Spark pug reach

[CrispinA10]

Plugs on my RGS replica (running in) are getting a bit black, so I thought I'd try a hotter plug. Was using Champion N4C and did an internet search for a hotter version (when really I should have posted a query here!) and came up with Champion L86 C.
Four have arrived by post, so I gapped them up, whipped out the old ones and can see that the new ones are about five threads shorter.
Simple question: Can I use them, or should I relegate them to the lawnmower?
Obviously too long a plug would be a disaster, but what is the effect of too short? What should I be using in their place?
And no, I don't think it is running rich - I can swing air lever right over after a few minutes running and it seems fine, and no air leaks, although having said that it does sometimes kick back when starting.

[bsa-bill]

 too short would mean the plug centre electrode would not get the full cleaning effect as it would if it was sticking further into the combustion chamber, plus sooty deposits embedding themselves in the unused threads of your plug hole is not good
I have used L86 c in my Flash it is a short reach plug, your RGS I presume has an alloy head which needs a long reach plug.
So champion hotter then a N4 would be N5, I also like Bosch plugs W07cs is a long reach with a very fine central electrode that seems to keep clean

[CrispinA10]

Thank you for that - I hoped someone would know. Right. Off to order a pair of Bosch W07cs - they sound just the job. The bike does indeed have an ally head.  I assume that I am ok running on the short reach Champions until they arrive...

[bsa-bill]

I assume that I am ok running on the short reach Champions until they arrive...

I wouldn't, rethreading alloy head spark plug hole can be done but best avoided, clean up your N4s and run with them until new one arrive

[CrispinA10]

I'll just clean up the sooty ones. Thanks!

[Rgs-Bill]

   Not a good Idea to run with the short reach plugs in, they will foul faster, the exposed threads will just crud up on you, generally just make a total bodge of things.  Another thing the guys here in the states are doing, because our fuel is so  crappy, is go from factory recommended heat range of B-8ES, to B-7's , or even B-6's, however my 62 RGS runs better on the Bosch plugs, (they even self clean better) NGK anymore is a crap shoot for getting any quality in their plugs.  My bike also likes the cheapest made spark plug on the planet, it runs great on Champions ! !
   It has to do with the way that the porcelin part of the plug is swaged in to the metal housing, it does not have a narrow lip down inside of the plug that can catch the unburned combustion particles, and short the plug out.  NGK's foul way too easy, plus you might get one brand new one right out of the box that is a dud , their quality control has gone out the window.  I have better luck with Bosch and even Autolite plugs.