Topic: Transmission install and cam gear question - CB750

[Eydugstr]

I'm working on a '72 CB750 motor...Getting ready to install a new transmission (was slipping out of 3rd & 4th gear really bad...).  Is it possible to replace the transmission by flipping the motor upside and removing the bottom crankcase, without having to remove the pistons from the cylinders?

Also, can a cam gear from a '77 or '78 cb 750 be used on a stock '72 cam?

Thanks for you time.

[City Boy]

Hi.Yes ,the trans can be repaired without removing pistons or anything else in the top end.Yes,the cam gear from any year is usable on any year cam.

[Terry in Australia]

Yeah mate, easy peasy, I did this myself this weekend, a guy brought his K2 over because the countershaft seal (behind the front sprocket) was leaking badly, so I had to split the cases, but it's had a recent top end rebuild, so I didn't want to disturb everything up there.

Just flip the engine over and unbutton it, (all the external covers, gearshift mechanism, 8mm and 6mm bolts on top and underneath the cases) then just tap around the cases with a plastic hammer until they separate. Use some Threebond 1194 on the case halves, and while you're at it, replace the engine seals.

You'll need to re-torque the main bearing studs to 18 foot pounds (start at 12 and go up in increments of 2) and be careful when you take them out, I broke one and had a "heart in my mouth" time getting it out, but I was lucky and it came out in one piece. Here are some pics. Cheers, Terry.  

[Don R]

There are bolts that are hard to find, look carefully and take your time. We left the entire top end on to repair ours.

[Eydugstr]

Okay - TY City Boy, Terry, Don - Will tackle it this way, instead of risking buggering up the rings.

I am assuming that I'll need to replace both the mainshaft gear cluster (the one with the clutch) as well as the countershaft tranny assemblies - Is it okay to swing the mainshaft up and then lower the shaft until it clears the primary chains?

[Terry in Australia]

Yeah mate, you can remove and replace the entire transmission like that, no problemo. Cheers, Terry.

[yesteryear]

Just make sure when closing the cases you locate the gear change fork,test all gears 1 2 3 etc and replace the seal around the big dowel pin at the front of the crankcase.
Regards
yesteryear

[Eydugstr]

TY-will do.

[lucky]

I'm working on a '72 CB750 motor...Getting ready to install a new transmission (was slipping out of 3rd & 4th gear really bad...).  Is it possible to replace the transmission by flipping the motor upside and removing the bottom crankcase, without having to remove the pistons from the cylinders?

Also, can a cam gear from a '77 or '78 cb 750 be used on a stock '72 cam?

Thanks for you time.

You do NOT need to replace ANY of the gears.
You only need to replace the bad shifting fork.

It is not like a car it does not have syncros.

[Eydugstr]

Thanks for bringing that up, Lucky.  I'll double check the forks once I get the assembly out.  Unfortunately I think it's a gear, though.  Was in a rush getting the bike together for the SOHC rally, and wound up having to use a gear that was borderline according to the books.  Once I get everything apart will post some pics.

[mec]

I'm working on a '72 CB750 motor...Getting ready to install a new transmission (was slipping out of 3rd & 4th gear really bad...).  Is it possible to replace the transmission by flipping the motor upside and removing the bottom crankcase, without having to remove the pistons from the cylinders?

Also, can a cam gear from a '77 or '78 cb 750 be used on a stock '72 cam?

Thanks for you time.

You do NOT need to replace ANY of the gears.
You only need to replace the bad shifting fork.

It is not like a car it does not have syncros.

you need to replace the worn parts, shift fork as well as gears.

mec

[Eydugstr]

Okay...finally got everything apart.  Came away with more questions than answers, but figured I would post some pics and get some more opinions before I started replacing stuff...

After getting the cases split, started checking the shifter forks.  Granted, I didn't have a brand new set to compare them with, but when I bought another transmission (a '74) on ebay, the seller sent me the shifter forks as well.  So I started measuring and comparing them both...and the measurements came out to be almost exactly the same.  So I then started checking the tips of fork that go into the gear selector drum...both of them look to be in the same shape, no fried ends or gouges on the tips that go into the drum...
'72

'74


So I moved onto the gear assemblies.  The one on the right is the old assembly.


View from top


So far the small chips out of the connecting dogs are the only bad thing I can find, is this enough to cause a tranny to jump out of third/fourth gear?

[mec]

it is pretty sure, that this causes the tranny to jump out of gear.

you have to replace this gear anyway. the same to the gear next to this gear, where the dogs mesh, because it is worn as well.

mec

[Eydugstr]

TY, mec ...at least I think I'm on the right track.  One last thing, before I start putting it all back together...The shifter drums.  According to Hondaman's book, the K3 & later ones made it easier to find neutral.  Going by the pictures it's hard to make a judgement call on any of them.  Does anyone see anything that might be a red flag? The fork "paths" on the K4 & K2's seem to be "beveled" more at the top than the K3 - Is that just from extreme wear?

The one at the top is a K3, center is a K4, and the bottom one is a K2.