Author Topic: Swapping engine cases (Read 1257 times)

DickL · « **on:** February 07, 2016, 06:28:14 AM »

Is it possible to swap the engine cases between a 75 and 70. I recently bought a 1970 CB750 that has a 1975 motor. I would like a 70 motor on the bike. Can I buy a 70 case and swap it and rebuild the 75 engine with the 70 case. case? Is it physically possible. Thanks and I think this a great board!

Cqyqte · « **Reply #1 on:** February 07, 2016, 06:36:54 AM »

I am thinking not, just reading the changes from the earlier CB750 engines through to the K5's I suspect there will be interference with some pieces. But I could be all wrong... $:-\$

mystic_1 · « **Reply #2 on:** February 07, 2016, 07:05:20 AM »

Most of the parts will interchange just fine.

There were significant changes in the final drive shaft and bearing between these years so you'd need the earlier assembly.

Here's a little data I came up with while rebuilding my K0 motor using a replacement crankcase:

Quote from: mystic_1 on February 23, 2015, 06:35:01 AM

There are at least three variations on the CB750 final drive shaft. The differences are in the outboard bearing, and the auto-chain-oiling mechanism.

The new crankcase is machined to accept the later-style final drive shaft with the dual-row bearings, so I sourced the appropriate shaft and gears.

The earliest final drive shaft had a single-row bearing, with a notch for a half-circle locating ring.

The next iteration of the shaft had a single-row bearing with no notch, and used a full-circle locating ring outboard of the bearing.

The third iteration of the shaft had the double-row bearing, no notch, using the same full-circle locating ring.

In all three instances, the location of the outer surface of the bearing is unchanged. The third variation of the shaft moves the bearing inward to accommodate the additional width of the bearing. You can also see that in version 2, the shaft has been beefed up in between the bearing and the gear.

As far as I can tell, the first two versions of the shaft should be completely interchangable. The third variation of the shaft is different and cannot be used with the single-row bearing setup. The shaft required depends on how the crankcase has been machined.

Don R · « **Reply #3 on:** February 07, 2016, 08:46:42 PM »

Thanks for that info. I have an F2 trans that I'd like to use in a K2 case. The pics make it clear. I too have been watching for a case appropriate for a 14,000 to 14,500 or so frame number. (shameless hint)