Spark plug length

[muckland]

After reading several posts that recomend one plug or another, I made a list & went to my local autoparts store.The only plug on the list they had was the Champion J12yc .I pulled my old plugs & they were Autolight 65 with way longer threads. My 54 panhead has a magneto on it and I am wondering if, that is why it has the longer plug . I talked with my buddy at work and he said there are 2 schools of thought on the longer & shorter vertions of plugs . He said the longer ones fowl easier , which may be my problem cause they are black .

[Cotten]

Muckland!

No matter what the heat range of any particular plug,
you do not want threads exposed within the combustion chamber.

Not only are they a sharp edge that can get hot and cause pre-detonation, hard carbon deposits upon them can cause grief when the plug is removed.

....Cotten

[RUBONE]

Magneto has nothing to do with it! Do you have original heads or STD or some other aftermarket heads. Most aftermarket use long plugs. All original Panheads use short reach plugs. J12YC plugs are the equivalent of original H-D 3-4 heat range and correct for a Panhead. Autolite, Bosch, NGK, etc, all make plugs that work and are the same length as thr J12YC.
Robbie

[muckland]

What should I look for to figure out weather or not I have original or after market heads ?

[Hog54]

Well if its a 54 then thier should be a date cast in the heads up in between the pushrod tubes.

[muckland]

I looked for the cast date & could not find one. I will just measure the depth of the threads in the head .Thank you all for your imput.

[caschnd1]

Do you have a photo of the motor?

[muckland]

I will take some close up pictures this afternoon.

[muckland]

I took these with my cheap phone hope they are good enough.

[RUBONE]

Those appear to be '63-'65 outside oiler heads and would use a short reach plug, Champion J12Y or equivalent!
Robbie

[muckland]

Good deal! I try the ones I already bought. Thanks .