Topic: CB750K7 cafe racer build.

[stou]

My build is completed...almost, since we always have a little something to do! During the build, I took many pictures, so I tought it could be interesting to do a thread about it.

I bought this 1977 CB750 from a friend a little more than 2 years ago. He had this bike since something like 20 years under a pile of junk in is garage. Everytime I went at his home, I asked if he want to sell it to me. He always said that he want to do a project with it. But at one point during one of my visits, he decided to sell it to me!

The bike was in a very bad shape and not running. So I decided that this will be a cafe racer project.

Here's the bike when I bought it. It came with many old spare parts and another engine.









The first step for me was to clean it and have it running to pass the inspection to be able to put it on the road. Here in Quebec, we need to have the bike with no frame modification, with blinkers, horn, mirrors and not too loud exhaust to pass the inspection. So I decided to have it done before modified it into a cafe racer.

So I start with the carburetor, they was very, very dirty! And I bought a new battery.



I order a carb rebuild kit, and I borrow and ultrasound machine from a friend.

Before and after:







My wife made a seat cover, and I painted the tank to have a clean look to give me more chance to pass the inspection.



I went for the inspection and it went through easily.

[stou]

When I worked on it to have it running, I took the cylinders compression and it was very low. So it will need to be oversized. The front fork was also in bad shape. Scratches and rust on the fork chrome tube. I looked around to have it rechromed, but I found it expensive. The lower leg was not in good shape either. I decided to go for a front swap with an upside down fork. I found a GSXR 600 complete front end, at a not too bad price. So I started to work on that. After looking at the GSXR fork and CB neck, I decide to use the CB steering stem and install it in the GSXR triple clamp.

I grinded the stem of the CB triple clamp. It had a little bit smaller diameter than the one of the GSXR. I putted some welding on the stem and had it to the right diameter on my lathe. Then I welded a big washer at the buttom.







I also made an adjustable fork stopers

[stou]

I wanted to keep the spokes wheels and with 2 front rotors. I needed to mad some spacers the have the rotors place at the right place for the radial front brakes. I also needed to made some bushings to have the wheel centered.







When I putted everything together, I had a bad surprise. The calipers was rubbing on the outside wheel spokes. Not a lot but it wasn't acceptable. After looking at the problem, I decided to respoke the wheel with all the spokes on the inside of the hub. It worked!

[stou]

Next, I worked on the cafe racer back end. I bought a frame loop on the internet. I then cutted and welded the new back end of the frame. To make sure it was solid enough, I machined some parts to go in the frame.











Next, I made a seat form from some floral foam. This stuff is easy to work with.





Then I did it with fiberglass. The brake light and blinkers will be a LED bar that I bought on the internet.





The font will go into this part and the back will be hold with 2 bolts.

[SOHC4 Cafe Racer Fan]

Great job so far, Stou!

To date, this is the first front end conversion I've seen where someone has mated a modern front and calipers and still used the old CB brake rotors.  The modern calipers are designed to work with floating rotors, whereas the CB rotors are fixed.  I am not sure if this may cause any issues.  That said, you did a good job machining the spacing adapters for the rotors and the wheel.  You did a fantastic job on your fiberglass seat, too.

Looking forward to seeing the progression on this bike.

[70CB750]

Following

[stou]

Great job so far, Stou!

To date, this is the first front end conversion I've seen where someone has mated a modern front and calipers and still used the old CB brake rotors.  The modern calipers are designed to work with floating rotors, whereas the CB rotors are fixed.  I am not sure if this may cause any issues.  That said, you did a good job machining the spacing adapters for the rotors and the wheel.  You did a fantastic job on your fiberglass seat, too.

Looking forward to seeing the progression on this bike.

Thanks Cafe Racer Fan! You're right about the rotors, but we'll see how they'll worn out. It will be either the bolts or the hubs that will worn out.

I forgot, I also drilled the rotors. I thought it was to be very hard to drill but with a cobalt drill, it worked fine.

[stou]

The side covers was all cracked, so I start to look for new one. Meanwhile, I was thinking that I want something different. A day while I was looking at my old 1974 CR125 (that I restored), I had an idea!



I tried the side covers of the CR on the CB! I was looking good and the base shape fit well to the CB. So I decided to make some special number plate side covers for the CB!

I made molds from plaster


And made them in fiberglass a bit wider than the CR covers




Machined some aluminium to fix the covers in the CB holes

[calj737]

I think you’d better check for tire clearance on the rear with suspension compression. That hoop looks to land right atop the tire...

[physician]

Thats great! Where are you in quebec? Moi je suis de st-hyacinthe.

Great work so far. Looking forward to know about the rotor setup

[stou]

I think you’d better check for tire clearance on the rear with suspension compression. That hoop looks to land right atop the tire...

