CB750 or 900??

[Ben French]

Ok so here's the problem: I bought a wreck of a cb750 (1981) as a project café bike. cylinder block has 900cc on the casting near the base where the gasket to the crank case is. The guy I bought it off said that the 750cc crank case was interchangeable with the original 750cc cylinder block and head and the 900cc cylinder block and head. He said that the cylinder block and head had been swapped with the 900 so that the engine id number still matched a 750cc and thus registration, insurance etc would be cheaper (not that I care if that is the case or not). The engine is a non-runner, so far I have began rebuilding the carbs and in the mean time have managed to get the engine cranking over and firing a couple of times with starter spray sprayed into the holes where the carbs were. My question is, is it actually possible to run the 750 block with the 900 cylinder? or am I going to spend a long time rebuilding the engine just to find out that the reason he sold it to me is because he realized that the two aren't compatible!! has anyone hear of this or done it before??

 

[slowpoke69]

I know you can use the 900 cams in the 750, I'd call Ferracci in Pa, they don't do Hondas any longer, but he was THE man back in the day. The # is 1-888-FERRACCI, I'm sure SOMEONE will know, hrs. are 9-6 eastern Tues-Fri, 9-3 Sat.

GOOD LUCK, either is an awesome ride. I weigh 250 and my 750f/ss has NO problem brining the front wheel WELL off the road!

 

[65wonton]

I know this is a month or so old, but I'd like to share.

Back in the late 70s early 80s, I had a 750. I think it was a 73 or 75. I read of a fellow in CA that was racing Honda 4s. Following in his steps, I took the top off a 900 (jugs, pistons, rods, carbs) and put it on a 750 bottom end. I changed the cam, exhaust and rejetted the carbs. Everything bolted up into the stock frame, or if it was modded at all it was very simple changes. Man that thing would scream for a street bike of the day. I had more fun with stock bikes than I should have been allowed to have or should have occurred on the streets.

So I say you have one of these conversions.

Dave.

 

[amc49]

Old post. 750 cylinder will go on 900 crankcases but 900 cylinder will NOT go on 750 cases, the holes are too small and cases needs to be opened up. There's several other issues as well, the 900 parts are all longer to match the 900 crank.

DOHC parts are totally incompatible with SOHC parts pretty much across the board. I'd like to see somebody put 900 parts like cylinder on a SOHC, that would be good comedy. The carbs are like 1/2 inch bigger than anything a SOHC ever used as well. Won't fit in any form or fashion. You can't even apply 900 parts to a DOHC 750 like that, the stroke is so far off the pistons sit a mile in the hole and zero compression.

The only way OP post can happen is using a 900 crank in 750 cases that have been opened up at the cylinder holes to fit the 900 cylinder. Meaning a 900 in all parts other than cases alone.