Topic: CB 750 K0 — Me and the dog are building a bike. Finished!!

[calj737]

If you use Franks style of wells frame kit, then the center brace triangle also needs to be "removable" where it connects to the side rails. If you weld in the stock piece shown above, then how are you planning to connect it to the side rails? Something to think through...

As for the brat seat and tire clearance, you will have conflict under suspension compression. The flat stock brace that loops up and across allows for the inner fender and outer fender and clearance for the tire. Flat seats are not comfortable for long rides, offer little to no rider support during handling, and create space problems for your tire. All in all, a bad design and a poor modification.

Replacing the flat brace with a hoop solves the structural issue, but you still need to resolve clearance problems. So either the loop closed end needs to be long enough to allow the tire to pass within it, or the shock length needs to be increased, thereby changing some steering geometry. Or, you reduce the wheel diameter changing handling characteristics. Decisions, decisions, decisions.

[754]

The center brace gets welded back in , then cut on the sides to allow it to pull out.

I been thinking on good shocks or maybe most the top eye should unscrew.. Then you add longer snubbers reducing shock travel... ... Solves some of the proble.s with brat style seats and frame mods..
 Be a world better than struts... Which I have ridden a long miles day on.. Ouchh...

[dolamike]

My plan is to install the kit, weld the bracket, cut the bracket. Should work out, we'll see!

While I'm at it, I'm thinking about trying to weld the rear peg sections back on the frame. Anyone have any experience with this? I think it would give me more options for mounting exhaust etc. And if I wanted to do a restoration some day, I'd have a good frame to start from.

Anyone know of a donor frame with the correct sections? I'd love some help with this, the donor frame I have has the newer style.

[dolamike]

Good luck tonight! Looks like I found the missing frame pieces I needed to get the frame closer to original. Thanks gschuld!

http://forums.sohc4.net/index.php/topic,153257.0.html

I did some filing and clean up today as well, getting ready to start welding the bracket back in. Careful not to file the frame...!

[dolamike]

Project Update--Engine Tear Down

The frame is prepped and ready, as soon as I get the frame kit and rear peg sections she'll be ready for welding.

Time for the engine work.

I mentioned a few posts ago that I had a setback with the engine. As I tore down the bike and started to remove the chain I noticed something strange, the front sprocket was completely caked with oil and dirt, and as I poked at it to clean it I noticed the below...





It seems sometime over the last 45 years someone's chain broke and blew a hole in the engine cases. It was patched with fiberglass and eventually worked itself loose. Total bummer. I'm working with a forum member now to source some new engine cases, I will eventually rebuild this motor but for now I'll set it aside.

Luckily I was able to find an engine 2 1/2 hours from my house in pretty decent shape at a pretty good price. The numbers won't match the pink slip, and it's not the original motor but she'll be back up and running sooner rather than later. Today I did the tear down of the top end. the engine seems solid, PO says the compression was good and I didn't find anything too terrible when I opened it up.

My plan is to rebuild the top end, new gaskets and seals, piston rings and hone the cylinders.

Would you guys lap the valves? What's the best way to clean this up? Any advise would be much appreciated.

Photos below:

[mrbreeze]

It doesn't matter if the motor # is different as far as pink goes because it should be registered off of the frame # in Calif. If your new motor isn't out of a 77-78 shouldn't be any problems. They used a 630 drive chain so will need to swap sprockets to fit.

[dolamike]

Thanks for the info, it is indeed a 78 engine, looks like I will need to pick up a chain and sprocket kit. I live about a half mile from 4into1, they have this one...

http://4into1.com/chain-sprockets-kit-honda-cb750a-cb750k-1977-1978/

I might look around for a better deal if I can.

Thanks again!

[mrbreeze]

I'm not sure if you will only need sprocket or offset sprocket....I only know they are different. Someone more in the know will come along and tell you for sure. There are other differences in the head but that won't keep you from swapping the 78 motor into the older frame.

[kmb69]

Thanks for the info, it is indeed a 78 engine.....

Curious, what is the casting number on that 78 head?

[dolamike]

It says 392.

[kmb69]

Thanks. That's what I thought it should be but there are casting features on the bottom that looked similar to a 410. There are apparently variations in the 392 castings that I was unaware of.

[754]

Should work if you swap your sprocket shaft and the gears that drive it.. Or run the wider sprocket mount for room for bigger tire.
 Your old cases can be fixed.  If you find someone competent .
 The frame kit. Is on its way ...  I would get the sleeves  on before you weld the brace. If they dont slide on the new section, you may have to slit the lower part of brace..then weld later... Can be slight differences between frames.

