking multiple brand spark plug listings it seems that 74 and earlier plugs have short threads and 75 and newer have long threads.
My dad was the only owner and with only 14k miles I don't see any reason to believe that the engine was ever worked on.
The cylinder heads clearly have short threads and so that is the plug I will be using...unless there is something else I should know.
I'm sorry about turning this into spark plug science but I did warn everyone in the title.
Thanks...

 

When Harley upgrades a component they don't throw all the superceded parts in the bin, so a transition year machine might have the remaining stock of components installed.

 

I have a '75 FXE, inherited from my brother so I know its history. It has short-thread heads.

 

I've seen one long reach plug in one head and the short in the other, from the factory like that. 74 through 79 were interesting times for shovels, most were as they should be but during transition years mix & match games did go on. 78 & 79's were all over the spectrum.

 

Little confused.. I thought the 5r6 short plug was dedicated to the shovel and the RN12YC long was for Evo's.

 

Later shovels used it too.

 

According to Service Bulletin 680, new cylinder heads were used as of October 28, 1974 and they required a longer spark plug.





But if the bike was produced before October 28 then originally it would have had short plugs and that seems consistent with the short-thread heads you have.

Maybe the bike is an early-75 model. Is there a sticker on the front R-H downtube of the frame? It may have the ‘Date manufactured’ on it indicating month and year.

Also, if it is an early-75 model then I imagine the VIN may be fairly low? For 1975 models the VINs for the Shovelhead family began at either 10000 or 10001. (FX/FXE/FL/FLH VINs were grouped together.)
Eric

 

Look in the plug hole, it's pretty obvious which is which, easy to tell.

 

This is exactly the kind of information I was hoping someone might have...the service bulletin is really informative to read.
My dad's FXE is missing the sticker with the date of production but the VIN sequence is 11824 which seems pretty low.
Thanks again.

 

Ok, so I took my plugs out this morning and they are the short ones. I also measured the plug hole and the thickness there is 9/16". Does this mean I have older heads on my 77 FX? And does it really make any difference?