Burman CP gearbox seizing up

Helpful information and requests for assitance and advice
Post Reply
Invicta
Member
Posts: 171
Joined: Mon Jan 29, 2018 9:42 pm
Location: Kent , England

Burman CP gearbox seizing up

Post by Invicta »

Having finally got round to fully reassembling my 1949 Burman CP gear box, when I fully tighten the nut on the kickstart ratchet end of the main shaft, the gearbox seizes and the main sprocket and main shaft will no longer turn. Backing the nut off slightly frees it all up again . Obviously this nut has to be tight so has anyone any idea what is causing this problem . Other than replacing the lay shaft and main gear sleeve bushes all of the gears and shafts are original parts. :(
g80csp11
Member
Posts: 1567
Joined: Mon Jan 01, 1990 12:00 am
Location: WEST SUSSEX UK

Re: Burman CP gearbox seizing up

Post by g80csp11 »

have you used gaskets on the build or silicon sealant , they are needed to ensure correct clearance
User avatar
ajscomboman
Member
Posts: 3963
Joined: Mon Jan 01, 1990 12:00 am
Location: HAMPSHIRE UK

Re: Burman CP gearbox seizing up

Post by ajscomboman »

I've had this before and it's all down to the rachet gear and bushing. It'll not be visually obvious and I had to work through each bit piece by piece until I sussed it. It may have been the sleeve on the ratchet gear was a smidge too short and loaded the mainshaft. I honestly can't remember exactly as it was a while ago.
Invicta
Member
Posts: 171
Joined: Mon Jan 29, 2018 9:42 pm
Location: Kent , England

Re: Burman CP gearbox seizing up

Post by Invicta »

Hi, Thank you for your replies.
I just used a standard gasket and nothing else in the join between the gear case and the mid section.

The kickstart ratchet does protrude slightly over the shoulder of the splined end of the main shaft, so that the nut is tightening down on the ratchet pressing the ratchet against the sleeve, which in turn is pressing against the inner race of the bearing, but as they all rotate together I can't see why this would cause everything to lock up when the nut is tightened, or am I missing something ? If the sleeve was a tad shorter then the nut would tighten against the shoulder on the end of the mainshaft instead. Is this how it is supposed to be? If so that would probably give the main shaft a bit of end float, which from what I have read on the forums most owners seem to have. With things as they are with the main shaft locked up against the bearing there is no possibility of any end float at all.
On stripping the gearbox down this nut was extreemly tight . I had to hold the clutch end of the mainshaft in the vice and hammer on the end of a spanner to shift it, but as I said in an earlier post this may have been due to the thread being belled out by a poorly adjusted clutch causing the clutch actuating plunger to rub against the end of the mainshaft. :(
Invicta
Member
Posts: 171
Joined: Mon Jan 29, 2018 9:42 pm
Location: Kent , England

Re: Burman CP gearbox seizing up

Post by Invicta »

Thank you ajscomboman and g80cps11 for pointing me in the right direction. I removed the gasket , which is 0.017" thick and reassembled without it. The kickstart ratchet nut was tightened up and everything turned freely. I carefully measured the end float at 0.015" , as without the gasket the nut was tightening against the shoulder on the end of the main shaft and not against the sleeve, which must be the way it is supposed to be. Looking at diagrams of the gearbox it appeared to me as though the third gear fits up against the inner face of the bearing. It does not, and if the kickstart ratchet sleeve is protruding over the end of the splined shoulder of the main shaft , tightening the nut loads up the bearings at either end of the main shaft stopping them from turning. My solution will be to either make a thinner gasket or grind a few though off the end of the ratchet sleeve to make it a tad shorter. What a difference a few thou makes. :oops:
User avatar
ajscomboman
Member
Posts: 3963
Joined: Mon Jan 01, 1990 12:00 am
Location: HAMPSHIRE UK

Re: Burman CP gearbox seizing up

Post by ajscomboman »

:thumbup: :thumbup:
g80csp11
Member
Posts: 1567
Joined: Mon Jan 01, 1990 12:00 am
Location: WEST SUSSEX UK

Re: Burman CP gearbox seizing up

Post by g80csp11 »

i have to say that ive seen different lengths of sleeve and have picked and mixed to give clearance i wanted
sounds like you are on the right track
User avatar
ajscomboman
Member
Posts: 3963
Joined: Mon Jan 01, 1990 12:00 am
Location: HAMPSHIRE UK

Re: Burman CP gearbox seizing up

Post by ajscomboman »

g80csp11 wrote: Mon Mar 13, 2023 3:57 pm i have to say that ive seen different lengths of sleeve and have picked and mixed to give clearance i wanted
sounds like you are on the right track
Yep me too that's why I was convinced that the rachet gear area was the issue.
Post Reply