G: Gearbox (Single - Burman) Oval Slots

[davg1607]

The kickstart on my Burman G97 gearbox sticks in the down position when starting meaning that I have to hook it up with my foot to have the next swing. Also, the quadrant sometimes jumps off the ratchet pinion at the bottom of the stroke.

I think it's related to worn kickstart casing slots, some of which appear to be oval in shape (see picture below), perhaps meaning that the outer case moves slightly during the swing causing the kickstart shaft to stick in its bearings. This could also be the cause of the quadrant jumping I guess.

I don't have a solution to this other than to procure a new outer casing for the gearbox but would appreciate your thoughts.

Dave

 

[chankly bore]

This stud is a "special". It carries a 5/16" Whitworth thread at both ends, but the shank diameter is 21/64", precisely because of your "problem". The V.O.C. part, PR50-60X/9 has this diameter right, but is usually made about 1/4" too short. I've told them, but I'm not sure they listened. Saying that, are you sure everything else, such as mainshaft end float, spring, bush and ratchet are tickety-boo? I also fit small hex nuts on the two 5/16" Whitworth threads that retain the cover.

 

[oexing]

The kickstartcover should be doweled to the gearbox, at two places so they can take the loads when jumping on the kickstart lever. You may find two bolt holes suitable to this job. Then drill and ream them for taking a hollow dowel. First do it at one bolt hole, drilling a bit deeper into the thread and ream for the dowel. With that in place do next hole. Nearest size looks like 3/8 for the dowel with 5/16 through hole. I´d have a 10mm reamer certainly . . .

Vic

 

[chankly bore]

The threads are too shallow and supported by too little "meat", in my idiot opinion, to allow for dowels, hence the stratagem detailed above. If you have to "jump" on the kickstarter of a Vincent single then mechanical correction or stern re-education is certainly required. Incidentally, Dave, you'll be lucky to find any 70 year old outer casing better than what you seem to have. Also there are a couple of different types, to accomodate different widths of kickstart return spring.

 

[oexing]

Well, I am not clear about what the threads got to do with setting hollow dowels at their place. Reaming the cover through holes into the gearbox main case 1/4" deep for a 3/8 dowel should take most of the heavy push on the kickstart lever. Sure I got no idea about what meat around the bolt holes is there for the dowel, but 3/8 thinkable ??
Jumping on the kickstart - I don´t on any bike, but you need a good shove for the miserable Lucas or so I imagine . . .

Vic

 

[Pushrod Twin]

There is not an over abundance of material to counterbore around the case studs, even for 3/8" dowels, which would have very thin walls. Although we no longer regard it as "best engineering practice" it was not uncommon for engineers to use "fitted" bolts with close tolerance holes, even in aircraft. These have all had close to 70 years of hard work, and in many cases, (pun intended) abuse with the nuts loosely neglected etc so it should come as no surprise to discover the bolt holes are oval. I have 3 Comet boxes to administer to over the next few weeks, I will give some thought to dowels, at both ends of some studs, as they pass through the sandwiched centre casing .

 

[oexing]

Thanks for the photos, I see not a lot of meat around some holes. Even thin-walled hollow dowels with 1 mm wall thickness bear a lot of side loads so maybe I´d try the bigger hole at 10 o´clock position looking at the ball bearing place and go for 3/8 or 10mm size, for a 8mm stud. Same for the long bolt at bottom- with a slotted screw in it ?? Maybe make a stepped bolt there with 6mm outer half, well, 1/4 is allright. So a hollow dowel would do easily. For finding the correct place of the oval hole a mill would be good and a center finding clock. So then the corresponding gearbox part can be the datum for placing the worn cover over it and bore the holes for accepting the dowels. Most of time simple screws would do for gearbox covers but when a kickstart shaft is inside with bushes there is substantial side load from this and dowels would be the way to do a good job.

Vic

 

[greg brillus]

Even the twins kickstart cover only uses one dowel and its on the opposite side to the kick start shaft.........mostly relying on the other screws to clamp the lot together........some of us have seen the effects of a loose cover and how it wears away at the face of the mating G50 plate......All these issues are not a problem for bikes that are well maintained and cared for.......sadly this isn't always the case.......years of neglect and abuse.

 

[davg1607]

What a remarkable resource these Forums are and thank you all for your responses, it's a splendid community.

Unfortunately my 73 year-old Comet hasn't always been 'well maintained and cared for' so I guess one must expect these problems. I ordered a new stud (PR50-60X/9) from the VSC as recommended by Chankly Bore and it is slightly smaller in diameter than 21/64" but I will try it in the case and let you know how I get on. I suspect that it won't resolve the problem and I'll have to look at other solutions as suggested by Vic.

@Pushrod Twin - I'd be interested to know how you get on with your 3 Comet boxes.

Many thanks again for your responses and support.

Dave.

 

[davg1607]

Quick update on my problem which has now been resolved with your help.

On further inspection, it was not only the kickstart cover that had an oval slot but also the case itself. So, I drilled out the case and cover to 3/8" and fashioned a steel 3/8" sleeve to fit with an internal diameter to firmly hold the stud - see pictures below. This now firmly holds the cover and there is little or no movement.

I did have to ream the new outer kickstart bush to allow the kickstart to run smoothly but all good now.

The 'jumping' on the kickstart swing that I experienced was unrelated to this problem. When the box was rebuilt the worn ratchet pinion (PR50-38BA) had been replaced but not the driving ratchet (PR50-41BA). They looked like they engaged cleanly but a new driving ratchet resolved the problem - I guess it's a fine mechanical tolerance.

Thanks again for your help, my restoration continues and I'm now closer to starting the bike for the first time in my ownership.

Cheers,

Dave.