Are K100 and K75 frames interchangeable provided the year is within the same group? Thinking about a future project building from the ground up...

I think you'll find that 'missing' cylinder will make the mounting points not able to be aligned between K75 & K100. K100 frames will fit the K100-16V, K11 and K12 engines, however.

I remember posts on the engines being a fit but am I right in thinking the K1200 6 speed gear box [as opposed to the 5 speed K100/K1100] has a different frame mounting point?

Yes, I can see how that may present an issue. Thank you for your response!

92KK 84WW Olaf wrote:I remember posts on the engines being a fit but am I right in thinking the K1200 6 speed gear box [as opposed to the 5 speed K100/K1100] has a different frame mounting point?

You can attach a five speed intermediate case to the K12 in place of the six speed version, use various pieces of both K11 & K12 clutches, as well as a merging of sprag clutch pieces, and the earlier K gearbox works just fine (on K12 motors) in a K100/K11 frame - I used a Paralever final drive and K1100RS gearbox on mine.

clark_sas wrote:Are K100 and K75 frames interchangeable provided the year is within the same group? Thinking about a future project building from the ground up...

As far as I know, all the K100 frames should be the same, and all K75 frames should be the same.  However the engine mounting points should be different.  4 cylinders as against 3 cylinders. 

However browsing the Haynes manual (2 valve models) it gives the overall length as 2220mm (87.4in) and the wheelbase as 1511mm (59.5in).  Assuming this is for both K100 and K75 models I am unsure if there are differences in steering geometry or elsewhere. 

Perhaps one of our members who own both a K100 and K75 can race out to the garage with a tape measure and provide the definitive answer?  A cold schooner of Tooheys Old on the Kosciusko Run for the winner. 

If you look at a K75 frame, the front down tubes that go to the front engine mounts are at a shallower angle to allow them to go back further to the shorter K75 engine.
There will obviously be other differences but without getting the hacksaw and brazing torch out, the two won't mix.