This is a question I've got 2-different answers within the past 24 hrs.

We are talking H-D spark plugs.

Which Is HOTTER, the LARGER or smaller number. 3-4, 4, 5???

I had The Dealer tell me Larger
I had a Jobber tell me smaller

and to add to the mix, with NGK plugs, the larger #, the hotter.

Can anyone clear this up?

 

Hey Skip, where you at buddy...

 

The heat range numbers used by spark plug manufacturers are not universal, by that we mean, a 10 heat range in Champion is not the same as a 10 heat range in NGK nor the same in Autolite. Some manufacturers numbering systems are opposite the other, for domestic manufacturers (Champion, Autolite, Splitfire), the higher the number, the hotter the plug. For Japanese manufacturers (NGK, Denso), the higher the number, the colder the plug.

Cut and paste job, got me straight too.

 

Hot plugs have a relatively long insulator nose with a long heat transfer path. Cold plugs have a much shorter insulator nose and thus, transfer heat more rapidly

 

The heat range of a sparkplug is typically specified as a number, with. some manufacturers using ascending numbers for hotter plugs and some using descending numbers for hotter. plugs.

 

Irionhead
I remember Champion spark plugs catalog had a heat chart in the back explaining numbers and letters on plugs.

 

I's rite cheer!

HD plugs are labeled 2, 3, 4, 5, with the lowest number being the hottest. There are also a couple special purpose plugs, 3-4, & 5-6. Used to be that 4 was used for break-in, then switch to 5 for normal use. 3 might be used if a lot of slow speed driving was done. Sometimes the bike'll run better with different heat ranges in each cylinder, with the front cylinder using the colder plug.

More porcelain = colder plug, but that's the porcelain up inside the plug body, where you can't readily see it.

 

Thanks guys and especially to Skip for clearing this up.