Topic: Resto for cb750k8

[HondaMan]

I see that a lot in the K7/8 and the F2/3 engines' clutches. I suspect it was part of a quick-and-dirty heat-treat that was done to the hubs. Some are only yellowish, most look like yours.

About the rear wheel: these clutches do not disengage like an automotive dry-plate clutch, never. Instead, when you pull the clutch lever, a few moments later the spinning hub lets oil that is collected in the inner parts of it (and the inner edges of the plates) slide in-between the cork squares and the metal plates, which disengages the friction. It is only a partial disengage, though: I have not ever been able to spin the rear wheel on a not-running engine by pulling in its clutch lever. Occasionally I have been able to hand-turn the rear wheel while someone else held in the clutch lever on a warmed-up engine, but with considerable drag.

Nature of these beasties.

[letmwhirl]

I see that a lot in the K7/8 and the F2/3 engines' clutches. I suspect it was part of a quick-and-dirty heat-treat that was done to the hubs. Some are only yellowish, most look like yours.

About the rear wheel: these clutches do not disengage like an automotive dry-plate clutch, never. Instead, when you pull the clutch lever, a few moments later the spinning hub lets oil that is collected in the inner parts of it (and the inner edges of the plates) slide in-between the cork squares and the metal plates, which disengages the friction. It is only a partial disengage, though: I have not ever been able to spin the rear wheel on a not-running engine by pulling in its clutch lever. Occasionally I have been able to hand-turn the rear wheel while someone else held in the clutch lever on a warmed-up engine, but with considerable drag.

Nature of these beasties.

Very good to know. Thanks millions for your response!

[Dime]

Great work! Lots of great info being brought up. Taking notes

[letmwhirl]

Update...
I have managed to get the engine taken down to the cases. All exterior housings and covers are awaiting work from the coatings company. Here is a link to his webpage when interested... https://www.thermotec-coating.de

Rims, hubs and spokes still at the bike shop. He has honed the cylinders and ck'd them,,, all ok. Pinstons are still there.
I could tell that someone else was in the motor before.. The head gasket area was scraped in areas where I know I had not been. Also there was a recessed area in the middle of the cylinders just under the head where somone drove a tool in between trying to get them separated.

I have the fork lowers back from coating in addition to those previously mentioned.
I have the following list of parts ordered with delivery status

Ordered    Received   Item           Cost+Shipping           Seller
            
11/4/24   11/5/24   3 Oil Filters          21.01            u-k motors
11/13/24   11/15/24   Air Filter                  17.57           motorrad ketten
11/13/24   11/27/24   4 Carb Kits          20.79           gettingrich6
11/13/24   11/16/24   4 Intake Boots         184.93   classic-bike-parts
11/13/24   11/16/24   4 Carb Boots           54.11   classic-bike-parts
11/19/24   11/21/24   3.5mm hose             7.80   orbach-motorrad
11/21/24   11/28/24   Tire Label                   12.49   hielorojo
11/27/24   12/1/24   Brake Piston&Seal   63.00   classic-bike-parts
11/27/24   12/1/24   Mstr Cyl Kit            39.90   classic-bike-parts
11/27/24   12/1/24   Fork Seals                     5.21   classic-bike-parts
11/27/24   11/21/24   Handle Bar           67.79   andy.cepok
11/26/24   11/27/24   Motor Screw Kit           17.85   screws4bikes.de
11/28/24   11/30/24   Engine Gasket Set   74.80   andy.cepok
11/28/24   11/30/24   2 Dowel Pin             7.60   andy.cepok
11/28/24   12/1/24   Tacho Seal                9.49   andy.cepok
11/28/24   11/30/24   Shock Grommets   17.99   classiccyclecity
11/28/24   11/30/24   Engine Seal Kit           27.49   mvh_racing
11/28/24              Reg/Rec                   28.79   mosel505
11/25/24   11/27/24   Coating                   300.00   thermotec-coating.de
12/2/24   12/4/24   Starter Relay           13.59   motorcycleparts
11/30/24   12/4/24   m-unit                   302.90   caferacerwebshop
12/4/25     12/5/25    Pri Chain Set              156.50     BikeSide.de

More later...

