So I have something only the detail freaks would notice or appreciate. A set of 8 Leonard 14 F "High Compression" spark plugs. They're all side gapped, which according to an image search is how they were made. They're 14mm and will fit my flatty once I clean the threads. Frankly I can't remember what I took them out of but they were too kool to toss away and actually fire despite the existing crud. I'm well aware of the 'useless w/out pics' rule so I'll take some later or tomorrow and add them here. I also did some searching for history and all I found was that they're in the Smithsonian in some way and listed/cataloged, but no story. When all else fails the best source is, you guessed it, right here. I'll take all the info that's fit to print. I promise, pics soon. No really, I will. Quick edit: the Smithsonian has their catalog on file in the the library. Didn't see anything about actual plugs.
Ah I get it, I think. The side gap clears the block/valves/pistons on high compression engines? I had to shim up the plugs in my 60 to keep from closing the gap.
I might A-B-A test these plugs and see if they actually make a difference. Yes or no, I think they're kool. Found some old boxes online and they refer to the holes under the wire end as "air cooled". Kooky...
Just so it's clear, I dont expect to set an El Mirage record with spark plugs. I thought they're kool, I'm a fan of side gaps, and to see one done that way from way back when is intriguing to say the least. I guess their history will remain fleeting and mysterious. I'ma run em anyways even if it doesn't make a diff. Should be a nice mate to the Holthouse coil. Lawn mowers, WTF...
Bringing this thread back to the top, just discovered this thread looking for info on Leonard High Compression 5-HD plugs I found on a flea market $1 table.
Well I still have em, still gonna run em, still think they're kool as can be. These came off an old Packard and the guy wanted AC or Autolite (either is correct). I had to have them, "...it's you or toss em in the trash, I don’t want em.." so they're ready for my flatty. Still never found any info or historical references. Should look kool as fk under some Rajah plug tips
Found a little snippet from a FB spark plug collectors group. Looks like my 5-HD was primarily used in Boat Motors and Scooters
Bosch plugs for O-T 356 Porsches were 'side-gapped'. Electrode ground was shorter, pointed to the side of the electrode. Would have run them in my flatty, but who'd know??? A framed pic on the firewall might be appropo...
For a bit of reference, I side gapped some NGKs in a hopped up 3cyl 2 stroke. Probably about a 165HP motor. All I could do is make back to back tests down the road. With the side gap plugs the thing went into notable over rev. I had to make a clutch adjustment to compensate. What's the point? In a small motor it made a difference I could feel. What I didn't like was that keeping it on the ragged edge wasn't sustainable when your 80 miles away and ready to burn shit up. Conversely I did it to my racer BBC. The use of side gaps in that one did nothing. Ran the exact same ET/MPH. I attribute the result to some fairly superior ignition to begin with. MSD vs old CDI 2 stroke. Back to these Leonard's, I think the kool factor makes the juice worth the squeeze.
I tried these "surface gap" plugs in my H-2 Kawasaki drag bike. They were supposed to be anti-fouling plugs, but I didn't have any fouling problems, I was looking for more power. They didn't make any more power. After I lightened the magneto, it wouldn't fire them any more, so I ran regular plugs. It ran a little faster, because of the lighter rotor.
So looking at the top of the plug, the wire was inserted in the hole under the cap and then the cap was tightened? My Dad had a Heath henderson 4 airplane conversion and I think thats where I saw those plugs. It was a Henderson 4 with the transmission sawed off and a prop hub fitted. Lippy
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