From:Dave Hennessey
Subject:RE: Spark Plugs & Point Gaps Date:Mon Apr 8 18:30:40 2013
Response to:18367

You probably thought this was an easy question, but it's a slight can of worms.

The information I have comes from the 1948 Model 125 Rider's Handbook, 1955 Hummer Supplement, 1959 Model 125 Rider's Handbook. 1963 Pacer/Scat Rider's Handbook, 1959 Service Manual and 1966 Service Manual.

The 1959 and 1966 Service Manuals say #4 is standard, so this probably applies to all Model 165s and the 1960-1966 models. Probably the same for Model 125s and Hummers, but I can't find any references in the literature.

Almost all the literature says the proper plug depends on your style of riding (see below).

1948 - 1959 all models .025 to .030
1960 - 1962 not sure - probably .040 to .045
1963 - 1966 .040 to .045

I don't have any literature for the 1960-1962 models. These are 165cc versions of the Hummer engine. 1963-66 are 175cc. I would suspect .040 to .045 for 1960-1962 also.

Model 125 and Model 165 (battery) .020
Hummer and 1960-1966 (magneto) .018

Mostly I have used modern Champion H-8 or H-8C plugs. I have used Harley #3 plugs, since I have a few of the early "take-apart" models.

For my style of riding - infrequent and typically slow speeds - it really doesn't matter, and I've never noticed either burned or fouled plugs.

If you're planning to do a lot of riding, you'll have to experiment. I'd start with a Champion H8C (cheap and common), and "read" the plug after some use. Then decide whether you need to go colder or hotter.

Infrequent use, slow speed - hotter plug
Frequent use, high speed - colder plug


Wet, black - bad rings, way too much oil in gas, or ignition problems

Dry, black, sooty - carburator too rich or plug too cold

Light brown dry, glassy - carburator too lean or timing advanced

Rusty brown to tan powdery deposit - just right

HARLEY PLUGS - low# is hot, high# is cold.
It appears that #3, #3-4, and #4R are still available. I think the #4R is a #4 with a radio-interference suppressing resistor. Better check with them. I was never sure what a 3-4 was.

CHAMPION PLUGS - low# is cold, high# is hot
It appears that your choices are limited to the H8C and H10C (hotter). Resistor RH8C/RH10C or old stock H8/H10 are fine too.

The Rider's Handbook says "clean with a sandblaster, found in nearly every service station". Yeah right. But you can buy one for $20 on

Hope this answers your question.


P.S. Both Service Manuals also say "it is not uncommon for best results to be obtained with plugs of different heat ranges in the front and rear cylinders, with the front usually the colder".


First time to the exchange as I just inherited a 1957 165cc from my dad. It has been sitting since the 1970's so I have some work to do. I am boring out the head to .060 as its currently at 0.040. I am having trouble with the simple task of figuring out what spark plug to use. It looks like it should be a #4 but the last one in the bike had a #5. It also looks like the gap should be .025-.030. Any suggestions on a more modern plug or if #4/#5's are avaible?