Already checked! Those Progressive shock have only 3.8" of travel. There's enough of clearance between the hoop and the tire ;-)

[stou]

Thats great! Where are you in quebec? Moi je suis de st-hyacinthe.

Great work so far. Looking forward to know about the rotor setup

Thanks Physician! Je suis à Laval, Quebec

[stou]

I didn't like the tank of the '77 - '78 with the cover on top of the gas cap. So I decided to remove the cover and install a flush mount gas cap. I ordrered a kit and started to work on the tank.



I cuted the cover to be able to position the cap at the right height. I then did some weld tacks, removed the cover and welded the cap on the tank. The welds must be well sealed to avoid gas leak from the top.



Here's what it look like once welded, sanded and primed.

[calj737]

Thats nice work. Did you create/leave an overflow tube from the top to bottom? I'd hate to see the fuel expand in the tank and destroy your paint later...

Respectfully, seeing the bike in primer from the side, the tail section shape does not blend well visually to the tank. It might look more "integrated" if the two shapes had a better geometry relationship. As they are, they appear to be 2 separate pieces put together on the same bike, not 2 pieces designed for the same bike.

A really good trick that has been used is to get a spare tank, cut off the back, then use that as a form for shaping the cowl. Makes a nearly perfect extension with its symmetry.

Just friendly input.

[stou]

calj737, no there's no overflow tube. There's no need of it because everything is sealed. If there is a spill, it will run ou from the gas cap. I did a few pressure test to be sure everything is really sealed.

I understand what you wrote about the tail section. But the front of the tank is high and what I looked for is to get some regular shapes from front to back. I didn't wanted a high front tank and a small back end. Once painted and the seat on, I think it give a nice overall shape. But everything is a question of personnal taste.

You can see a little bit better what I mean on this picture.

[calj737]

calj737, no there's no overflow tube. There's no need of it because everything is sealed. If there is a spill, it will run ou from the gas cap.

This is my point. If it runs out of the cap, it will run over your tank, not thru it. You will just need to be careful to avoid spillage.

[754]

As long as it's vented, and he only fills to bottom of filler tube  he will be better off than our 76 and down tanks.

Because of prepay, i over filled my tank a few times this year, scary mess with a hot motor.
One time I stopped after fueling, had a bite asked for directions and the gas expanded and overflowed.. messy.

 You great job on the bike you think a bit like me I think, the Elsinore cover idea was good.   The way you made the pegs for the grommets is what I would use on an alloy sidecover.   I did a big brake setup for a wire wheel, using wheel like yours for Bert in Quebec using F forks a few years ago.
 Keep up the nice work

[stou]

Thanks 754! By the way, I made a good use of your frame kit!

For the sidecover, as you can see I made some groves at the bottom of the pegs to be sure the fibeglass retain them strongly. I puted the pegs in the grommets, then I used some hot glue to position the side covers at the right place on the pegs. Then, I used fiberglass to fix permanently the pegs. It look strong enough.

[SOHC4 Cafe Racer Fan]

What color do you plan on painting the bodywork?  If the color of the tank and seat match the frame (at least where they come together), the visual gap of the seat to the frame will blend in.  Otherwise, the seat may appear like it is propped up an inch or two off the frame.

[stou]

Like I wrote in the first post, the build is already completed, so the paint is already done. I don't want to put a picture of the paint now in this thread, but the color is dark and it is blending very well with the frame.

[SOHC4 Cafe Racer Fan]

Like I wrote in the first post, the build is already completed, so the paint is already done. I don't want to put a picture of the paint now in this thread, but the color is dark and it is blending very well with the frame.

Ah, missed that.  Okay, we'll see it sooner rather than later.

[stou]

Engine rebuild.

I started by powder coat all the casing in wrinkle black. A big masquing job !





I rebuild it with new bearings (some), new seals, new cam chain. At this point, I checked the primary chain and from the Honda shop manual, it was in the specs. It had 67mm of travel at the chain tender and the shop manual indicate this is good under 70mm. So I didn't replace it since it's an expensive update. (I will regret it later)



I had the cylinders oversized and I installed the 836cc kit


I also had the valves guides replaced and valves seats machined. I installed new valves and seals. The springs was in the spec.






I then putted everything back together.

[SOHC4 Cafe Racer Fan]

Did you treat the inside of the engine with any coatings.  In your photos, the inside of the crankcase looks gold. 

I've heard of builders using red Glyptal to seal cast metal and ease oil flow, but I haven't seen anything in gold.

[stou]

No, I didn't do any coating inside the engine. It was already gold like that.

[algophobe]

nice build, only wish you put more detailed pics of your process. lots of fabrication points one can learn from that.
btw, do K7 gas caps seal as poorly as K2s? I know old bikes don't have overflow channels like the newer bikes but i'm curious the slight leak is intentional? (a noob question)