Tach cable --> tach drive gear destroyed

[Testarossa]

Had to replace the mud-soaked Veglia tach on my 850T, so I bought a Vega replacement and installed it with new tachometer cable. Didn't work. The cable engages the tach and spins it nicely using an electric drill, and seems to lock into the drive unit on the timing case -- that is, it won't twist from the upper end when screwed down properly with the olive in the correct place. But when the engine runs, the cable doesn't spin in its sheath. Drive unit seems properly seated against the cam drive worm. What am I missing?

[wirespokes]

Seems the only conclusion is the drive isn't turning. Did you check to see it turns? Rotate the engine and see if it moves?

Of course you removed the cable from the sheath and know it's not broken...

[Testarossa]

So here's why the tach won't work.  170 miles after firing up the rebuilt T. Now I have to worry about steel bits in the oil, and in the pump, and bearings. I think I really screwed up.




Will drop the pan in the morning, look at the screen and magnetized drain plug, and see if I can retrieve the thrust washers on a magnet. Worst case I'm back to another engine strip.

 

[bigbikerrick]

Darn, hate to see that, Testarosa! Do you know the cause of the gear stripping like that?
Rick.

[Testarossa]

I don't know what caused this other than possible assembly error by me. How is it possible to misalign these parts?

Good news -- one of the thrust washers came out with the drive gear -- it fell onto the carpet and I didn't spot it until this morning. Frankly I'm confused about how many thrust washers there are supposed to be -- some pix show one, some show two.

Dropped the pan and using a strong magnet recovered .4 gram of steel powder/shavings from the sump and screen. I have no way to know how much came out with the oil. The damaged drive gear weighs 19.2 grams -- it would be useful to know the weight of a new undamaged drive -- that would tell me roughly how much material was ground off. Can someone weigh a new one?

I may be able to use a bore scope to examine the worm drive on the camshaft. I sent an oil sample off for analysis.

If it turns out that I got most of the material out, and the oil isn't loaded with steel, I might just button it up and ride it until something dire occcurs.

[pete roper]

You didn't have the speedo drive out of the gearbox at the same time and accidentally swap them did you? The toothforms are different.

[wirespokes]

This brings up a question I have. As far as I can tell, the speedometers are all geared the same, meaning, any Tonti speedometer will work on any other Tonti bike. I'm used to the beemers that match speedometers to the final drive. All transmissions are the same.

If the Guzzi final drive ratio is changed, does the speedometer drive gear get changed to match?

Pete - is that what you're talking about - the speedo drive gears not being the same? Could you explain?

[Testarossa]

You didn't have the speedo drive out of the gearbox at the same time and accidentally swap them did you? The toothforms are different.

Pete, that's brilliant. I did the engine and gearbox builds separately, but a bought another used gearbox for parts so I think I had two speedo drives and may have popped the extra into the timing case.  Will look around and see if I can find the third drive and figure out if that's the correct tach drive.

Meanwhile I'm left with the question of how much steel dreck is in the oil passages. If you have a tach drive gear lying about, can you weigh it? After going through the sump again I wound up with about .6 gram of tiny steel bits. I'd like to think that's most of it. Now I very much regret not installing an oil filter.

Whatever I decide to do about the engine, I'll now plug the tach drive hole and use an electric tach.

[pete roper]

Does this engine have an oil filter? If so I'd just clean out the sump, add new oil and go. If it's filterless it might be a bit more of an issue.

Compare the third drive to the one that stripped. If it's different? There's your problem. If the Speedo is working then that one must be OK. If you haven't moved the bike yet lift the back wheel off the ground and with the gearbox in neutral very gently try and turn the wheel. If it meets resistance and 'Springs back' against your attempt to turn it stop immediately as you've got the tach drive in there and it's trying to destroy itself and the worm.

Guess how I learnt that.........

[Antietam Classic Cycle]

Pete, that's brilliant. I did the engine and gearbox builds separately, but a bought another used gearbox for parts so I think I had two speedo drives and may have popped the extra into the timing case.  Will look around and see if I can find the third drive and figure out if that's the correct tach drive.

Meanwhile I'm left with the question of how much steel dreck is in the oil passages. If you have a tach drive gear lying about, can you weigh it? After going through the sump again I wound up with about .6 gram of tiny steel bits. I'd like to think that's most of it. Now I very much regret not installing an oil filter.

Whatever I decide to do about the engine, I'll now plug the tach drive hole and use an electric tach.

I have a tach drive gear or two laying around. I'll dig one out and weigh it when I get a chance.

I doubt that any of the sheared off bits would be small enough to fit through the very fine mesh of the oil strainer, so you'll most likely be fine.

[Testarossa]

Thanks, Charlie. I pulled the speedometer drive and weighed it at 19.9 grams. Since I recovered .5 to .6 grams I may have .1 or .2 grams of powder floating around. Hard to say exactly because of the weight of oil sticking to the weighed bits.  The remaining material is small enough to pass through the screen and pump. Pete, it does NOT have an oil filter.

As you surmised, it was a speedometer drive in the tach drive hole. Fortunately it's also a speedometer drive in the gearbox case.

The speedo has behaved normally and of course I turned the engine by hand to circulate oil and met no resistance, so I was pretty confident that was the case. Because I had extra parts, before pulling the speedo drive I was able to assemble the damage part with its thrust washer into a spare case-end with the drive worm held by the spline seal, and convince myself that I could in fact withdraw the drive without displacing the thrust washer. And in fact was able to do that. I know it's still there because on reinserting the drive it stuck out the same distance from the top of the case.

What concerns me is what even a pinch of finely powdered steel could do the the big-end bearings. If it's not going to be a serious problem I'll just ride it. I am going to attach a small strong magnet to the screen to catch whatever bits float through, and will inspect in another 200 miles. Or may be safer to epoxy a strong magnet to outside of the drain plug. The magnet inside that thing is pretty weak. I don't know how much additional metal came off the camshaft worm drive, just assuming that the cam is of much harder steel than the tach drive.

[pete roper]

To be honest I think Charlie's 'Don't worry, be happy' summation is the right one. The screen is designed to protect the bearings and while, undoubtably, engines with proper filters last longer I don't think that there will be enough tiny stuff floating around to destroy the bottom end.

With the sump off get a tube and funnel and pour a litre or so of Kerosene into the timing chest through the tacho drive to flush any gribblies out of the timing chest, clean the sump, bolt it back on, install the correct drive, fill with oil, go ride. You'll be golden!

Pete

[Testarossa]

That's my plan, then! Thanks for the moral support. Will take this bike to the Ouray NAR.

[Testarossa]

And by the way, I found the actual tach drive so everything's copacetic.

[Muzz]

I have to admit to having to look that one up Seth.

Pleased you found the right one.