Smith Tacho Drive Gearbox Rebuild

Hi, all...been unable to locate much info on the subject in a search here and online, but when my Atlas engine is running (g15-cs model), with the cable removed you can see the gearbox tacho drive barely creeps slowly around. Not at all synched to the engine and somewhat irregular. Assuming something is really torn apart inside.

Tach itself is frozen and awaiting a repair...it sheared off two cables before I realized the issue, probably causing whatever issues the drive gearbox now has.

So...are parts available for a rebuild, or do I need to source a new one? Appreciate any insight you can provide. (Also, can it be removed and reinstalled without removing the timing cover...?)

Thanks!

I recently had to rebuild my tachometer drive gearbox.

See this thread ...... https://www.accessnorton.com/NortonCommando/tach-drive-gear-and-spindle.28859/#post-440750

And this one .....https://www.accessnorton.com/NortonCommando/smiths-1805-tach-drive-rebuild.27592/

Also check here: http://www.speedograph-richfield.com/html/rgb1507_1508_series.html

My Atlas required a Smiths BG 1508-05. The part number will be found on the mounting flange. Removal of my gearbox required removal of timing cover first.

There are a few NOS drives around ..... they are pricey. Parts are unobtanium. Your best bet may be with the link above (speedograph)

Slick

Thanks!! My best bet, I think, is a new repop drive unit and I'll see if I can repair the original over time...

AgentX said:

Thanks!! My best bet, I think, is a new repop drive unit and I'll see if I can repair the original over time...

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The drives from speedograph are said to be made from new old stock Smith's parts inventory. At least that is how I interpreted an email reply from them when I asked if they were repops.

The link put up by marinatlas seems to be a good source for repops if the guy is still in business.

Good luck with it.

Slick

marinatlas said:

http://velobanjogent.blogspot.com/2009/09/smiths-tachometer-drive-gearboxes.html

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I have seen this link everywhere, and it is very informational...but does anyone have actual sales site or contact info for the guy? Appreciate it!

texasSlick said:

The drives from speedograph are said to be made from new old stock Smith's parts inventory. At least that is how I interpreted an email reply from them when I asked if they were repops.

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Also great to know; thx!

I found out the problem with mine was simply a loose push-on cover...PO had safety-wired across the front to prevent it from migrating totally off, but it had loosened and allowed the gears to slide out of mesh or something. I pulled the cover out entirely from behind the wire, held light pressure on the gears with a dental probe, and cycled the kicker...works fine. Tapped cover back in place with a brass drift and tiny hammer and now it works great.

So, no rebuild/replace for now. Any advice on keeping the cover in place? I was considering a small crumb of JB weld on opposite sides of the cover to resist any future, unwanted migration. I could instead stack shims of some kind underneath the existing piece of safety wire crossing over the cover...? Use a spring-loaded center punch or a small cold chisel to stake it?

I press fit and red Loctited my cover. So far so good. After review of the issue of scarcity of parts, I am considering staking the covers with a punch.

Glad you got it sorted without a global chase for parts.

Slick

texasSlick said:

I press fit and red Loctited my cover. So far so good. After review of the issue of scarcity of parts, I am considering staking the covers with a punch.

Glad you got it sorted without a global chase for parts. Slick

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Gently heat the cover whole drive in a low oven temperature and pour liquid two pack epoxy while cover is held flat while it cures. HTH

Wow, that's a great idea and a thorough fix...

Well, we'll see how that works. Nickel plug on the side for fun (over brass cover), epoxy to hold it in. Just epoxy on the bottom side, as using a nickel there was flush with the edge of the drive body and didn't give a good place for the epoxy to grab. I put some divots around the edge of the body with a spring punch to give the epoxy something more to flow into and grab.

Didn't use an oven, but warmed the drive and the epoxy both with a heat gun.

OK, this thing keeps blowing its covers off, despite epoxy. I am sure the camshaft nut is tight on there, and even spaced the drive out a hair to ensure its spade end wasn't riding too hard against the camshaft nut.

How much grease, if any, should be in there? I figured the amount I put in at first was the culprit in creating too much pressure, but even after not re-applying any, I lost it after two spins round the neighborhood. (Luckily just the cover, and not the gear...)

