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Clunky Gearbox #664385
08/18/16 1:17 pm
08/18/16 1:17 pm
Joined: Feb 2016
Posts: 58
Victoria, BC,
S
Stephen Hill Offline OP
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Stephen Hill  Offline OP
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Joined: Feb 2016
Posts: 58
Victoria, BC,
Now I have sorted out the clutch and starting issues, I have been riding my 1956 DB34 with a STD T gearbox. The bike shifts nicely up and down, except for 2 to 3, or 3 to 2. There is a crunching/grinding sound when I shift between these gears. It will crunch regardless of which speed I am going, although at higher speeds the crunch sounds worse. It also doesn't seem to matter how I rev the engine between shifts, it still crunches. The clutch is working well, and is not dragging.

Any suggestions on what could be causing this?

Stephen Hill

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Re: Clunky Gearbox [Re: Stephen Hill] #664396
08/18/16 2:15 pm
08/18/16 2:15 pm
Joined: Feb 2003
Posts: 2,951
Ohio
R
Rickman Offline
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Rickman  Offline
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Posts: 2,951
Ohio
You may find you'll need to take the 'box apart, and see if the gear dogs and their holes have rounded off too much.

If so, this means replacing gears...

Maybe the proper shimming hasn't been done inside the trans?

Are you SURE the clutch isn't dragging???

... Maybe a chain stretched too tight? [unlikely...]

The higher speeds thing may be the dogs and their holes rattling against each other faster...

.... Uhm, is it possible you may have a bent gear shaft?

Re: Clunky Gearbox [Re: Rickman] #664421
08/18/16 7:06 pm
08/18/16 7:06 pm
Joined: Feb 2016
Posts: 58
Victoria, BC,
S
Stephen Hill Offline OP
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Stephen Hill  Offline OP
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Joined: Feb 2016
Posts: 58
Victoria, BC,
Clutch is definitely not dragging.
Primary and drive chain are not too tight.
Does worn dogs and holes make for a noisy shift?
It may need shimming. Is this related to crappy shifting between some gears? In which case, does a crappy shift between 2 and 3 identify where the shimming is required?

Perhaps the broader question is worth asking: is the STD gearbox normally crunchy between 2 and 3?

Stephen Hill

Re: Clunky Gearbox [Re: Stephen Hill] #664433
08/18/16 9:33 pm
08/18/16 9:33 pm
Joined: Feb 2003
Posts: 2,951
Ohio
R
Rickman Offline
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Ohio
Mine aren't, unless the clutch is dragging....

The noisy shift, in my opinion, something is still holding the rpm's in the trans going, not allowing the gears to "float" easy so that they easily go into place when the clutch is released.
Maybe the cam plate is worn...
..... Maybe you need to be certain you firmly make the shift??? These aren't as smooth an action as just about any modern trans... They don't *snick* into place.

If the contacting surfaces/edges of the 'corners' of the dogs and holes are rounded/worn enough, they may tend to try to shove their way out of the "locked together" aspect. This may be why the undercutting of dogs was developed?
Possibly the shimming process... I've never heard of what indicates where a shim is needed...

I hope you never break a dog off!

It will be interesting to find out what IS going on with your trans!

Re: Clunky Gearbox [Re: Stephen Hill] #664449
08/19/16 12:10 am
08/19/16 12:10 am
Joined: Feb 2012
Posts: 1,266
Middle East,
Kerry W Offline
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Kerry W  Offline
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Joined: Feb 2012
Posts: 1,266
Middle East,
Something isn't right inside the box. gearboxes aren't my speciality, but I'd be suspicious of the gear dogs/holes rubbing together during the shift i.e. s spacing (shimming issue).

You'd need to lift the gearbox out to do it, but a peek through the inspection hole in the rear, while turning he mainsheet and moving the lever might be illuminating...and them looking for evidence of the noise on the dogs on the gears (which can appear to be relatively insignificant). When you pull the internals out, keep very careful note of what spacers are where.


