Board index Flathead Power-Technical Questions, Answers, and Suggestions Big Twin Flatties Sparkplug suggestions for 1945 U with a 80cui bore

Sparkplug suggestions for 1945 U with a 80cui bore

Posts: 2
Hi there, I'm almost finished building my beautiful flathead. I've done a few alterations, and am curious to know which sparkplugs you guys would reccomend and why. Specs. are : 12V electrical system, Ed Fralick's Electronic ign. Quickstart2000, Crane 2,8 ohm coil, and a dell'orto carb. I've had the engine rebuilt by one of the best mechanics/machinists there is here in Denmark. I'd really appreciate your advice in this, thanks guys:)

Posts: 83
I had, and have, good luck with the NGK AB7 for the original 18mm plug holes.


Posts: 575
Location: devon,england
has anyone ascertained yet wether the ab7 is a resistor plug ,i.e incorrect for use with a magneto? regards jib
Dude, check out that jibhead, he's crazy. Hasn't been sober for 40 years

Posts: 1676
Location: Interlaken, NY USA

I've gone to using the 18mm14mm adapter and using NGK B6L's, which seem to work well.
The motor is currently 84" 4 1/2" stroke with about .035" popup relief in the heads and the usual 3 7/16" pistons from Ted.

Posts: 2
Cool, I was actually considering the Ngk's since thats what the bike was fitted with when I got it home to Denmark.( It hasn't been running for 20 years, just sat in a garage in Kenai Alaska). Was also considering the autolite's, my friend's got the sweetest '41 Chief which runs with the same carb as mine, and he uses autolite... Anyways, I don't know a lot about plugs, but have been told that sometimes you have to adjust the gap yourself? Is there a "standard" point gap, or does it change from plug to plug?

Thanks a lot guys,
-Emil, Cph. Denmark

Posts: 1676
Location: Interlaken, NY USA

It's always a good idea to check and adjust the gap to suit either the factory specs, if ignition is stock, or to whatever the maker of the aftermarket ignition recommends. Plugs do come set at some setting, but it might be more suitable for modern electronic ignition, which typically uses larger gaps than older points style ignition.

Posts: 251
Location: Hudson, Florida
Doc have you had any luck with the Champion D16 plugs, I run them on my Knuckles. Do the Knuckles and Flatheads use the same 18mm plugs?

Posts: 1676
Location: Interlaken, NY USA

I used D16's for years in the flatheads and the knucklehead. Also tried the NGK's in the Knuckle, A-6's, which have worked fine, but to tell the truth, that set of plugs is probably 15 yrs old at this point. I also tried the D14 at one point in the flatties, using a 16 in the front and a 14 in the rear. Thats before I read about Manifold leaks here. :(

Posts: 172
Location: San Diego, CA
Autolite 3116

Return to Big Twin Flatties