Different sized spark plugs

Kwality · Post by **Kwality** » Tue Apr 07, 2015 9:30 am

Hi all, First time poster here.

I've recently acquired a C77 (C78) in Perth, Australia that I am in the process of recommissioning. The previous owner was not mechanically inclined and as far as I've been able to ascertain never had the bike running.

Aside from some obvious wiring gremlins and a few missing bits and pieces the bike seems to be in fairly good order, however this evening I removed the spark plugs to undertake a compression test to find that they are different sizes!

The left side housed a 12mm, short reach NGK (it was an NGK D8HA). The right side housed a larger (14mm) diameter plug (B7HS), of the same thread reach.

Presumably a previous owner cross-threaded a plug, then had the head machined to the next sized plug.

I've not heard of this solution before. Has anyone here had experience with this sort of thing? Is it a reasonbable solution? I ended up doing a compression test and both sides produced similar results. Should I be worried?

Cheers,

Leigh

G-Man · Post by **G-Man** » Tue Apr 07, 2015 2:21 pm

Leigh

Welcome to the Forum!

I've seen it done but you could probably get an adapter made to bring that side back to 12mm if you wanted. Early Dreams had 10mm plugs....

G

Kwality · Post by **Kwality** » Tue Apr 07, 2015 7:35 pm

Thanks for the reply G-Man. I'll investigate that option.

Cheers,

Leigh

rrietman · Post by **rrietman** » Tue Apr 07, 2015 11:38 pm

Hmm; I wonder, the physics/resistence/ etc. you are running on one coil for both plugs. probably not a real issue, but an interesting question.
Randy

G-Man · Post by **G-Man** » Wed Apr 08, 2015 4:07 pm

Randy

If the gaps are the same, it shouldn't matter what size the plug threads are....

G

rrietman wrote:Hmm; I wonder, the physics/resistence/ etc. you are running on one coil for both plugs. probably not a real issue, but an interesting question.
Randy

G-Man · Post by **G-Man** » Wed Apr 08, 2015 4:09 pm

Leigh

Before I managed to pick up the correct 10mm plug head for my early CB72 I was looking at making some reducers from 12 to 10mm out of old spark plugs. There was just enough room to put a 10mm thread inside the 12mm plug having removed the porcelain part.

G

Kwality wrote:Thanks for the reply G-Man. I'll investigate that option.

Cheers,

Leigh