Doherty twist throttle

The MK0 has the same Doherty twist throttle that both of my MK1s have. The rubber grips are in great shape so I will use them. One problem I noticed awhile ago is that there is no adjustment for the cable which is about 3/8″ too long. Both ends have soldered balls so I did not want to cut the cable to shorten it. Both of my MK1s have replacement cables that I cut too length. So I had to turn the throttle maybe 1/4 turn before the throttle shaft would even move. With the twist throttle WFO I was getting only 3/4 throttle at the carb. The bike was plenty fast at 3/4 throttle so I did not mess with it.

I really wanted full throttle and I had noticed before that my 04-257 twist throttles looked a lot like the 1962 Doherty. I grabbed one and soon realized they are almost identical except for the rubbers and the adjuster on the 04-257.

I removed the adjuster and compared it to the old Doherty. Turns out all I have to do is tap the old casting and then I can fit the adjuster to the original throttle. So I measured the threads and found that they are M9 x 1.00. I have a fairly complete metric tap set: M6, M7, M8, M10, M12, M14 and M16 in fine and coarse pitches. I do not have a M9 x 1.00…… So I now have one ordered.

Of course I am annoyed but now I decided to find out just how similar the new and old throttles are. Turns out they are interchangeable with some minor deburring. I installed the lower adjustable piece on the old Doherty top piece and it fit.

This will work until my new M9 x 1.00 tap shows up. The adjuster now allows me to get full throttle on the scooter.

Brake rebuild

The front brake worked quite well but it was not aligned very well so one of the shoes wore crooked. I was able to buck the loose rivets to get another 1/32nd of life out of the worn shoe. I decided to clean and paint the backing plate and put the brake back together as is because I want to use the bike soon for Morel hunting. The backing plate is pinched between the fork and the end of the 5/8″ diameter axle. The hole in the backing plate is 1/2″ and the bolt going through it is 3/8″ so there is a 1/8″ gap which the axle had fallen into on one side making the shoe alignment even worse. I found a white 1/16″ thick O-ring that I will use to center the brake backing plate better than it was.

The shoes are difficult to install with a nasty learning curve which reminds me of the first 20 times I struggled attempting to re install Rokon front miter boxes until I figured it out. Cusswords and big honking snap ring pliers finally overcame the heavy springs.

I cleaned up the hardware. The threads are 3/8-26 BSC (British Standard Cycle).

Tires

The 58 year old Firestone Power Implement tires are in excellent shape with little wear and no cracking. I set the MK0 front tire up against my MK1 Firestone to show how much more tread they have. These are 4 ply rated and probably would work great on an off road lift truck since they are rated for 1280 pounds each. They require no air pressure. I did not remove the axle and bearings on the front wheel. I plan to use them as is since they are still running smoothly. I did have to remove the rear axle since it was bent.

Hand clutch

This post details the assembly of the hand clutch to the Albion 3 speed gearbox. This is the heavy duty version dual disk Albion clutch. It uses cork for the friction material and new corks are available in the UK. The intermediate fixed disk was missing so I had to grab one from my stash.

Engine and Albion installed

I put the fluid drive on first. I cleaned it the best I could short of sand blasting. I was afraid sand would get inside. The chain and inner cork disk went on next.

Inner cork disk

This is the fixed middle disk that was missing. That meant that the inner and outer cork disks were rubbing against each other. They do not appear to be damaged.

Middle fixed disk

Next is the outer cork disk.

Outer cork disk

Now the outer fixed disk.

Outer fixed disk

Return springs and bolts.

Springs

Finally the clutch cover.

Cover

Fork installation

I would have liked to use both original kingpin bolts but I can’t since I had to drill and tap out the bottom mounting hole on the fishmouth to 9/16-18 UNF. Both of the original bolts were made from 5/8″ hex head bolts. They both have a center drill hole on one end which means they were modified in a lathe using a tailstock. The top bolt is drilled for safety wire and was centered on the hex end and the bottom bolt was centered on the threaded end.

I like the bottom bolt better since it has longer threads. I will use this bolt in the top but it has a few problems.

First it was not fully threaded which means it had locked up on the threads instead of the 5/8″ diameter shoulder. I suspect this was a contributing factor into how badly the bottom hole was messed up. I used a 1/2-20 UNF die to cut threads up to the shoulder.

The next issue to address is that the shoulder is not long enough to allow the bolt to seat on it. The bolt hex will lock up on the bushing instead. This will unloosen the bolt when you turn the handlebars to the left. It will tighten when you turn to the right. No problem as long as you plan all your rides using right turns only.

