The BSA A7-A10 Forum

Technical => Clutch, Primary, Gearbox => Topic started by: KeithJ on 13.09. 2020 20:39

Title: Loss of first and second gear
Post by: KeithJ on 13.09. 2020 20:39
Only had 3rd and 4th to come home with.  Made a nasty meshing sound trying to get in to any other.  Could only push it with the clutch in.  Have stripped it all and it looks like the mainshaft had "seized" in the plain bushes in final drive gear.  Done abut 5000 miles since being rebuilt.  It was assembled using an SRM final drive nut with oil seal. 
Two question:
Was the use of the nut with an oil seal likely to have caused this?
Would the "seizing" give the symptoms I have experienced?
Thanks
Title: Re: Loss of first and second gear
Post by: berger on 13.09. 2020 21:10
definitely the symptoms and the seal shouldn't have any effect. was there enough oil in the box, seems strange they should pick up after 5000 miles. has the bush with holes in that should line up with the holes in the sleeve gear spun and blocked of the oil holes.
Title: Re: Loss of first and second gear
Post by: KeithJ on 13.09. 2020 21:13
Yes, recently checked oil so OK.  Good points regarding the oil holes and will check.
Title: Re: Loss of first and second gear
Post by: morris on 13.09. 2020 21:32
As Bergs wrote, seal shouldn’t have influence.
My bet would be to little tolerance (.003 minimum) between bushes and shaft. Were they reamed?
Title: Re: Loss of first and second gear
Post by: KeithJ on 13.09. 2020 21:43
A few years ago since the rebuild but pretty sure bored to size.  Surprised they did not seize before if though.  Will check them.  Thanks
Title: Re: Loss of first and second gear
Post by: bsa-bill on 13.09. 2020 21:54
can be caused by a broken selector return spring
Title: Re: Loss of first and second gear
Post by: Swarfcut on 13.09. 2020 21:59
 KeithJ.     The original design has two bushes, 67 3073, the one innermost to the gearbox is drilled to match oilways in the  final drive sleeve gear. Just as bergs explained. From my experience original bushes are relieved where they join to form a small oil reservoir.  I have seen boxes where these two bushes have been replaced with a single bush, or pattern bushes with no relief butted together but this seems a bad idea as there is nowhere for oil to accumulate to aid lubrication.

 Effectively sealing the shaft end with the SRM nut certainly stops the drips, but also prevents any through flow of lubricant, which is actually desirable (if messy) on a design with concentric shafts running at different relative speeds in the intermediate gears.  On a tight shaft this may be just enough to give a seizure. Something has changed if the box has covered some 5000 miles without problems.  More common to tighten up in the first couple of miles with an assembly problem.

 With the shaft seized, mainshaft and sleeve gear rotate as one, the normal mode when in top gear. Third gear introduces a change in relative speeds, but not as much as in second or first gear. So a partial  intermittent seizure could still give you third gear. 

  With the clutch pulled in pushing with the box in top gear (mainshaft and sleeve locked as normal or seized) would be possible.  Probably not in any other gear with a true seizure.

Swarfy.

 Additional. Re reading added posts, an internal fault due to inadequate lubrication, or an external problem, as suggested  (chain tension) are good places to start looking for a solution.

