Layshaft speedo gear removal

[RichardL]

Hi all,

I am in the process of trying to replace the speedo driving gear at the end of the layshaft and do so without removing the gearbox from the bike. I removed the oil tank to allow a better angle for drifting out the drive pin through the layshaft. It's still quite hard to get a good angle. I chiseled off where the drive pin was peened over and, basically, turned into a rivet. I have a few of other questions with which I could use some help.

First, does anyone know the actual diameter of the drive pin through the layshaft? Knowing this, I could fashion a proper drift. I have seen the pin (67-3128) for sale and it's cheap enough, but I would like to get busy before it could arrive.
 
Second, how much end play should be allowed in the layshaft? I have about 0.065". The only thing I could see that might control this is the layshaft thrust washer. I don't think I would have fogotten it when I put the gearbox together back in '05, but I guess it's a possibility.

Third, any other tips I should know? Am I attempting the impossible by trying to do this with the gearbox in the frame?

Any advice would be appreciated.

Richard L.

[RichardL]

Answering my own questions, maybe in reverse order:

It is NOT impossible to get the layshaft speedo gear out with the gearbox in the bike. Hooray! I did it!  

Here's how: Like I said, I used a sharp (and sacrificial)  tool-steel wood chisel to remove the peened-over portion of the pin on both ends. The pin, being softer than the layshaft, I used a 7/64" drill bit to pilot into both ends of the pin without too much concern for the layshaft. Because there was no straight-in angle for the drill, I made the bit operate in a slight arc (two broken bits is a small cost). After making the pilot holes, I went up to an 1/8" bit and drilled until I thought I was through the thicnkness of the layshaft. Tried my 7/64" drift again and, voila, the pin started moving.  

For those still interested, the pin diameter is 1/8".

The question still out there is how much end play should be in the layshaft. Like I said, mine is about 0.065" I'm now thinking this is not a big deal, after all, the transmission gears are meant to slide around on the shaft and the speedo gears shouldn't care if the spirals move back and forth across eachother. Any other opinions?

Richard L.
 

[alanp]

Richard, this reminds me of when I went through the same process a year or two back to replace the speedo drive gear. Unfortunately, the pin hole of the replacement gear was in a different position to the old one and also therefore the drive shaft. Since they were both hardened steel I had to give up and put the old one back. I hope you are luckier!!!
Alan

[RichardL]

Alan,

That is a very good warning, because the speedo gear I'm set to buy is a different part number from what my parts book says. SRM tells me the parts are interchangeable, but now I will ask them to measure from the end to the hole center.

Thanks very much for the heads-up. As you can see, putting the old gear back is not really practical. How did it get this way? That's a very embarrassing story that had me talking to myself back when I wrote the posts at http://www.a7a10.net/forum/index.php/topic,940.msg6327.html#msg6327 , which are tedious while trying to be funny.

The untold and embarrassing end of that story is that the drive gear I was forcing into the layshaft gear was, unobserved by me, spiraled in the opposite direction from mine (I think it might have been from an A65, but don't really know). I do these stupid things just so others can feel good about themselves. No need to thank me.

Richard L.

 

[trevinoz]

Richard,
              hopefully they won't give you the one that suits the STD2 or RRT2 box. Same number of teeth but different diameter.
Trev.

[RichardL]

Trev, et al,

I'm afraid I've been a bit of a dolt about my gearbox. Though I knew it was a Sc T2, I didn't bother checking the BSA gearbox chart for the correct speedo gears. My parts book assumes that everything on the bike is '55, but the Sc T2, according to the chart, was '57 Spitfire, only. The chart says to use a 42-3033 layshaft speedo gear, which is the one I am considering. Now, here is where I get in a twist. I have already purchased what was advertised as a 67-3175 driven speedo gear and it appears to match the slightly damaged one that was in the gearbox when I bought the bike in '73.