No generalisation is wholly true, not even this one.
Oliver Wendell Holmes
Re: Clunky Gearbox [Re: Kerry W] #664531
08/19/16 6:01 pm
08/19/16 6:01 pm
Joined: Feb 2014
Posts: 477
Cork Ireland
C
chaterlea25 Offline
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Posts: 477
Cork Ireland
Hi All,
When the BSA pre unit gear boxes change from second to third and back it involves both selectors moving at the same time
one gear pair engaging and one pair disengaging,
I have found some of these gearboxes can "crunch" on 2-3 and 3-2 changes
They are very robust boxes and usually perform well even when well worn ??
On the STD box when powered by an A10 or Goldie motor the weak (wear) point are the layshaft and sleeve gear bushes to a lesser degree.
The layshaft can be machined undersize and custom bushes made.
Kickstart quadrants and pinions suffer normal wear,
there are two gear tooth forms, do not mix them up

Quite a high % of these boxes can be noisy in third gear an odd one noisy in second, for no apparent reason and it doesn't seem to do any harm
Once the kickstart ratchet nut is tight and the small bearing in good order the mainshaft position is fixed
The sleeve gear is held in position by the bearing and sprocket nut
The layshaft end float is controlled by the bush faces and the washer inside first gear
If the detent plunger adjuster is screwed in too far changes will be notchy, around 3 threads showing outside the locknut should be good
Camplates and selectors seem to last forever

John

Re: Clunky Gearbox [Re: chaterlea25] #664592
08/20/16 5:12 am
08/20/16 5:12 am
Joined: Dec 2005
Posts: 42
San Diego, Calif.
C
ChuckB Offline
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Posts: 42
San Diego, Calif.
[/quote]there are two gear tooth forms, do not mix them up

Could you elaborate on this? How do you tell them apart?

ChuckB

Re: Clunky Gearbox [Re: ChuckB] #664633
08/20/16 4:44 pm
08/20/16 4:44 pm
Joined: Feb 2014
Posts: 477
Cork Ireland
C
chaterlea25 Offline
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Posts: 477
Cork Ireland
Hi Chuck,
Basically, the gear teeth are either pointed or flat topped to look at
If they are mixed mechanical marmalade ensues eek eek

John

Re: Clunky Gearbox [Re: chaterlea25] #664806
08/22/16 3:55 am
08/22/16 3:55 am
Joined: Dec 2005
Posts: 42
San Diego, Calif.
C
ChuckB Offline
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ChuckB  Offline
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Posts: 42
San Diego, Calif.
Thanks John,

This is simple enough, and I surely wouldn't want any avoidable mechanical marmalade.....

Chuck

Re: Clunky Gearbox [Re: Stephen Hill] #664885
08/22/16 4:25 pm
08/22/16 4:25 pm
Joined: Aug 2015
Posts: 1,268
Bolton Lancs UK
A
Andy Higham Online content
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Andy Higham  Online Content
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Posts: 1,268
Bolton Lancs UK
An often cause of a clunky gearbox is too thick oil. It sticks the pinions to the shafts causing them to rotate too fast/slow to engage smoothly. It is a common belief that thicker oil has better lubricating properties, this is complete boll0cks. Modern thinner gear oils give more protection than the treacle of 50 years ago


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Re: Clunky Gearbox [Re: Stephen Hill] #664992
08/23/16 11:31 am
08/23/16 11:31 am
Joined: Feb 2016
Posts: 58
Victoria, BC,
S
Stephen Hill Offline OP
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Stephen Hill  Offline OP
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Joined: Feb 2016
Posts: 58
Victoria, BC,
Interesting idea: too thick oil might be a cause of gearbox clunking between shifts. This would be simpler than tearing down the gearbox!
Gearbox has 90 weight gear oil in it. Is this the right oil, or could this be too thick??

Stephen Hill

Re: Clunky Gearbox [Re: Stephen Hill] #665001
08/23/16 1:19 pm
08/23/16 1:19 pm
Joined: Feb 2016
Posts: 58
Victoria, BC,
S
Stephen Hill Offline OP
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Stephen Hill  Offline OP
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Joined: Feb 2016
Posts: 58
Victoria, BC,
Oh dear, an oil thread.
Some earlier words of wisdom from Kommando (below) suggest 50 weight engine oil is no lighter than 90 weight gear oil.
In which case, what would be a "thinner" oil?
I recall using a Redline synthetic gearbox oil in my BMW which improved the shifting remarkably. Might be worth a try. Viscosity was 75W80.

Stephen Hill


"The manual was written before EP gear oils had been developed, if you look at the BSA and Triumph handbooks in the 60's as the years move on they drop 50W and change to EP90 on gearbox oil recommendations. 90W gear oil is the same viscosity as 50W engine oil so as an oil it gets where it needs to in exactly the same way, the difference is the additional additives that give it the EP or Extreme Pressure bit, these protect the gears on the contact points. Same as the monogrades engine oils which changed to multi-grade again as they became available."


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