 
Title: Re: Loss of first and second gear
Post by: ironhead on 13.09. 2020 22:49
G'day Keith.  How tight was the primary chain? An overly tight chain puts a lot of load on that bush.
Title: Re: Loss of first and second gear
Post by: KeithJ on 14.09. 2020 06:35
Difficult to gauge how tight so could have been just too tight but sounds likely to have been a key issue.  ATB. Keith
Title: Re: Loss of first and second gear
Post by: morris on 14.09. 2020 08:00
Further to Swarfy’s thoughts, I must admit that I added the SRM nut only after having ran it for about 1500 miles. This may have given the bushes some time to bed in.
Another thought is that the bushes I installed were a bit shorter than the original ones which left a gap of about 2-3 mil between them. Personally I think that’s a bit of an advantage as it will hold some oil for lubrication at start up. *dunno*
Title: Re: Loss of first and second gear
Post by: KeithJ on 14.09. 2020 16:29
Closer inspection shows it has two bushes, with a gap between them and the oil holes still line up.  Problem appears to be the inner bush was a few thou proud of the gear.  I think over time the mainshaft has rubbed against the small projection producing a very small radial "bump".  Only extended in to the bush by 15 thou or so but just enough to bind on the layshaft.  Have scraped the bump by just adding the slightest chamfer on the leading edge.  Shaft goes in OK, all clearances are what they should be so just need to assemble it and sort out some speedo drive leaks.  So my fault I guess for not making sure the bush was flush or just sub flush.  Pleased nothing more serious.  Have found some other bits and bobs to do.

So next thought 20w50 or EP gear oil?  Have used both over the years but since they started making multigrade EP think engine oil is likely to be better?

Thanks for the input.
Title: Re: Loss of first and second gear
Post by: morris on 14.09. 2020 19:14
Penrite’s gearbox 30. Started with engine oil first but got a lot of foaming. Ok on my beer but not for oil...  *smile*
Title: Re: Loss of first and second gear
Post by: chaterlea25 on 14.09. 2020 23:32
Hi Keith
Check the mainshaft for straightness

John
Title: Re: Loss of first and second gear
Post by: Beeza on 15.09. 2020 09:41
Hi All, just seen this, and maybe check the selecter shaft has not moved into/onto the selector pivot.
If the shaft grub screw comes loose the shaft can%u2019t go past the gear box sprocket, but can move timing side wis and foul the selector arm.
Hope this helps.
Thomas

Now I think of it, precise symptoms
Title: Re: Loss of first and second gear
Post by: KeithJ on 15.09. 2020 10:08
Grub screw still tight and selector shaft in place. Mainshaft running within a thou.  So looking positive. Thanks
Title: Re: Loss of first and second gear
Post by: Beeza on 16.09. 2020 09:09
So I take it that this is a STD box?
So any main/lay shaft interference could not possibly interfere with gear selection!
Correct me if I’m wrong please, everyone!!
As I understand you have been able to get to the grub screw in the gear box housing by taking the entire primary side off to check the fact that the selector shaft has the grub screw and it is fitted well into it’s little slot, and punched to prevent loosening?

For this one I am very interested to
find out the cause, other than what I have previously experienced.
Big  Cheers
So So interested
Title: Re: Loss of first and second gear
Post by: KeithJ on 16.09. 2020 15:15
So I take it that this is a STD box?
Yes, correct.
So any main/lay shaft interference could not possibly interfere with gear selection!
Correct me if I’m wrong please, everyone!
All I can say is the "binding" of the very end of inner most bush on the output gear was enough to prevent getting first and second gear.  It also needed the clutch pulled in to push my bike.

As I understand you have been able to get to the grub screw in the gear box housing by taking the entire primary side off to check the fact that the selector shaft has the grub screw and it is fitted well into it’s little slot, and punched to prevent loosening?

I took off the gearbox cover and then the inner.  Could not see anything obviously wrong.  Selector shaft was not loose.  Removed the outer gear on the layshaft and still could not see what was wrong.  Only problem was I could not pull the mainshaft out of the gearbox.  Removed gearbox completely off bike.  Undid grub screw and removed selector shaft.  Pulled out all the bits I could and still the mainshaft would not slide out.  Used some delicate use of a hide mallet, managed to tap the mainshaft out of the final drive gear  Then removed final drive gear and examined the bushes.  Found the shaft would go virtually all the way in from the clutch side but not enter from the gearbox side.  Measured and looked to find the deformed end of the inner bush. 

For this one I am very interested to
find out the cause, other than what I have previously experienced.
Big  Cheers  ;)


So So interested

Reply in bue.  Hope that helps