CB100/125 Big Bore Old Skool Cafe Road Racer Build

kittykatjaz

New Member
Hi Everyone,

What better way to introduce myself than with my little cafe racer build. I have been into bikes since a kid and always loved the GP bikes of the late 60's and 70's. Even owned and raced a few.

After years of wanting to I am undertaking a custom build of what was a basket case CB100 frame with CB125 engine I picked up a few months back. The bike is an old bucket racer with an engine modded with a 62mm piston, XL head, custom crank and cam. Unfortunately the big end is blown and the engine incomplete.

The goal is to build a custom bike that is compliant with NSW AU registration without needing engineering by leaving the frame and running structure as original as possible and doing all the ancillaries within the Australian Design Rules to the year the bike was built.

I plan on making as much of the gear as possible using aluminium. It is either 5052 sheet, 6061 plate, 7075 round. So far I have made the seat/tail piece, combined rear guard and electrical tray which is hidden under the tail piece, fuel tank, rear sets and various other bits. I am just starting on rebuilding the engine.

The wheels I will rebuild using alloy valanced rims with stainless spokes.

I don't expect a powerhouse of a machine, I have a first gen CBR900 for that. This is a fun toy for scooting around the local area :)

Jaz
 

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kittykatjaz

New Member
Figuring out the rear sets was a tough one. I wanted to the mountings up high away from the centre stand mount instead of low like many of the others that have been done on the CB125. The pegs have to be folding for the use of a kick starter. The pegs and top supports are made from 7075 alloy which has a very high tensile strength. There is a lot of thinking required to make the first set of folding foot pegs that work well. I was surprised at level of geometry required.

I still have to make the gear lever and associated linkages and will wait until the engine is in the frame to do this.
 

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stroker crazy

crazy as a fox
Welcome aboard kittykatjaz!

Brilliant start to your build thread; tank and seat etc. look real good and the footpegs leave me speechless.

Can't wait to see what you do with the motor!

Crazy
 

kittykatjaz

New Member
The little tiddler is starting to take shape now. The engine is assembled, wheels done, controls connected and I am about half way through making the new alloy front guard. Everything that has been fabricated has been done in my little workshop and it has been an enjoyable experience. Once the front guard is finished I will start on the exhaust.
 

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kittykatjaz

New Member
The engine took a bit of work to get together. I had to buy a couple of parts engines to get all the missing bits of the gearbox, kick starter and all sorts of other parts. Then clean it all up and determine what to rebuild or replace. I replaced the big end and con rod. The crank halves were scored so I machined them and fitted thrust washers. The crank has been lightened somewhat when compared to an original. New 62mm piston kit sent from the UK which will make the engine close to 150cc. New timing chain and guides, new genuine seals, main bearings and lap the valves. Once all prepared then assembled. The head is ported though I have no idea what grind is on the cam. It could be standard but it looks like it has been welded and ground. Guess I will find out when it is started.

I fitted a new Mikuni VM28 flat slide carb. I also have a Dyna S ignition to go in.
 

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Luugo86

'73 CB350, '78 XS650 Cafe Killer
Those are some TiG-Tastic welds. Well done man. May I ask what guage sheet steel you used to form the petrol tank and seat ?
 

kittykatjaz

New Member
Luugo86 said:
Those are some TiG-Tastic welds. Well done man. May I ask what guage sheet steel you used to form the petrol tank and seat ?
Thanks.

The alloy is 1.6mm thick 5052 grade.
 

kittykatjaz

New Member
Wheels were fun. I purchased a couple of second hand valanced rims being 1.85x18 for the front and 2.15x18 for the rear. Also new stainless spokes and nipples. It took a bit to clean the rims and hubs up. Fitted new wheel bearings then laced the wheels up. To get them straight I also made the wheel truing stand.
 

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Luugo86

'73 CB350, '78 XS650 Cafe Killer
Do you have any detail on how you welded in the fuel petcock bung on the tank?
 

kittykatjaz

New Member
The top triple clamp was a pain. I decided to machine off the original handlebar mounts and weld up the rear holes. I don't know what type of alloy was used in the casting but it was crap to weld. As soon as the torch lit up it just splattered everywhere. After mucking around with tig settings the best I could get was a barely passable result. I am not game enough to hit it again.
 

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kittykatjaz

New Member
Luugo86 said:
Do you have any detail on how you welded in the fuel petcock bung on the tank?
Sure have.

I bought a second hand RGV250 petcock then machined a thick 6061 alloy plate drilled and tapped to match. I welded the plate to a small box section that sits as the lowest point of the tank. The weld is not pretty there but it is solid.
 

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kittykatjaz

New Member
Doing the rear brake cable setup took a bit of thought. I made the brackets, adjusters, clevis and cable. To make sure the cable is going to be safe I made a tool to birdcage the inner cable before soldering. All the local bike stores looked at me funny when trying to buy cable ferrules and nipples. Finally I had to order them in. I have also had to shorten the front brake cable and clutch cable using the same technique of birdcaging the inner cables and dipping in hot plumbers solder.
 

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kittykatjaz

New Member
For the speedo I am fitting a Daytona Velona with an active pickup fitted to read from the front sprocket bolts. I welded an alloy block in the side cover then machined it to fit the active pickup. It has been machined to give the sensor 1.5mm clearance from the sprocket bolts.
 

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Luugo86

'73 CB350, '78 XS650 Cafe Killer
Thanks for the info on the fuel petcock man, I appreciate it. Was it difficult to weld that alloy block to the cast engine side cover? Those engine covers are really porous metals, just curious how it acted when you put an arc on it. You must have a pretty nice shop man lol.. all that machining.
 

jpmobius

where does this go?
DTT BOTM WINNER
Some nice work there. Especially like the eccentric peddle adjuster - often overlooked. I often bore a hole axially through the threaded section of a stainless bolt and silver solder the cable to it. Stronger than the cable itself, and makes a handy adjustment to the cable length when you don't want to have the adjustment at the housing as you have nicely done.
 

kittykatjaz

New Member
Luugo86 said:
Thanks for the info on the fuel petcock man, I appreciate it. Was it difficult to weld that alloy block to the cast engine side cover? Those engine covers are really porous metals, just curious how it acted when you put an arc on it. You must have a pretty nice shop man lol.. all that machining.
Welding the side cover was not too bad as I was not worried about the looks of it. I hit it with a die grinder first, then stainless brush then acetone. The only bit that splattered was the lower part but I only needed enough weld to securely hold the block in.

My tools are good and equipment well setup but workshop is a cramped mess.
 
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