Topic: Early 750 output shaft bearing design change

[slyguy]

hey guys....I got a ko, wasn't the single row early style "not up to the task" hence the dbl.row and when did that take place? K2-K3-vin point? To use this dbl row in a ko, the "newer"shaft would be as well? 

[GammaFlat]

This is an interesting story related to me by HondaMan.  It seems that folks loved to "trailer" CB750's.  There were hitches that would hold the front wheel and drag the rear wheel.  We had one when I was a kid.  Much like flat towing a car, the transmission is not lubricating the main output shaft very well and you end up with bearing failure. 

Honda engineers got wind of the bearing failures and were not aware of the real reason they were failing.  They bumped up the spec of the main output shaft bearing to "fix" the problem.  Well, it didn't fix the problem and cost you about 1hp.  The "single row" bearing is fine for up to 90 hp or more.  Not to worry .

[slyguy]

so what was the "real" reason they failed? Thanks..

[HondaMan]

so what was the "real" reason they failed? Thanks..

They were being towed behind cars with the front wheel up and the rear on the ground. Since the 750 is a dry sump system, it ran the final shaft without oil pressure, burning up the bearing and then the seal. Honda mistook this as a bearing failure that they didn't understand well, so they beefed it up with the dual-row version, just in case it was because Americans were flogging it more than other country's riders (we already held the 750's world records for broken chains, cases, sprockets, and rear spokes by then).

The other bikes out there in those days were all wet sump designs. When they were lifted up to the 'Tagalong' carriers, the oil fell to the back of the engines and oiled those bearings just fine. That's the bikes they were designed for: the 750 was not one of them. But, folks never read the instruction sheets that came with the Tagalong: it said, very clearly, "Disconnect the drive chain when towing the motorcycle". Problem was, in 1971 Honda introduced the endless drive chain to help stop the 750 chain breakage issues, so this was not an easy thing for 750 owners to do. So, they didn't.

Folks today often don't quite understand this (sort of) situation with the bike because they don't understand the phenomenon the 750 was in the day: on any given summer's day from 1970 to 1975, you couldn't throw a dead cat without hitting a 750 somewhere. The streets were crawling with them, and every night you heard the bumblebee sound of groups of them, out riding together. If their owners couldn't ride them on the family vacation, they trailered them along, or bought the Tagalong (or its copies) to hook to the family station wagon's bumper to roll it along with them. It was a most unique time in motorcycling history!

[DH]

Not to hijack, but did this change affect engine case dimensions
(wider) in the front sprocket area? Is that why a lot of us with later model bikes can't fit an o ring chain without it rubbing? (later 750k)
IE k3-k6...

[Retro Rocket]

Not to hijack, but did this change affect engine case dimensions
(wider) in the front sprocket area? Is that why a lot of us with later model bikes can't fit an o ring chain without it rubbing? (later 750k)

The later 750 K7 and K8 have a 10mm wider output shaft, an O ring chain will fit easily and won't rub, its the smaller 500 and 550 that have problems with O ring chains rubbing.......

[slyguy]

so what was the "real" reason they failed? Thanks..

They were being towed behind cars with the front wheel up and the rear on the ground. Since the 750 is a dry sump system, it ran the final shaft without oil pressure, burning up the bearing and then the seal. Honda mistook this as a bearing failure that they didn't understand well, so they beefed it up with the dual-row version, just in case it was because Americans were flogging it more than other country's riders (we already held the 750's world records for broken chains, cases, sprockets, and rear spokes by then).

The other bikes out there in those days were all wet sump designs. When they were lifted up to the 'Tagalong' carriers, the oil fell to the back of the engines and oiled those bearings just fine. That's the bikes they were designed for: the 750 was not one of them. But, folks never read the instruction sheets that came with the Tagalong: it said, very clearly, "Disconnect the drive chain when towing the motorcycle". Problem was, in 1971 Honda introduced the endless drive chain to help stop the 750 chain breakage issues, so this was not an easy thing for 750 owners to do. So, they didn't.

Folks today often don't quite understand this (sort of) situation with the bike because they don't understand the phenomenon the 750 was in the day: on any given summer's day from 1970 to 1975, you couldn't throw a dead cat without hitting a 750 somewhere. The streets were crawling with them, and every night you heard the bumblebee sound of groups of them, out riding together. If their owners couldn't ride them on the family vacation, they trailered them along, or bought the Tagalong (or its copies) to hook to the family station wagon's bumper to roll it along with them. It was a most unique time in motorcycling history!

