Smith Tacho Drive Gearbox Rebuild

AgentX said:

my drive is 2-1 off the camshaft nut, so it's 4-1 at the tach head which is what the tacho calls for.

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The drive gearbox in the photo is marked BG1508-05 which would normally be a 1:1 reversing drive to match a 2:1 RSM3003/00 tach.

Hm, hang on here...

AgentX said:

I assumed 1508 series were all in the same housing, though...could there be a compatibility issue between housing and gears, perhaps?

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That, I don't know.

I apologize, the bike does call for a 1-1 ratio drive, which all three of my drives are, and drive the speedo appropriately (before blowing up, anyhow).

All you can do at the moment is eliminate that there is no end float on the camshaft. There should be a top hat p/bronze brush that butts up on the camshaft shoulder on the o/s crankcase. If anything is badly worn there, then you will get afloat. Otherwise it's the g/box that has been badly put together, or the drive nut is loose. Or even it's a pattern but that is slightly overlong.

Last time, I spaced out the drive from the timing cover to eliminate the possibility of a hard fit between the camshaft nut and the drive, and I took a skim off the spade just now during reassembly.

I just disassembled the whole thing again, cleaned to surgical-ish, and checked fit of the gears. Reassembling with only a light coating of light grease and it turns fine, very lightly, and no thrusts I can feel, though if you catch the output gear on something, the larger gear does want to ride up and against its cover...maybe I left some grit inside last time or something.

My only question now: is there anything I'm missing in the assembly of the drive? It's just the two gears inside the body and the covers, right? No other spacers, washers, discs...?

video-1587416205 by Mick Doul, on Flickr


94100744_635854227271019_7295360092398944256_n by Mick Doul, on Flickr

I am amazed that those right angled tacho gear drives are so expensive. I never realised until I bought that tacho for my friend and we could not use it. I kept it anyway. I love those RG250 tachos, they look so original on a race bike. It is under my bench where I am keeping it as a spare. A lot of guys in historic racing use Scitsu tachos - they look so unoriginal. To them it obviously does not matter. But I try to keep my bike looking authentic, even when it is not.

The RG250 gear will mate with British stuff, or does it need modification?

(Edit: Found your previous posts on this...looks cool!)

Perhaps it is time to invest in a new drive.

I am waiting on a new one from Richfield Speedograph. I took advantage of the good exchange rate to buy one for a spare, but if my original continues to weep oil, I may fit the new one and keep my original for the spare.

I expect the drive to arrive any day now. When it comes in, I will give it my evaluation and post it here.

BTW .... the drive including Her Majesty's Royal Mail, was $137.

Slick

Slick, looking forward to hearing your report. I was thinking the same, if this latest effort doesn't hold tight.

Regarding your oil leak, I'd attempt the epoxy fix recommended by Bernard. I used the heat gun to warm both the drive and the epoxy during mixing, and it was very easy. I used JB-Weld brand clear 2-part epoxy I found in my garage. That should seal it up nicely while also holding the cover in place. Maybe do that once you've replaced with the new unit, then have it in reserve as a fully-functional spare.

AgentX said:

The RG250 gear will mate with British stuff, or does it need modification?

(Edit: Found your previous posts on this...looks cool!)

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Reversing an RG250 tacho is really difficult. You undo the two screws that hold the drive on the back and turn it over, so it is on the other side of the drive shaft. The only thing I don't like about the RG250 tacho, is it has a small Suzuki 'S' printed on the dial inside it - the unit cannot be taken apart to black it out. Apart from that, if you have a classic racer, the RG250 tacho really looks the part.
There is a problem in modern historic racing, many guys these days don't know what the read deal looked like. It amazes me, the O-ring mount on the RG250 tacho is exactly the right size to fit the standard plate on the MK3 Seeley frame. All I did was drill and tap three screw holes to secure it.
It is a really minor thing, but that tacho unit looks so right. My friend ended up with an after-market head which works, but it looks so wrong - and I did not have the heart to tell him so. He thinks it is lovely.
Yesterday, I watched a video of the McIntyre Manx racing. The tacho was not the original Manx unit - looked completely wrong. But that bike should look totally original. I'm not a purist, but something with a history like that Manx, should look correct in every detail. - Am I being silly ?

The last RG250 tacho I bought, came from a wrecker in Adelaide - cost $50. I found the bike was being wrecked, on the web. - Simply rang up and gave the guy my card number, and he sent it. You have to be careful, the later RG250 had an electronic tacho. You want the one which is cable driven.
I now have three of them. The other two cost about $20 each.

That's very cool.

There's a German company that makes cable-driven instruments, too...I have their tiny speedometer on my Royal Enfield, along with a smaller electric VDO tach in a housing that looks pretty minimal and cool.


In any case, I am fingers-crossed here that my Smiths repair saga is at an end and my trouble-free Smiths use saga is beginning. I think I had several problems.

First, the original timing cover which forced the bad angle on the cable. I only just now found the spade end of the cable was bent at a slight but discernible angle. Straightened it and put away as a spare, and replaced with a new one. I think this bend was the cause of many of my problems.

Then, the possibility of some foreign matter in there and over-packing with grease.

Now I think it is all sorted, and a warmup 20min test ride showed much less vibration in the cable, a consmuch moreistent needle position (there was slight wavering before...I have no basis to know what's normal for this type of tach...) and most importantly, no expulsion of gears. The epoxy had 24 hours of set time; I risked it figuring I'd ride before some rain set in here.

Here is my try at preventing the tach drive gear box from spilling it's guts on the road ... again

First drill two #57 (0.045") holes diametrically opposite in the edge of the housing. The cap must be underflush by a minimum of 0.062".



Next make a 'V' shaped wire clip from 0.040" music wire.



Then, snap wire clip into holes ...



I warmed the housing and flowed blue Loctite into the cap seam, removing the excess. Hopefully this will cure the oil weep. I can always go to the epoxy if it still weeps.

Slick

Neatly done...hope it solves all the problems!

Is there a possibility that the inner cable is a bit to long? This would put a vibrating pressure on the gears. It might explain the pushing off of the cover and wobble of the tach needle.

texasSlick said:

Here is my try at preventing the tach drive gear box from spilling it's guts on the road ... again

First drill two #57 (0.045") holes diametrically opposite in the edge of the housing. The cap must be underflush by a minimum of 0.062".

View attachment 15791

Next make a 'V' shaped wire clip from 0.040" music wire.

View attachment 15792


I warmed the housing and flowed blue Loctite into the cap seam, removing the excess. Hopefully this will cure the oil weep. I can always go to the epoxy if it still weeps. Slick

Click to expand...

I went and drilled 3 holes in mine an used lockwire

franko said:

Is there a possibility that the inner cable is a bit to long? This would put a vibrating pressure on the gears. It might explain the pushing off of the cover and wobble of the tach needle.

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Franko, how far should the inner cable project from the housing on the drive end?

(I know it's 1/2" at the instrument end, but have no idea about the drive end...)

The inner cable does not project from the housing, but there is an adapter that fits in the cable housing. The adapter extends 7/16" from the end of the cable housing.



Slick