Dolls head gearbox advice

Previously robert_tuck wrote:

I don't know anything about the Doll's Head box but I had a similar problem with the AMC box that would not stay put. I found that the top bolt was siezed in the case and was not allowing the box to slide in the plates ,all the adjustment did was to twist the box and give the illusion of being adjusted.Then the chain pull would move it around. Had to take the box out to free it off.

Thanks Robert.

Previously derek_ambler wrote:

Hi David

Suggest you look at: www.norton.uk.com.expert.htm

George Cohen has a specific article on this particular problem and his solution.

Regards

Derek Ambler

Thanks Derek, very helpful indeed.

Thanks Derek, very helpful indeed.

Previously norman_lorton wrote:

If there are oval holes on the gearbox and frame at the bottom then it has been running with the bolts loose at some time in the past. It is complicated to fix properly as both the gearbox and frame need boring oversize and a larger diameter stud replacing the worn stud.

The top fitting has the added complication of the two aluminium gearbox flanges that point up becoming a loose fit on the iron frame flange. Again this can all be re-machined and a shim inserted. George Cohen's words are a good description of what to do. He warns of the risk of cracking those aluminium flanges.

There is something you can do if the wear is not too bad. When the bike is running the rear chain imposes more pull on the gearbox than the primary chain, so the gearbox always tries to move backwards (not forwards!). If those bolt holes are worn then the clamped faces will never hold the gearbox still - it will move back to take up any gaps around the bolt shafts and thus remove any primary chain slack.

So you need to shove the gearbox as far back as possible before tightening. Get someone to really heave on the final drive chain (pull up or push down) as the bolts are tightened. If the primary chain is now too tight undo it all and move the adjuster, and go through the process again.

You may see the gearbox twisting as the rear chain is heaved on - this is showing you the extent of wear in the fixings.

Norm

Thanks Norm, very good advice. i will try this first.

Previously dave_graham1 wrote:

You could probably bore and sleeve the bottom mounting holes so you can keep the same size stud. People tend to over tighten the top mount and break one off, as Norman has mentioned. These ally flanges aren't flexible, so if there is a gap between the flanges and the frame, doing it up tighter won't clamp it, it will only crack the ally. You will need to make a shim to slide between the flange and the frame lug so it is a tight fit, the bolt will pinch it up then.

Dave,thanks for your advice. I will try this.