[letmwhirl]

Another Update...
Today, one of my good friends and I installed the AMC20 rear axle in another friends very beautiful '85 Jeep CJ. We will not test drive it until late spring when weather is much nicer and no salt is left on the roads.

I was able to get the mirrors, switches and levers off of the old handle bars today. I also rebuilt the master cylinder.. So a little progress..

[letmwhirl]

Yet another update…
Things are progressing slowly now.. after getting back parts from the bike shop last week the coating guy has completed a few parts as shown in photo. Some parts were previously mentioned. The engine cases will not be coated until sometime in January. Upon receiving them, I will need to ck the crank and rod clearances and make any necessary adjustments before assembly.

Also, if anyone has this badge that attaches under the headlight please let me know. Thanks to all.

[letmwhirl]

Oops.. forgot to add photos from wheels and tires.

[denward17]

Looking good whirl.....

You could try paint stripper on that badge, then sand the letters with 1500/2000/2500 grit to see if they could be polished.

Repaint the badge behind the letters?

[letmwhirl]

Looking good whirl.....

You could try paint stripper on that badge, then sand the letters with 1500/2000/2500 grit to see if they could be polished.

Repaint the badge behind the letters?

👍🏻great idea, I think I will pick up some stripper that will not harm plastic. Then go-to-town on it.

[denward17]

I don't think that badge is available anymore, but not sure.

Orion Silver is a good color if need to spray silver....see badges I just painted.

[RAFster122s]

The orange citristrip is likely a plastic safe stripper, test on backside and rinse thoroughly.

[RAFster122s]

The plastic modelers will sometimes have access to inexpensive platers for plating/replating plastic. Some guys are setup to do in home plastic plating. Might want to pursue that through plastic modeling forums if you cannot find an acceptable solution.

[letmwhirl]

I just found that it is plastic covered with foil that is either heated or vacuumed down over it. PITA to remove. Great tip on Plastic Modelers. I will see if I can find one here. Thanks so much.

[HondaMan]

About reassembly: the 2 oil port holes on the cylinders' deck will need thicker O-rings than those found in your gasket set. Use size 2.6x10.8, Parker #02-111. If you're weeks away from that, PM me your address and I'll send you a pair. I also have an Oring 'set' for the little caps on the engine (not found in any gasket set) if you're interested: I could send those at the same time, but you'll need to donate about $7 (USD) to the forums for that kit: it's something I make for their support.

[letmwhirl]

Hello all! I hope you all had a very nice Christmas and a wonderful New Year’s celebration.
I picked my crankshaft yesterday from the bike shop after he polished the journals. He said I need 1 size over for the center (#3) crank journal and 1 size over for all Rod journals. Also the other four crank bearings should be replace as is.

I have included a snapshot of what bearing colors were in the engine upon disassembly. If any of you have these bearings or know where to get them please respond.

Happy 2025 everyone!!
Ronnie

[letmwhirl]

Here are the crankshaft measurements after using a micrometer set. The case had 5 B‘s stamped for the main journal O.D.

The color of the bushings are indicated by the letter G or B adjacent to the journal and measurement.

So what would be the bushings I should order??

Ronnie

[letmwhirl]

I have ordered 6 brown and 4 black main journal bearings. I have used plasti-guage to measure the 4 existing brown rod bearing clearance. Take a look and give me your advice if I should replace. There exists a couple shiny spots as well.
I loaded the pictures in sequence from piston 1 to 4.

[denward17]

I wish I could help, but haven't used plasti-guage before.

Following to learn.....

I see it appears you are measuring the width of that substance on the bearing, what does that tell you?

[BenelliSEI]

I wish I could help, but haven't used plasti-guage before.

Following to learn.....

I see it appears you are measuring the width of that substance on the bearing, what does that tell you?

Den…. You lay a strip of the plastigauge (imagine a thin piece of plasticine) across the crank journal and torque down the bearing. A bit of oil lube is important. When dismantled, you compare the width of the “squashed” strip to the chart on the packaging. That tells you the bearing to crank clearance. Surprisingly accurate product that has been around forever……

[letmwhirl]

As I see it.... and of course, please correct me if I have stated something inaccurate.

