my shovel has two types of spark plugs why ?

[Jailbard]

Hi there Anyway at the risk of sounding dumb, can someone xplain why Ive got two different plugs. The front one is shorter than the rear.( yep its my first shovel ) Cheers
Oh yeah its supposed to be 1981 fl

[pappyfreebird]

r you sayun front cyl has short n rear cyl has long? then id be checkin real fast to make sure that long range is a deep thread ..look in both plug holes ,,early heads had short plugs n later heads had long range..possible to have n older er newer head..then thats way ya run it..many two plug heads run a long n a short depends on yr of heads n who did the second hole fer ya..1st werry would be make sure long aint in a short hole...am thinkin 72 -73 sum where in there was last of shorties

[randallwhitman]

Pappy is just a LITTLE hard to understand.  If both of your heads are stock and original, they should have the same plugs.  If one is longer than the other, look at the carbon deposits on them to see if either one is too short or too long.

[Purplepandan]

My 76 had SHORT plugs!

[Jailbard]

Hi brought the bike of Ebay when it arrived it ran rough so i cleaned the carb ( it was prertty dirty)
then went to change the plugs, found the front one shorter than the rear.
tried putting a longer one in, it wouldnt thread down. put the short one back in.
seems to run ok havent taking it for a run yet, thought id find out abit more about it first
Had a look and yep theres only two plug holes.
Sounds like I may have two different heads, is there any way I can tell if this is the case .
Would appreciate any help, Cheers

[Speeding Big Twin]

IIRC my 1974 FL had short plugs. My 1941-1984 H-D FL parts catalog lists several plugs available for use 1948-1974 but also a change to plug 5A6 which can be used 1975 to early 1978.

My 1971-1984 H-D FX catalog lists several plugs available for use 1971-1974 but then jumps to plug 5A6 for use late 1977 to early 1978. However, plug 5A6 is also listed as part of an FX tune up kit for use 1975 to early 1978. The first electronic ignition for both the FL and the FX series appears for late 1978.

The photo below shows a style of casting date code which appears on some heads. Left shows 2 69 to represent February 1969 and right shows 3 69 for March 1969. However, some date codes don't necessarily equate to the exact model year of the bike they appear on. Have you got date codes on your heads? And what year and model is your bike?     Eric

[Unclelaffey]

I won't hazard a guess about what plugs you need. But I do know if someone ran a short reach plug in a long reach head for any period of time, the bottom threads could get so carboned up that it would it not be possible to seat a long reach plug because the threads wouldn't cut through the carbon buildup.  Might have to chase the threads with a tap to clean them out. Just take a length of wire and bend a short 90 degree leg on the end. Insert the bent end of the wire in the plug hole and pull it back until the short leg hits the inside of the head. Mark the wire at the outside of the plug hole. Then compare the threaded part of the spark plug to the distance on your wire between the short leg and your mark. Do the same thing with both heads. If they are the same, then they should take the same plugs. If they are not the same, then you have another set of problems.

[Speeding Big Twin]

Jailbard, I just spotted that you mentioned it was a 1981 FL. My catalog lists plug 5R6A for use 1980 to early 1982 -- FLH, FLH-80, and Classic, if all is stock.     Eric
 

[harpo1313]

possably,the seller knew it was runnin rough and swapped in an oddball plug to get her runnin,hey,its a possability that would be good for you,pics?

[stroker800]

   I would say you have 2 differnt years of heads,,,,66-77 used a short reach plug,,when the motors went to 80" ..78-84,,they started using a long reach plug,,,,DO NOT use long reach plugs( more threads) in the early heads...the threads will carbon up and when removed will tear out the threads in spark plug hole....I run early heads on my shovel,they work fine when not OVER TIGHTENED..
Dave

[Jailbard]

Hi
Just want to thank everyone for their help much appreciated.
I have identified the heads as '66s.
Now I know which plugs I should be running.
Cheers Mike