Is there any other reason this could be happening? Again, I've used two completely separate gear sets...

AgentX said:

OK, this thing keeps blowing its covers off, despite epoxy. I am sure the camshaft nut is tight on there, and even spaced the drive out a hair to ensure its spade end wasn't riding too hard against the camshaft nut.

Again, I've used two completely separate gear sets...

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Were the gear sets matched?

Slick

It's been with matched and unmatched sets both. However, with the loss of one (or two?) I'm not sure where I am with that right now.

Edit: Also, the nickel/epoxy plug came off in a previous iteration and the one I lost today was pure epoxy, poured so it came over the top of the rim and ostensibly held on to the rim of the drive body as well as the interior. I also used a spring punch around the rim of the interior to help give the epoxy plug even more grip, flowing in and hardening into the divots.

The straight cut gears should not develop a thrust force. However, if the gears are unmatched, perhaps they might.

Slick

What's your opinion on greasing them? Should there be any packing of grease inside, a light coat only, or none to speak of? Do you think excess grease could cause the issue, or am I barking up the wrong tree?

I don't know how to fix a tacho gear box. But on both of my race bikes I use the tacho off an RG250 Suzuki. They are a dead spit ringer for what was used on Italian race bikes of the 1950s flexible mount and all. I bought one for a friend who has a Triumph. There is only one Triumph tacho drive which suits the 2 to 1 ratio of the RG250 tacho. It is the one in the shaped timing chest of the pre-unit triumph. We looked at getting a simple right angled drive of 1 to 1 ratio, but they were too expensive. The engine drive turns at half speed, the tacho is 2 to 1, so the gearbox on the engine needs to be 1 to 1. And that is what the gear drive on the Atlas engine probably is.

When I rebuilt my drive, I packed it with Mil-G-3454B. I used that mostly because it was what I had, and because it was of a consistency that I thought was suitable.

I do not think grease is causing the covers to pop off. I think you are getting a thrust force.

IIRC, I used 0.040" discs as covers. I measure approx. 0.065" from the rim of the housing to the cover. This is barely enough room to drill 0.040" (#60 drill), three equally spaced holes which would allow you to safety wire the covers in place. Sealing such a thin disc is another problem. I tried red LocTite on mine but I have an oil weep.

My drive does not have an O ring at the drive end of the input shaft, just a scrolled shaft that is supposed to pump oil back out. If your drive is similar, the invading oil should be sufficient to adequately lubricate the drive.

I have two spare cover discs, 0.028". If you want to give the safety tie wire a try, I can put these in the snail mail.

Slick

Tacho gearboxes have always been problematic, simple cause of prevention is to light heat the g/b and pour some epoxy resin while its flat on both discs.

Bernhard said:

Tacho gearboxes have always been problematic, simple cause of prevention is to light heat the g/b and pour some epoxy resin while its flat on both discs.

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Slick-certainly something is causing a force against the covers...just can't pin down what or why, or how to fix it. Just keeps happening no matter how hard I put those covers on. Appreciate the offer of more covers. Will let you know if I need them.

Bernhard, thanks--but the gearbox had epoxy on both covers, mixed and poured with heat, cured for days before use, and still blew the cover off on the second or third short test ride out. It was the side cover in this case, but I've also lost the bottom cover in a previous occasion, which also lost the gear. And in this case, it wasn't the metal coin plugging it with the added epoxy--that blew up, and I refitted everything with just the epoxy alone this time.

Acotrel, 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.


I'm really at a loss. Don't want to keep banging my head against the wall but I can't think of a single thing I'm doing wrong to make this happen. I can't see a way it could be endemic to the drive body but it's happened with matched and unmatched gears both. Don't think I have any more previously-matched gears at this point. The cover had safety wire across the cover on it when I got it, but it had enough space between cover and wire to allow the gear to displace a little and cease to function. (Safety wire now gone to allow my attempts at plugging with the nickel, then the epoxy...)

Tach was just rebuilt by Mark Bohman, whose work has been widely praised in this forum so I doubt it's coming from the tach itself...