I used my toy lathe to make the shoulder longer by turning about 1/32″ off the bottom of the hex.

The next issue is that this kingpin was made from a 5/8″ hex head bolt that measures .611″ under the hex. This is too loose in a .625″ ID bushing so I decided to bore out a FB912-6 562″ ID diameter bushing to .612″ in my toy lathe.

Perfect fit!!!

Albion

Since the Albion is working correctly I decided to not rebuild it at this time. I cleaned it up on my Scotchbrite wheel and tightened up the bolts. It is leaking at the kick start and input shaft so I will keep an eye on the fluid level. There is nothing weird about it. We have seen plenty of these handclutch units before. I will detail the clutch sometime soon.

Albion right side shift lever

We have seen these before on handclutch MKDs. They cut off a BJ 11 left side shift lever and brazed it to a EJ 47 side lever. Later Nethercutts and Rokons use a longer EJ 47c side lever. It was painted Hunter green on the EJ 47 end and still had the original chrome on the BJ 11 end.

It is fastened with a EJ 48 nut that has a 3/8-26 BSC (British Standard Cycle) thread.

The old MK0 picture looks like a black G 91a shifter knob was used so I went with that. Both of my MK1s had wooden G91c shifter balls.

Front chain adjusting bolts

I noticed when I picked the bike up that the front wheel was near the bottom of it’s adjustment. It also sat higher than my MK1s so I decided to take out a link or 2 to lower it so I could get my feet flat on the ground.

Turns out the adjusting bolts only had 3 or 4 threads engaged. I removed 2 links which dropped the bike 1/2″ and replaced the original bolts with my A1400 bolts that are fully threaded to allow more adjustment.

I rode the bike this way until I completely took it apart. I will see if I can put the originals back in since they will have another 1/2″ of engagement. They may not work since they are not fully threaded and used much thicker nuts instead of jam nuts.

Bushing facing and reaming.

I used one of my toy lathes to face off .031″ on both bushings. The flanges now measure .094″ thick.

I installed the bottom bushing using a vise. It was a heavy press fit and it squeezed the bore of the bushing down to .621″ The kingpin bolt is .624″ so I had to ream the bushing to .625″ to allow the kingpin to fit. Fortunately my .625″ reamers have a .750″ diameter shank which fits perfectly in the upper bore of the fork. This allows me to align ream the lower bushing to the upper bushing. I could not find my 3/4″ die holder so I used a 1″ die holder with a 3/4″ to 1″ bronze bushing that now appears to be seriously jammed in my die holder. After reaming I installed the top bushing and took a measurement of 4.496″ between the bushings which gives me .002″ clearance between the fork and the fishmouth.

Fork

The fork cleaned up nicely in preparation for painting. I half expected to find a crack or two but was pleasantly surprised to find no cracks. It is quite different from later forks. It is 3/4″ wider and has 7/16-20 UNF nuts welded in the top to accept the handlebar mounting bolts. I used a 7/16-20 UNF eyebolt from an old Plymouth Barracuda to hang the fork for painting. The horizontal braces were not painted green underneath so I suspect they were added when Jim Cavanaugh upgraded the bike.

The bottom bushing was a heavy press fit which will require reaming to accept a kingpin bolt. I was able to push the top bushing in with my thumbs and it accepts a standard size kingpin bolt nicely. I am not surprised that the fork will not fit onto the frame fishmouth without some work. The fishmouth measures 4.494″ across the mounting bosses. The fork measures 4.434″ inside the new bushings. I will have to reduce the flange thickness of each bushing by 1/32″ to get this to fit. The next problem is the welded on bungs with the bores each measure .660″ thick. They need to be .625″ or thinner to accept the FB1012-6 bushings correctly. .660″ means the kingpin bolts will lock up on the bung instead of the bottom of the bolt shoulder. All these bad dimensions explains why I see so many hammer hits on the fork. Instead of correcting these issues someone just beat the fork on. Thinning the bushing flanges and extending the kingpin bolt shoulders even more on my extended kinpin bolts will fix this problem. What is truly amazing is neither Nerthercutt or Rokon ever figured this out. Instead they jammed set screws into the kingpin bolt threads to keep them from backing out. Looks like they gave up in 1986 when they switched to the bearing head steering forks.

3/8-24 UNF nuts are welded into the bottom of the tubes to accept the chain adjusting bolts. The brake anti rotation bracket was welded on about 5 degrees out of alignment which wore one of the brake shoes crooked. I straightened the bracket with a crescent wrench.