Thanks;) yea I was a teenager with my 1st bike, a ct90 basketcase..lol but became friends with a mc mechanic in school whose brother in law had a shop which also catered to the street racers so I kinda remember alot of broken cases. I also remember at some point Honda added the case guard, will they bolt on/fit my ko? seems like good-cheap ins....

[Bodi]

two things:
- the early CB750 chain failures were primarily caused by poor battery vent tube location and/or owners fiddling with that tube: it could drip battery acid directly onto the chain. The acid eaten chain would soon part.
- 70s chains were pretty feeble. Even small bikes went through a lot of chains as they would stretch - often unevenly - and ruin the sprockets as well. Also, the market was full of 2-stroke bikes - X6, A7, RD350, H1 - that would power wheelie easily; Honda 750s wouldn't do that with their forward mounted heavy 4-stroke motors, so wheelie envious owners would do clutch dump wheelies that are pretty damn hard on the chain. Current chains are much stronger than ones from that era. Get a decent quality (not the very best $$$$ ones made for 200hp superbikes, that's overkill) modern chain and it's extremely unlikely you will ever snap it with the 750's horsepower. Just watch out for the battery vent tube location and routing!

The case design is poor at the sprocket, it does pretty much guarantee severe damage if a chain breaks. The "fix" steel plate doesn't cure it, if a snapped chain gets kinked under power... in the space something is going to break. There isn't room between the sprocket and the case to fit a thick enough steel plate to really prevent case damage.

[slyguy]

Thanks guys for the clarification on this matter If I did want to...does the later dbl roller require the newer shaft as well. I believe these swaps would be up to 1976 too, correct?

[754]

I will check my R C catalog,they offered the bearing upgrade.
 The info you need will get on here soon.
 The only possible issue I see is that the seal may not be fully seated.

[HondaMan]

Thanks guys for the clarification on this matter If I did want to...does the later dbl roller require the newer shaft as well. I believe these swaps would be up to 1976 too, correct?

If you make the swap, it requires both the bearing and the final drive shaft together. Some cases have both grooves for the retainer, but the real early ones do not. On those, the case site is also not wide enough to support the wider bearing, so it's moot.

The 1-row bearing is entirely up to the 70-something HP that comes from most of these bikes, even with a big-bore kit. If you did a big-bore kit with hi-CR pistons, bigger cam, more breathing, lower gearing, and a bunch of other stuff, you might be able to finally exceed the 1-row bearing's torque capability, but on the street this would still require 90 MPH+ speeds for a long time to do it in.

Come to think of it, even on my own K2, which toured flat-out for most of 1972-75 with 2-up and all her junk aboard, mine never even broke a sweat! It was MOSTLY over 90 MPH when we'd hit the big roads.

[754]

 This out of the RC catalog
  SUPER DUTY COUNTERSHAFT BEARINGS
 Double row ball bearing to replace stock unit. This bearing will take three times the thrust of the standard bearing. Eliminates broken bearings and possible case damage. Will fit all 750, but must use K1 shaft machined to accept new bearing.
 SEND MOTOR NUMBER WHEN ORDERING

[slyguy]

Excellent info, Thanks again guys! By the way, I have 2 E-1033xxx sets of cases with the 1/2 moon retainer only. hey, where might be a good place to get both the inner & outer bearings on that shaft? And maybe 1 dbl row bearing? (I had p/u one eng. that had no shaft or outer bearing with very little damage to the case/bore).

[HondaMan]

Excellent info, Thanks again guys! By the way, I have 2 E-1033xxx sets of cases with the 1/2 moon retainer only. hey, where might be a good place to get both the inner & outer bearings on that shaft? And maybe 1 dbl row bearing? (I had p/u one eng. that had no shaft or outer bearing with very little damage to the case/bore).

I have one of the inner bearings. Would have to check to see if I still have the extra shaft and outer bearing. The inner one is new, the outer one is used.

[754]

Sounds to me like RC was cutting the shoulder back on the shaft, lraving seal in same place.

[HondaMan]

Sounds to me like RC was cutting the shoulder back on the shaft, lraving seal in same place.

I heard tell someone say that one day, in a YouTube video I stumbled across several years back. You have it right, it seems! It was a California cafe' bike builder's video, said he was using old R/C parts they had found.

