My CR750 replica

[Nap Bollemeijer]

I hope I'm allowed to post this thread here, if not or when it should be replaced please let me know.
My intention is to build a CR750 / CB750R replica like the one Dick Mann rode and with which he won Daytona in 1970.

There are a few different type of "Kits" you could buy back in the day to build a CB750R like the Novice kit, Sports kit and RSC kit.
Depending on the kind of funds available and your personal power needs.
My goal is to build the bike in the background with as many as the original parts as I can find and afford.


I started with a beat down K7 as my base, removed all the unnecessary brackets and chopped the frame.
After that I've made the footrest brackets from mild steel and welded these to the frame.

[Nap Bollemeijer]

Most items are different from the normal production bikes.
The front forks from the production racers have 4-bolt magnesium lowers, the first kit bikes had these as well.
Although these changed quite quickly to regular K0 type forks which were adapted. 

For the 4-bolt lowers the nut and axle were also different as you can expect.
I did ran into one of the original early type front axle nuts:

[Nap Bollemeijer]

The front wheel had the same disc as the regular CB750 but are turned down to 4mm in order to save weight and a dual setup.
For the right hand brake there is a kit bracket, which is just a mirrored left hand bracket.
I'm a teacher in mechanical engineering so when I couldn't buy one I decided to make my own.

[Nap Bollemeijer]

A few years later the possibility arose to buy a NOS one so I did off course.

[Nap Bollemeijer]

The front hub on the production racers were smooth in the middle, made from magnesium and had a different look.
For the right hand side brake disc to be placed outwards 2mm there was thicker right side.

As I can't buy the correct style hub I turned a standard type hub down in the middle and placed a spacer between the hub and disc.

The production front wheel has an 18" DID aluminum rim, there were different kit versions it started with 44701-300-970 but was later replaced by 44701-300-971, there is even mention of using 42701-300-970 (rear rim as a front rim)
I'm not sure which number corresponds with which size and if they are front or rear wheel but these were the first DID choices: Aluminium rim 1.85B - 19 (front) and Aluminum rim 2.15B - 18 (rear)
Some kits had the same rim diameter and size in front and rear while others obviously varied and had different numbers front and rear, I chose to use the 18" front and rear wheel size with the slimmer front wheel.

[Nap Bollemeijer]

The top bridge for the CR750 was smooth and had to be fabricated as well.
I took a regular K7/F1/F2 style top bridge, cut the excess lugs off and welded a piece of aluminium in the gap.
Then filed and sanded everything until I was satisfied with the result, the homologation shows the RSC style top bridge which I tried to mimic.

[Nap Bollemeijer]

The bore of the CR was enlarged to 69,5mm to get 749cc.
Originally that was the only difference in cylinder blocks, these had different types of cam chain tensioners.
(The parts depicted aren't mine unfortunately)

[Nap Bollemeijer]

However there was an updated cylinder block and different type of cam chain tensioner.
I got the chanche to buy a NOS cylinder block, cylinder gasket, top piston ring, cam guide and 2 used pistons (A and B).
Perhaps I will be able to buy the remaining items in the future to put this together, if not I will fabricate the missing components for this setup:
(The first pictures of the complete setup aren't mine, the rest is)

[Nap Bollemeijer]

The frame has a few modifications, one of them is the right hand gear shift.
As most riders outside of Japan were used to English machines the racers were fitted with the possibility of a cross linkage and extended brake cable.
Initially I made my own parts by lack of the real thing, but I was able to buy the correct engine hanger bracket and bearings.

[Nap Bollemeijer]

All these parts are rare as hen's teeth and I've been collecting them for 9 years, but the hunt is far from over.
I hope you guys enjoy the pictures and if there's a request for more detailed information or a side by side comparison with other standard parts just let me know.

[Sam]

Im enjoying it for sure! Keep it up!

