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How to: Velocette MAC gearbox into a KSS

March 7, 2010
tags: , ,


31velo The early Velocettes had a top-mounted gearbox, which was typical practice for motorcycles in the 19teens and 1920s.  With the introduction of the M series MAC, MSS and MOV in the mid 1930’s, they updated the frame and gearbox to have the more modern mounting arrangement with mounts on top and bottom of the gearbox casting.  This method is much stronger, and easy to adjust when setting primary chain tension.


Early gearboxes can be hard to find, and the 4 speed versions are even more rare than the 3 speeders.  With all that in mind, the previous owner of Kim’s 1930 KSS adapted a MAC gearbox and clutch to fit the early frame.   This involved cutting off various lugs from the box, and welding on a top plate, complete with two mounting studs, and an adjuster stud which points towards the rear of the machine.


A bit of trivia:  Veloce’s engineer Harold Willis invented the positive stop gearchange.  Previous to his invention, all riders shifted carefully, as too much shift lever movement would jump forward not just one gear, but two.  He was able to see the advantage in the now-familiar one-click-down to get one gear, two-clicks-down to move two gears.   That little touch of having the shifter return to the same spot after each shift was worth a huge amount of time on the isle of Man TT races.  Very quickly, the spring centered arrangement was adopted by all motorcycle manufacturers with any sporting intentions.   Indian Chiefs stayed with the hand shift however…  🙂


mac gearbox 001

The photos show the work required, I can supply dimensions if you would like to do this mod yourself.  Be sure to use Helicoils for the stud threads, and lock the studs well with Loctite.  While you are at it, I recommend welding on the frame strengthening plate described in my earlier article.


We have modified the layout to use shouldered studs, and also to use castle nuts and cotter pins to lock the adjustment.  It does make it a bit harder to adjust, but there is now no problem with the hardware coming loose, which leads to the gearbox sliding to and fro due to chain tension.


mac gearbox 002Aluminum alloys that would be suitable for this part would be 6013, 6061 or 6063.  Each is good for machining and for TIG welding.

The plate on the top of the box is pretty simple, with a groove cut down the center to fit the dimensions of the Veloce frame.  Coutour the bottom to fit the box casting, add two helicoils, then weld the round bar on the back with a drilled and tapped hole for the adjuster.  Once you have cut off the unneeded boss on the bottom of the casting, Bob’s your uncle.


mac gearbox 003

mac gearbox 004

mac gearbox 005

mac gearbox back silencer 001


velo kss gearbox 005

velo kss gearbox 001

3 Comments leave one →
  1. louis poleet permalink
    October 13, 2013 6:35 pm

    Hi Bro can email me the dimensions

    • October 13, 2013 10:05 pm

      Sure Louis. I’ll need to measure the bike after work tomorrow. The general shape of the aluminum plate is seen in the pics, and the slot that is milled into the top should be a sliding fit on your frame rail.

    • October 14, 2013 7:45 pm

      Hi Louis. The studs are 7/16″ diameter. I used coarse threads on the bottom, into heli-coil inserts in the aluminum. The studs are fine threaded at the top, and cross drilled for a cotter pin and castle nuts are used. The studs are about 1.875″ center to center. The slot milled in the aluminum is about 1.9″ wide and 1/4″ deep. Check your frame on that 1.9″ number to make it a good sliding fit. good luck, Pete.

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