BSA Motorcycles 1950s

Today in Motorcycle History

Beeza Bill's BSA A10 Gearbox

How I converted a Scrambles cluster to Close-Ratio:

I have always run STD boxes in my A10s. So when the opportunity came up to buy a swing arm Scramble box at the right price I grabbed it. Third gear main shaft was blown and I thought I might do some experimentation. I had always dreamed of a Day close-ratio or a RRT2. The DAY ratios interested me most, as they appeared to be half way between STD and the extreme RRT2. A good compromise for the street.

I noticed in the SC cluster, that M/S 2nd gear looked like L/S 3rd gear in a STD cluster [both 19T]. Sure enough the STD L/S 3rd would fit in the SC M/S 2nd position but not vice-versa. The SC M/S 2nd gear dog center diameter is smaller and prevents it from engaging with a STD L/S 1st gear, which is the same as SC L/S 1st gear. I thought will SC L/S 3rd gear, 21T fit in the SC M/S 2nd position? The answer is Yes! I calculated the ratio and got the same as a Day 2nd gear 1.460. Problem was I didn't have a 22T gear to match in L/S 2nd gear position. I made a gear with SC M/S 3rd gear 67-3305 OD and tooth form and all other dimensions as SC L/S 2nd gear 42-3024.

While looking at the "CHART" I found that L/S 3rd Gear in an RRT2 cluster is the same gear as L/S 3rd gear in a STD box. I calculated the ratio and found it to be as RRT2 1.099 not 1.101 like the DAY ratio. While there is a 0.002 difference it was close enough for me! By using M/S & L/S 1st gear from a STD box and also M/S 3rd gear. I calculated 1st gear ratio to be 2.343 same as Scrambles. [This 1st gear ratio is the one I finally settled for, as it is more user friendly around town]. Not happy with this I sat with the calculator until I came up with my coveted Day 1st gear ratio.

To get the DAY 1st gear ratio, I needed a 26T L/S gear and a 17T M/S gear. While looking through an old Classic Bike Magazine, June 1982. I came across an article called "Smoothing the Goldie's flight" by Alan Cathcart. On pg. 22 I got a hint, an RRT2 gearbox had been fitted with an A65 bottom gear!!?? I checked the A65 parts book and L/S 1st gear in an A65 STD box is 26T! I quickly visited a mate carrying a L/S 1st gear and a M/S 4th sleeve gear from an A10 STD box. I confirmed dog engagement positions matched and so did OD and tooth form. I now had my L/S 1st gear!

Now I required a M/S 3rd gear with 24T. The SC L/S 2nd gear had 24T and the correct dog positions. IT was however 3mm thinner than the gear normally used in that position. This worked in my favor when modifying my M/S 1st gear. I used L/S 4th gear from an STD box as my M/S 17T 1st gear. I ground the small lip that pushes against the main shaft ring to a depth of 4 mm. Behind the M/S 1st gear a 4mmwide ring is fitted that presses against the inner cover bearing. The slots in my main shaft were ground longer to match the SC one. This cluster was being fitted into a ridged frame A10 with bolt on gearbox and an SC main shaft was never made for this model.

I now had my close-ratio cluster 1st 2.124, 2nd 1.460, 3rd 1.099, my final drive was 4.66 and I found it great on the open road! However 1st was a bit tall in town particularly when riding 2 up. While shedding a couple of stone might have helped! I opted to change 1st gear to SC ratio 2.343. This improved the takeoff performance very well.

A] Scrambles 4th gear main shaft 25 Teeth PT# 67-3311

B] Scrambles 4th gear lay shaft 18 Teeth PT# 42-3022

C] Scrambles 3rd gear lay shaft 21 Teeth PT# 67-3212. To fit in position C. Teeth on the front of the gear must be ground back a bit for it to engage correctly. This has been done to the gear on the left. [Also the engaging dogs diameter circle is larger then the original gear used in this position, causing increased loading on the dogs. While mine has not failed it is possible, you have been warned!]

D] This 22 T gear must be Made; it has the OD and tooth form of scrambles 3rd gear main shaft

PT# 67-3305. All other dimensions are as scrambles 2nd gear lay shaft PT# 42-3024.

E] Scrambles 2nd gear lay shaft 24Teeth PT# 42-3024.

F] Standard 3rd gear lay shaft 19 Teeth PT# 67-3198

G] Standard 4th gear lay shaft 17 Teeth PT# 42-3020 [The lip where it butts against main shaft ring must be ground back 4 mm measured from the end of the dog. A 4mm thick ring must be fitted between gear and inner cover. The modified gear is on the left.]

H] Standard 1st gear lay shaft 26 Teeth PT# 68-3151 [A65 STD cluster]

Main shafts

In the picture. STD Semi-unit Main shaft PT#67-3061 is at the top. Below it is a STD main shaft with lengthened slots to match the swing arm SC main shaft PT#67-3315 below it.[ semi-unit main shafts can be used in swing arm boxes if you wish to use an AMC clutch.]

NOTE: Gears E G H can be replaced with the following to give a 1st gear ratio of 2.343

E] Standard 3rd gear main shaft 24 Teeth PT# 67-3202

G] Standard 1st gear main shaft 16 Teeth PT# 67-3191

H] Standard 1st gear lay shaft 27 Teeth PT# 42-3097

I make and use a Roller Indexing Plunger in my gearbox as it gives a lighter shift, by reducing friction and will not damage cam plate.

All information is provided for your interest and is correct to my knowledge at this time. Any modifications done based on this information is done at your own risk, No responsibility accepted!

Thanks To Dave Kath of Nevada for giving me the updated "CHART".