73 top end on 68 bottom?

Hi,
New to this board. Just picked up two Commando project bikes. Bike A is a 72 matching numbers that has been mostly disassembled down to the frame. It was missing most of its front end parts but I have located everything but the headlight and fender. Its got a steel roadster tank and what I believe is the low compression engine as I can't find a trace of a "C" on the cylinder head. Bike B is a 74 rolling chassis which was missing its drivetrain. It came with a early 68 bottom end and a set of cylinders and head. The head has a code of RH6 on the top which according to Roy Bacon is a 73 high compression. I am assuming the cylinders are the same. My question is can I use these cylinder and head on the 68 lower? Also, are there lower compression pistons or thicker head gasket or base plate to lower the compression ratio. I would certainly be willing to give up a little power for something that works better with today's gas.
Thanks
Htown

Hello Htown,
Sounds like you're going to have way too much fun. Sorry I'm not one of the learned masses when it comes to Nortons and can't answer your questions with any authority. I'm presently doing a rebuild on a Mk3, my first Norton. A bit scary at first but it's all basic once you get into it. (apart from all the weird bolts) I don't know, or didn't understand, what's 850 and what's 750. I would think that if it's all 750 and all the holes and studs line up then it should be adaptable????
Like I said I'm knew to this Norton stuff but this site is invaluable for all sorts of information.
Graeme

I also admit to ignorance on the subject, but guess it would work.

I can't see any problem as long as allthe parts are in good nick. All 750 top ends are basically similar....you can also put the 750 top end on an 850 bottom end (I had a 750/Mk3 850 mix because I wanted a stronger bottom end), but you can't put an 850 top end onto a 750 bottom end....(The barrels won't fit in!)

I'd build it up and measure the compression afterwards, it might not be as high as you think!

Thanks for the replies. It looks like the openings on the cylinder match those on the cases. Also the heads have the same part numbers for the two years. There's an set of 850 cases on E-bay right now I'm thinking about and selling on the 68 ones. They already have the superblend bearings.

I think you will find the 850 cases command a higher price than the old 750 ones, I would keep your original cases and change the bearings as a matter of course, regardless of what is purported to be in another set. Just to reiterate; all of the parts you have should fit together with no problems, I have run a set of 1969 cases (with the cam shaft timed breather) with a hot cam and high compression head without any problems at all.