Author Topic: Clutch hub/center differences for CB750 years? (Read 1102 times)

fxef79 · « **on:** March 23, 2020, 05:39:58 PM »

I’m considering the clutch hub/center mods (drilling additional oiling holes). I figure safest way is to buy a used hub and drill it, in case I screw up, so I don’t screw up my existing one.

This is a 1972 engine. Is there any substantial difference in the hubs from 1972 through 1978? Supposedly the same part, but sometimes there are “unofficial/undocumented” changes over the years. There’s plenty of used hubs on eBay, of various years. Looking at a ‘78 hub in good shape. Just want to know before I pull the trigger whether there’s any particular reason a ‘78 hub wouldn’t match.

Thank you!

754 · « **Reply #1 on:** March 23, 2020, 05:45:35 PM »

77 and 78 clutch basket has difference I believe. And the clutch cover too because the late hub is longer.
I think it involves a different plate that the other may not use.

HondaMan · « **Reply #2 on:** March 23, 2020, 05:59:50 PM »

754 has it right: the K7/8 hub will not fit in the K2 clutch properly.
Modifying the hub for better oiling can be done 2 ways: one way is to add 4 more oiling hole. The other way is to grind a shallow "Vee" shape (as shown in my book) to the left of each of the 2 holes in the middle portion of each quadrant. This widens the capture area of the oil that slides along the inner surface of the hub, directing it into the holes faster. The inner 4 holes almost always have some flash casting over them that cuts off nearly 60% of the oil in the back of the hub, so use a small (tiny?) grinder bit to lower those to the same height as the hub surface.

There are many posts around here to show how this is done, some even copied from my book.

fxef79 · « **Reply #3 on:** March 23, 2020, 06:02:38 PM »

Quote from: HondaMan on March 23, 2020, 05:59:50 PM

754 has it right: the K7/8 hub will not fit in the K2 clutch properly.
Modifying the hub for better oiling can be done 2 ways: one way is to add 4 more oiling hole. The other way is to grind a shallow "Vee" shape (as shown in my book) to the left of each of the 2 holes in the middle portion of each quadrant. This widens the capture area of the oil that slides along the inner surface of the hub, directing it into the holes faster. The inner 4 holes almost always have some flash casting over them that cuts off nearly 60% of the oil in the back of the hub, so use a small (tiny?) grinder bit to lower those to the same height as the hub surface.

There are many posts around here to show how this is done, some even copied from my book.

Yes, thank you... I have been reading those, and those are the mods I’m planning to do. Thank you and 754 for confirming the hub differences.

So I’ll limit my hub selection to something K2-K6.

fxef79 · « **Reply #4 on:** March 23, 2020, 09:45:15 PM »

FWIW, from photos only, what are the hallmarks of a clutch hub to stay away from? Clearly I can't physically inspect something on ebay, and everyone describes their items for sale in the most positive terms. I would think obvious clues would be cracks, of course, and also significant wear on the splines from the teeth of the plates. Other than that, anything in particular to look out for?

fxef79 · « **Reply #5 on:** March 23, 2020, 10:42:42 PM »

Quote from: HondaMan on March 23, 2020, 05:59:50 PM

Modifying the hub for better oiling can be done 2 ways: one way is to add 4 more oiling hole. The other way is to grind a shallow "Vee" shape (as shown in my book) to the left of each of the 2 holes in the middle portion of each quadrant. This widens the capture area of the oil that slides along the inner surface of the hub, directing it into the holes faster. The inner 4 holes almost always have some flash casting over them that cuts off nearly 60% of the oil in the back of the hub, so use a small (tiny?) grinder bit to lower those to the same height as the hub surface.

There are many posts around here to show how this is done, some even copied from my book.

Book is on the way!

fxef79 · « **Reply #6 on:** March 29, 2020, 05:58:09 AM »

Received the “sacrificial” hub today. Before I start drilling or grinding on it, or on my currently installed hub, what’s the easiest way to confirm it is, in fact, a “K2 compatible” hub, vs later (77/78) hub? Critical dimensions I should check?

PeWe · « **Reply #7 on:** March 29, 2020, 08:01:01 AM »

The later K7/K8is around 2mm longer than the earlier

fxef79 · « **Reply #8 on:** March 29, 2020, 10:41:20 AM »

Quote from: PeWe on March 29, 2020, 08:01:01 AM

The later K7/K8is around 2mm longer than the earlier

Thank you.

scottly · « **Reply #9 on:** March 29, 2020, 01:05:24 PM »

I have a K7 clutch hub I can measure, so you can check the one you just got. In the meantime:
http://forums.sohc4.net/index.php/topic,7190.0.html

fxef79 · « **Reply #10 on:** March 29, 2020, 02:08:15 PM »

Quote from: scottly on March 29, 2020, 01:05:24 PM

I have a K7 clutch hub I can measure, so you can check the one you just got.