With the crankshaft bearing issue, this all boils down to 4 things, two sets of measurements and two charts in the Repair Manual. 1. Size of the Case or Rod bore where the bearings are inserted. 2. The size of the main or rod journals on the crankshaft. 3. Crankshaft Main Journal Chart 4. Crankshaft Rod Journal Chart
Once you have these measurements and referencing the two charts, you could concieveably determine the correct bearing size. Each case boring for the crankshaft has a size indication of A, B or C and determined at time of the boring and stamped on the front of the upper case. The Rod bores for the crank end are also measured and each rod has it's size stamped on side of the rod. These sizes are indicated as 1, 2 or 3. The crankshaft journals were also measured at time of machining and indicated on the crankshaft by a stamping near the primary chain teeth, but it is difficult to read and the journals wear over time. So it is best to measure and record each journal.

Refer to the book for correctly identifying which case-stamp letter applies to the appropriate journal, then go to the chart with letter and measurement and calculate the correct bearing.

These bearings are expensive. One has to weigh this cost against your findings, your pocket book and what lengths you are willing to go for this motor. In my situation, I have just purchased 10 new main bearings, based on the case stamping, measurements taken and reference to the bearing colors which were installed (no platigage of installed bearings prior to purchase). However, I will plastigage the new bearings when they are delivered.

I am leaning toward reusing the rod bearings since they are at the edge of standard specs. I do not ride hard and only once a week (if that) during peak season. I can use a little thicker oil if pressure is lower than desireable. Please weigh in if you have strong opinions against my feelings at this point.  Thanks to all for reading. I hope this is not 'boring'.. hahaha
Ronnie

[BenelliSEI]

The rod bearings are the ones that take a beating. Imagine hanging on to the big end of a rod going up and down 50/second (3,000 rpm). Opening a bottom end has become extraordinarily expensive, but now you’re there, I’d change ‘em.

[letmwhirl]

Thanks Ben for your recommendation. I will definitely take this to heart. I have done so much to this bike to get it clean, rust free and even more coming..

I was wanting to ask the forum, regarding the rod bearings. I will be flying back to the states and visiting my daughter in Spring, TX and my son in Seattle during upcoming March. If anyone has some spare brown rod bearings, I would be willing to entertain a purchase and shipping could be within the states. I can get them here in Germany but they are expensive and I have to add shipping, import and taxes if they come from the states. Just a thought...

I likely will not start on the assembly until April. That is about the time Spring starts up here and my wife will have Garden Honey-do's for me.. and likely more than I can handle in a month. That's life.

[Honda Hansel]

Hi Letmwhirl
Main and Rod bearings are readily available from CMSNL.com
all sizes available and colors
fast and easy shipping

Honda Hansel 

[letmwhirl]

Thanks Hansel.. I use the internet quite a bit looking for items. So well aware of cmsnl.. I have purchased quite a bit from them for this bike as well as a 93 xj900f which I am customizing. They have access to quite a good supply of classic parts. There are a couple resources in Europe as well as here in Germany. But these types are all retail.

I was soliciting for anyone who might have unused spares no longer needed. I will also look on ebay. (.com, .de, .it, .uk, etc)

Have a great day,
Go Texans!! maybe, hopefully.

[letmwhirl]

Engine  Build Update:
Finally received the main journal bearings and installed after another plastic-gage exercise. Clearances looked nice and close to one another. Rod bearings were in spec for being reused. The transmission gearing cleaned and reinstalled, along with main drive gear and kick start gear. Next day I test fit the upper case onto the lower case. Everything looked nice. Alignment pins and o-ring in place, and with Hondabond applied the cases went together after a little alignment with the transmission bearing rings. Torqued all bolts and moved onto the gear shifting mechanism.  Charging electronics installed on the left bank. I was set back a couple days waiting a new order of rubber seals. The original order indicated it covered 69 to 78 cb750k models, but when I tried the output shaft seal, I knew it was too small. Finally the new arrived and was installed then both left side covers were installed. Yesterday the rings were clearance checked and installed on the pistons, which were also installed on the rods. The studs were then installed in the upper case halves. Forgot to mention, the clutch assembly was also installed along with a cleanup of the points, points plate and mechanical advance. All installed with covers. Today I am lapping the valves 🤞. The cylinders are ready and waiting.