So, my questions are: Do you (or others) know why they would change the diameter of the layshaft gear? Is the driven gear also a different diameter? Did they just change part numbers for the fun of it? Does anyone have either or both of the speedo gears exposed such that they could measure their diameters for me and advise as to which gearbox they came from? Should I buy stock in BP?

Richard L.

[dpaddock]

Re Alan's problem -
The gear could be annealed, drilled properly and then rehardened.
The annealing and hardening temperature range is the same: 1450 to 1500 degF (medium to full cherry red). If the gear teeth are covered with boric acid, they won't scale. Quench in mineral oil. Try this on your old gear first.

David

[The General]

I have a 58 Golden Flash apart right now.
STD gearbox.
Gear you are replacing measures .468 inches in dia.
Gear it meshes with is .598 dia.

[RichardL]

General,

Very helpful information. Thank you. Mine, from the Sc T2, match these diameters. I don't yet have the layshaft gear but the driven I bought (67-3175 for standard gearbox, I think) differs in that it does not have the shoulder below the gear, as shown in the picture. Can you tell me if yours has the shoulder, or not? It appears I need only fashion a spacer. Much easier than if it were the other way around.

It seems I will ultimately learn each little detail of my A10, and many thanks go to all who have helped me, for years now.

[trevinoz]

With a bit of a brain strain on my behalf, you will find that some use a ten tooth driven gear and some eleven tooth hence the different diameter.
Trev.

[trevinoz]

I just had a look at an STD box driven gear. It has 11 teeth.
Trev.

[RichardL]

Trev,

Thanks, kindly. Sorry for the brain strain. I understand two pints of VB will cure that. Or, if that's not your thing, you could try chanting "Om." I'll take the former, thank you. Honestly, thanks for your time.

Richard L.

[The General]

My driven gear is still in the housing, but as far as I can see it does not have a shoulder.
The gear is about .675 long I can't really get an exact measure of it.

[RichardL]

Gents,

After much help from fellow members and a search for the correct part, which finally got me a used one, I have managed to replace my destroyed layshaft speedo gear. In the process of searching for the correct part, I bought two of the wrong type with the larger diameter, as mentioned by Trev. I installed the first one and managed to ding it up a bit getting it  out of the layshaft. When I received the second one I was diasppointed to see it was the same part number. What surprised me was the limited knowledge on the part of the dealers as to which gear worked with which gearbox, or that there was even a difference.

For future generations of our ilk, I have attached photos of the two gears. The larger diameter part is 42-3033 and the smaller, 67-3088. The difference in diameter is obvious on inspection, with 42-3033 being 0.550" D. (6 teeth) and 67-3088 being 0.468" (6 teeth), as noted by the General. Less obvious is the difference in distance between the back end of the gear and start of the pin hole. For 42-3033 it's approximately 0.300" with 67-3088 being 0.330". It's a bit difficult getting this measurement exactly, but the difference is enough to know it's not accidental. Furthermore. I had to do a bit of grinding on the back end of the gear to be sure the holes lined up.

All that said. exactly which gearbox gets which gear is still not clear to me. Mine is an Sc T2, so, let's say, Sc T2= 67-3088. According to Trev and the stock list at Len Haggis, RRT2 = 42-3033. According to Trev, Std2 = 42-3033. After these, I would just be guessing. For instance, I assume my parts book thinks my gearbox is an Std, so I would guess Std = 67-3088. For fear of leading anyone astray. that's it for my guessing. If someone has complete knowldge of which goes where, it would make a better list than my specultion here. Also, I am real curious why they are different at all.

Sorry about this being such a tedious read, if you've read this far and are disappointed with the ending. After the waiting, sweat, and money that went into this little part, not to mention your reading time, I thought a little more story and some findings would be worthwhile.

Richard L.

[trevinoz]

Richard,
                The reason there are different gears is because some boxes use a sleeve gear/layshaft gear with a different ratio to standard.
By changing the speedo drive set to compensate the speedometer can be the same type as standard.
    Trev.