[slyguy]

Does anyone have a pic of 74' cases at the output flange I could see? iirc it shows that outer bearing then a full retainer clip betweeb bearing and seal?
I also ordered my gaskets today! Vesrak kit and individual seals & head/cam pucks from the Honda dealer so my other question is...I read something from
hondaman about some o-rings being actually to small or thin in the kit(I believe cylinder or head studs? 1.9mm?) Thanks for any guidance:)

[HondaMan]

Does anyone have a pic of 74' cases at the output flange I could see? iirc it shows that outer bearing then a full retainer clip betweeb bearing and seal?
I also ordered my gaskets today! Vesrak kit and individual seals & head/cam pucks from the Honda dealer so my other question is...I read something from
hondaman about some o-rings being actually to small or thin in the kit(I believe cylinder or head studs? 1.9mm?) Thanks for any guidance:)

Do you have my book? It has all this stuff in it.

[slyguy]

No Mark I do not. So Honda stopped using the "nick"grooved bearing when approx.? K2? and then didnt they use a smooth bearing that was still narrow for a time before the dbl wide? I see the smooth still incorporated the "ring-bearing set" which I assume fit into both case halves and held the assy in...Thanks for all the informative stuff so far:) I just want to get a solid understanding of all these output shaft bearing design changes & when...

[HondaMan]

No Mark I do not. So Honda stopped using the "nick"grooved bearing when approx.? K2? and then didnt they use a smooth bearing that was still narrow for a time before the dbl wide? I see the smooth still incorporated the "ring-bearing set" which I assume fit into both case halves and held the assy in...Thanks for all the informative stuff so far:) I just want to get a solid understanding of all these output shaft bearing design changes & when...

The cases were grooved on and off until the K4. I am not sure why, but from these forums I think the K4 in places like Europe still received the [less expensive] single-row bearings for quite a while. I get e-mails from "over there" and "down under" sometimes, when they come across it in their engines and my book, and they have such things as single-row bearings with the retainer ring, and extra unused case groove, and the wrong output shaft seal because they bought the post-1973 Seal Kit. I think the US riders' abuse caused the 2-row bearing to show up here, first, and other places later on?

By the time of the K5, they were all dual-row everywhere, and the cases only fit that later version, having a much wider saddle for the bearing. Still, if you go to the trouble of adding the groove(s) and changing the output shaft-bearing set, you can reclaim the lost HP for your effort. It does create a lot of drag at high engine speeds, because the cases twist a lot near redline.

[slyguy]

Thanks, I'm trying to find out if/how I can run the dbl.roller bearing? My cases have the 1/2 groove in the upper case. I p/u a dbl roller and shaft from a 74'I believe which has the full ring which I assume the later cases were cut for?
I'm not sure how an old timer use to put these in the early cases? Unless he simply left the full ring out? Its so confusing/its so long ago/Honda made so many changes with these in such a short time.... After the grooved bearing, they went with a smooth bearing which was still thin and iirc had the full "ring" which I assume had to set in a groove all the way around the case....then went with the dbl roller.

[Dream750]

Perhaps this Honda service bulletin can help with the early/late bearing and crankcase part number changes?

http://manuals.sohc4.net/CB750SB/750_29.pdf

[slyguy]

Wow, thanks! This SB says it all!...fer now...lol

[Kickstart]

Perhaps this Honda service bulletin can help with the early/late bearing and crankcase part number changes?

http://manuals.sohc4.net/CB750SB/750_29.pdf

I don't think this refers to the single vs. double bearing change.
That bulletin was published in 71, and I don't believe the double bearing was introduced until 72 (or 73).

The bearing changed from a single bearing (6305) to a double bearing (5206). But the bulletin talks about a new 6305 bearing (which would still be a single row).

I hope the K2 I purchased is a single row... that's one of the reasons I got it.  If not, hopefully it has the extra groove.

[slyguy]

Thats interesting...I know after the single row grooved...they went to a non-grooved single row which moved the case groove to a full and to the outside of it which I assumed was the change at 1034807 until 10044805 then the double roller after that.  So more to my point..the mod to the dbl from RCengineering and others must have only been available to those 10,000 cases with the single row/non groove. So it seems I personally can now put this baby to rest since I don't have any of those cases/engines. Now I can continue to move in a forward direction with my engine refresh! Thanks guys for all the informative knowledge with this:)