Topic: Questions about DOHC 750 ccm engines.

[rasta]

Hi, I would like to ask you some questions about the 750 ccm engines, mounted in Hondas CB 750. I bought a custom motorcycle with an engine from 1982's Honda cb750k. Unfortunately, the rings or seals are leaking. I have to repair or replace the motor. And here comes my question. Can I replace the 1982 engine with the engine from the Honda CB 750 Nighthawk (1991-2003)? Is this a plug & play operation or requires a lot of work? I am asking about mounting places. Are they identical in these engines from different years? Is it better to replace the engine separetly, or together with the gearbox?
Forgive me my English.
Thanks in advance for your help.
I add my custom's image.
Mike from Poland

[MCRider]

Mike Welcome: This is the Single Overhead Cam forum. While we'd like to help the later DOHC bikes are completely different. You may have better luck at http://www.cb750c.com/  or seeing as how yours is a chopper:  http://deuceswild.yuku.com/

[Lostboy Steve]

Meanwhile... that's a pretty gnarly bike. I like it. Although the engine tone doesn't match the look i'm sure.

[Stev-o]

Hi rasta, that is a wild looking bike you have there!

I would look into repairing the motor you have before replacing it, although it may not be cheaper?

Good luck

[Grabcon]

I belong to both the SOHC and the DOHC forums. My guess is that the guys on the DOHC forum would recommend putting in a CB900F or a CB1100F motor in before messing with a Nighthawk motor or rebuilding the 750 motor. Those two will fit right in and have considerable more power than the CB750 DOHC or Nighthawk motors. I think the years on the 900 are 1980 to 1982 and the 1100 1983, tough to find that one.

Make sure that it is an F motor, a CB900C motor is a shaft drive motor and will not work.

[Ernest T]

Short answer is no, they wouldn't be a plug n play swap as the mounting set up is different.  There are a zillion DOHC motors and parts bikes for sale out there.

[weekend_junkie]

I belong to both the SOHC and the DOHC forums. My guess is that the guys on the DOHC forum would recommend putting in a CB900F or a CB1100F motor in before messing with a Nighthawk motor or rebuilding the 750 motor. Those two will fit right in and have considerable more power than the CB750 DOHC or Nighthawk motors. I think the years on the 900 are 1980 to 1982 and the 1100 1983, tough to find that one.

Make sure that it is an F motor, a CB900C motor is a shaft drive motor and will not work.

Excellent advice.  The CB900F was produced through 1984 in Europe, so there will be a good chance of finding a 900F motor.  The only concern will be the exhaust header as the CB900F and CB1100F cylinder blocks were taller and CB750F exhausts were often not as tall.  That exhaust looks somewhat custom, so expect 2cm less clearance between the collector and the frame.

[Danno]

check and see if it is really the rings or if the valves just need new spacers as the seats wear in the valves need to be reshimmed about every 10,000 miles  a motor needing a shim job will show low compression often fairly even across all cylinders I had a bike with this motor in it and got rid of it soon after