[Nap Bollemeijer]

On the CR750 they use a clutch lever bracket different from the production models (which houses electricity and switches).
I was able to buy a good reproduction bracket from the UK but when compared to other race brackets like the CR110 there was something missing/not right.
With the help of a friend we made several castings to correct the problem, along with a bunch of other parts as well.
Besides it always nice to have some spares...

As you can see I make 3D drawings of almost every (special) part which goes on my bike.
Part of this is so I can show my student real life items and processes, but also so that I can reproduce these in the future if necessary.

[Nap Bollemeijer]

The kill-switch and light on the standard CB's are located in the clutch lever bracket .
Because the CR doesn't need lights but only a kill-switch and uses a total loss ignition system without battery a separate kill switch is used.
Unfortunately I just missed out on the chance to buy a NOS one, but I do have the pics and perhaps I can reproduce these later on.
For now a standard item which is rewired should suffice.

The production CR750's had a clip-on which could only be taken off by removing the top bridge, even though most Honda production racers had already used hinged clip-ons.
I guess they chose for this because of the weight reduction they could achieve by making it a solid piece, there was also a lightning hole in the single piece clip-on.
In the kit versions the hinged clip-ons were made available, as well as a separate plastic cap to plug the open end.
(Those parts depicted aren't mine)

I made a solid version for my CR as I wanted to replicate the Daytona bike.
This is my trail version, I still have some tweaking to do.

[Nap Bollemeijer]

Because the CR has a full fairing and is developing more power an oil cooler was added.
The production racers had most probably 2 bosses welded to the frame on which the oil cooler was mounted.
For the kit versions the same oil cooler and hoses were used but with a special bracket which mounts the oil cooler to the horn mounting point under the steering stem.
I still have to make or buy the correct style oil cooler but bought a NOS kit bracket.

[Nap Bollemeijer]

The CR750 had a special front brake master cylinder, which I think has a slightly tilted reservoir as well as an adapted piston diameter.
Because the clip-ons tilt the master cylinder there is more risk of sucking in air whilst cornering, but this is an assumption as I've never had one in my hands.

Running from the master brake cylinder is a special brake hose, which is shorter.
I had the chance to buy 2 of them still NOS, just for fun and comparison as I don't intend to use them for safety's sake.

The brake hose comes to a brake splitter (joint, 3 way) and continuous with another special kit brake hose to the standard steel tube connecting them to the calipers.
Factory production CR's had the brake splitter mounted directly to a single screw under the steering stem.
Kit bikes used the standard steering stem and were supplied with a different style bracket.

Funny thing is that because later kit versions made use of standard parts which changed over time the joints changed as well.
At first I thought I was scammed because the joint I bought looked like an F2 type, but a careful look at the package showed it was never opened and genuine.
So I decided to keep and open it, after all it makes sense because racing parts always end up in standard bikes if proven effective.
(the pictures of the first 2 types of joints aren't mine as is the brake master cylinder pics. The NOS parts are in my possession)

If there is an interest in the numbers corresponding to the story and parts depicted I can add the numbers in the previous posts.
Here are the part numbers used in the CR's front brakes besides the standard part which are re-used, those with a star I've got NOS:

*45115-300-030 Holder, Caliper (standard item, but an additional one is required and comes with the joint right holder)   
*45116-300-030 Pin, Holder (standard item, but an additional one is required and comes with the joint right holder)            
45117-341-700 Bolt, Adjusting (standard item, but an additional one is required)         
45119-300-020 Spring, Adjust (standard item, but an additional one is required)               
45120-300-970 Disk, Front Brake               
45125-300-970 Hose B, Front Brake      
*45127-300-970 Joint, 3 Way         
*45128-300-970 Hose A, Front Brake      
45130-300-970 Clip               
45146-300-000 Packing, Hose (standard item, but an additional one is required)         
45200-300-970 Caliper Assy., Right      
*45204-300-970 Joint, Right Holder      
45228-300-970 Pipe, Right Front         
45245-300-970 Clip, Right Brake Hose   
45500-300-970 Cylinder Assy