[dolamike]

Just curious, the hole in the case goes across both upper and lower case where they meet. In the past when you have seen a weld fix the problem was the case permanently welded shut? Or were both the upper and lower halves patched and them machined to seal?

I did some reading, looks like I will try the 10mm offset rear sprocket from Cycle x, did anyone find a 520 front chainring for the 77/78 motor? 18 front 46 back will be a gear ratio of 2.55, cool?

[kmb69]

.....
Or were both the upper and lower halves patched and them machined to seal?
.....

That's the best way to fix them if you can.

[754]

 Cheaper to run spacers and bolts than buying a sprocket, or change sprocket carrier.. Chain will be very close to shock.

[HondaMan]

I'm intersted in hearing opinions on the flat "Brat" style seats that seem to be very popular on these bikes these days. Pic below...



I'm considering one for my build which seems to mean that I'll need to chop the fender mount at the back of the frame. Pic below...



It looks to me like I'd be losing 4-5inches of tire clearance in doing so. Does anyone have first hand experience with this? Is your back tire hitting the seat pan?

Ive seen other options on ebay that aren't as thin but seem to keep the fender mount and tire clearance:

http://www.ebay.com/itm/1969-1978-Honda-CB750-K0-K8-low-profile-cafe-racer-sport-seat-CODE-D1029-/111797923851?hash=item1a07acf00b:g:wQsAAOSwrklVN4F1&vxp=mtr

Any opinions on these? Any info would be greatly appreciated. Thanks again!

They make my nuts hurt, in even a short ride.
Just sayin'...

[dolamike]

The frame kit arrived. I think this is going to work out nicely, thanks to everyone for the help and guidance on this project!

[700504]

Looking good, you sure move fast!

[dolamike]

Quick question, will the 71-76 carbs work on a 78 motor without too much frustration? Should I try to track down a set of 78 carbs to keep everything simpler? I know Honda made some changes on the 77-78 carbs...

Any help would be greatly appreciated!

[Roach Carver]

yes they will. different intake rubbers and i am not sure what you have to do with the airbox. probably get one that is for the older carbs as well but short answer is that it is definately do-able.

[seanbarney41]

Looks like you have most of the right parts to use 71-76 carbs on a 77/78 head(based off first picture in thread)...yes, airbox is different for 77-78 carbs.  You must use one from 71-76.  I would not make an effort to use the 77/78 carbs.  The 77/78 carbs are more complicated mechanically, harder to get parts for, and offer little, if any, performance improvement.

[dolamike]

Quick question, while prepping my engine for rebuild of the top end (CB750 K8) I had trouble removing two dowel pins on top of the cylinder block. I need to replace them and I have them measured at just under 12x14mm. The ones I see online are 12x15mm.

Did the size of these change after 1975?

Photos below... Any help is much appreciated.

[dolamike]

Just to close the loop on the dowels, I found this thread with info from HondaMan with some history on them:

http://forums.sohc4.net/index.php?topic=91874.0

This lead me to here:

http://www.hondapartsnation.com/oemparts/a/hon/506b8ca9f870023420a1e916/cylinder

It looks like the part is still available, it's #3 in the diagram. I ordered all 8 new, there is no length mentioned, just "PIN, SPECIAL KNOCK (12MM)"

I'll measure them when they arrive and update then.

[dolamike]

Build Update:


I've had some time to think things over, my initial thought was to build this bike as a Cafe / Brat in an effort to learn more about these amazing bikes. I have an all stock CB750 K6 that I love to ride and was the inspiration behind learning more about these bikes.



As I have gotten more into this build my thinking has changed. I think that a better project at this point would be to try and bring this K0 back close to stock, semi-restoration. I do want to get it on the road sooner rather than later, there will be things that take longer so I won't let them hold me back. Restoring the original engine being one of them.

The theme for this build will be "Do No Harm." The things I do to bring this bike back to life will be in an effort to create a platform to continue to refine the restoration over time.

in that effort I will be posting in the parts wanted section from time to to for things I feel are key to keeping the project true to it's intent. The first of which is below.

Thanks for all of the help and support as always.

http://forums.sohc4.net/index.php/topic,153547.0.html

[754]

Good to hear..if you dont go all out, can be done quicker, and enjoy it longer.. Things like them 187.00 seats..