The BSA A7-A10 Forum

Technical => Clutch, Primary, Gearbox => Topic started by: bakerlonglegs on 31.01. 2019 21:19

Title: New gearbox sprocket very tight
Post by: bakerlonglegs on 31.01. 2019 21:19
Hi,
Please can I pick the collective forum minds once again?
I’ve bought a new gearbox sprocket during the rebuild as the old one has a damaged tooth. The new sprocket only goes a couple of mm onto the shaft splines before sticking.
The gearbox has had a lovely rebuild so I don’t want to do anything rash and stupid that might damage it.  Time to stop and ask advice!!
Is a new sprocket supposed to be an interference fit?  Can heating the sprocket damage seals inside the box and/or should I be forcing it on somehow?
Any thoughts appreciated!
Title: Re: New gearbox sprocket very tight
Post by: coater87 on 31.01. 2019 23:34
 A splined fit should not fall on, everything needs to be straight and lined up to work. the parts can drag on each other while putting them together but it should not require any tool besides your hands (or very reasonable taps from the grip end of a screw driver).

 Pull your pieces apart and see where they are interfering, should be a few new witness marks.

 Eyeball and use a cheap calipers to measure your old against your new to figure out what is what.

 and you are right, dont pound on gearbox shafts. Best that can happen is you dont damage a cog or pooch out a case.

 Lee

Title: Re: New gearbox sprocket very tight
Post by: muskrat on 01.02. 2019 01:55
G'day bll.
As Lee said it should slide on with little effort. Look for marks.
It may be a case of "pattern parts strike again". Some times an old worn sprocket that's loose on the spline can mushroom the end of the shaft. Time to get the cubit stick (verniers) out and do some checking. If this is the case a light dressing with a file only at the end to 3 or 4 mm in. If it still won't fit dressing the sprocket with a file a very little at a time, we don't want it loose on the shaft.
Cheers
Title: Re: New gearbox sprocket very tight
Post by: duTch on 01.02. 2019 06:00

 In addition to the above, have you tried flipping it over out-side-in just to see if it goes on ?

 
Quote
......Time to get the cubit stick (verniers)........
I like that *smile*
Title: Re: New gearbox sprocket very tight
Post by: BSA_54A10 on 02.02. 2019 08:34
Have a close look at the splines.
Some are milled with a square cuter and others are milled with a sloped sided cutter

Or to put it another way,
The tops of some splines are like pryamids cut short while others are like key slots.
Lots of cheap parts have the wrong proifile of splines.
Title: Re: New gearbox sprocket very tight
Post by: Rex on 02.02. 2019 11:38
Pattern parts again. Gently run a Dremel sanding drum around the inside of the sprocket and do regular trial fits to see when it fits right.
Title: Re: New gearbox sprocket very tight
Post by: bakerlonglegs on 03.02. 2019 11:26
Thank you very much for all your advice, always appreciated!!

I followed the collective wisdom. I measured the splines (they are the correct size), used some copper slip to highlight how far it was going on and where it was rubbing. Finally, I dug out my dremel with a sanding drum fitted and very gently buffed a tiny amount off the corners of the splines (can barely tell by feeling along the edge). And hey presto, with slight hand pressure it now fits! 

Cheers all! You saved a gearbox from weapons-grade muppetry.....  *smile*