That would be awesome!

Quote from: scottly on March 29, 2020, 01:05:24 PM

In the meantime:
http://forums.sohc4.net/index.php/topic,7190.0.html

^^^^^^^^^^ This!

This is exactly what I get the second hub for, so I can try my hand at doing this exact mod, but without dorking up my working hub if I go completely apesh*t with the drill and/or dremel.

I have a problem with not being able to get into neutral when it's running and heated up. I understand this mod can help with that, as well as replacing the 4 o-rings discussed here (three on the oil pump, and the countershaft oiler):
http://forums.sohc4.net/index.php/topic,78643.msg877816.html#msg877816

Just got my o-rings and oil pan gasket in the mail today so I can do that part. Awaiting delivery of Hondaman's book (this is worse than waiting for Christmas to open presents!) so I can see the detail of his mods to the existing oil holes in the hub.

fxef79 · « **Reply #11 on:** March 29, 2020, 02:11:03 PM »

If it were me riding this thing all the time I wouldn't be so anxious to sort out this neutral thing... I can hold a clutch for a long time (try riding around Daytona beach during bike week on a '74 Sportster - never could get neutral to work right on that thing). Or just remember to slide it into neutral while still rolling to a stop.

But this is the wife's trike, and I don't want to worry about her having that trouble.

scottly · « **Reply #12 on:** March 29, 2020, 03:55:21 PM »

From the top of the splines to the friction surface I measured 1.730".

fxef79 · « **Reply #13 on:** March 30, 2020, 09:18:19 PM »

Quote from: scottly on March 29, 2020, 03:55:21 PM

From the top of the splines to the friction surface I measured 1.730".

Measured the hub I got, which is supposed to be a '72.
top of splines to friction surface I measured 1.610" (40.90mm).

Your 1.730" is about 43.9mm... so consistent with "around 2mm longer" (3mm actually).

So I'm comfortable what I received is properly an earlier hub.

Feel better now about pulling the existing one and beginning to drill/chamfer one or the other. Although I'm still going to hold until I get Hondaman's book to see his approach to the "v-slots".

fxef79 · « **Reply #14 on:** April 05, 2020, 07:01:08 PM »

Wrote this reply 3 times, and every time when I post the whole thing goes away... Might be annoying, but I'll break it apart so as not to lose all of what I typed.

First, found out by digging in parts box I got with this trike I already had another hub. A whole clutch pack, in fact (hub, 7 square corks, 6 metal discs, and rear plate).

Presumably this is the clutch the PO told me he had replaced after riding the trike once, and before he sold it to me. Or maybe he just got it with the trike. Dunno.

It measures the same as the hub I bought... splines all measure out at from 40.90 to 40.99 mm.

fxef79 · « **Reply #15 on:** April 05, 2020, 07:04:17 PM »

Pulled apart the clutch on the bike... had some "dangit!" moments and some "WTH" moments.

First, the lifter plate has the tip of one arm broken off. Although, it works, so not sure just how long I can live like this. Interestingly, that arm that has the broken tip also had a bolt in it that feels "tighter" than the other 3 bolts. The other three bolts loosened to "finger loose" before this one did... and this one bolt is also lighter colored than the other three.

fxef79 · « **Reply #16 on:** April 05, 2020, 07:08:47 PM »

Next "WTH" moment... ALL the cork discs in the clutch are slant cut... and facing different directions. The inner two and the outer two spin one direction, and the middle 3 the other direction.

BTW, also this pack, and the one in the parts box are 7 corks and 6 steel discs, with no "double disk" nor discs with larger tangs, and the basket does not have wider slots to accommodate larger tangs... so all of that, plus the hub spline measurements (the one on the bike same as the other two) confirms an "early" clutch. That's what it "should" be... but the rest of the frankenstein nature of this engine now makes me wonder what I'm going to find when I open things up.

fxef79 · « **Reply #17 on:** April 05, 2020, 07:14:18 PM »

Next - the hub that came out of the bike has casting marks like "R|K" and a "3". So does the hub I just received. But the hub that was in the parts box has different casting marks. It has "R|K" flipped the opposite way, a "4", and then in the next quadrant "300".

Is this significant in any way?

fxef79 · « **Reply #18 on:** April 05, 2020, 07:20:40 PM »

Wow... why are ALL the CB750 clutch lifter plates for sale in Germany?

bryanj · « **Reply #19 on:** April 06, 2020, 12:03:43 AM »

I would say i got a couple i think but problem is i cant get to them for 11 weeks!!

fxef79 · « **Reply #20 on:** April 08, 2020, 06:36:44 PM »

Got confirmation today I have one on the way from MI. Actually a whole clutch, basket, hub, sprockets, disks, springs, and the lifter plate.

News:

Author Topic: Clutch hub/center differences for CB750 years? (Read 1102 times)