Thorneycroft 154 Tachometer take off

[SiandSue]

The tachometer take off on my Thorneycroft consists of a mechanical angled drive unit by AC and an electrical generator by VDO which powers the tachometer. The mechanical drive unit was broken when I bought the boat and had been removed. I have managed to source a new one but unfortunately the part that connect the drive to the generator is missing. Does anyone know where I might be able to source one.

[Tony Brooks]

I am afraid not but if you had asked before buying I think most of us would have advised you to buy an alternator driven one and if necessary get the alternator modded to drive it, but your alternator may already have a tacho (W) terminal.

 

You could try ringing Calcutt Boats and ask them but I don't hold out much hope.

 

 

I am also unclear as to what is missing. Is it the gear that fits inside the engine, the piece that bolts onto the engine and holds the gear in place plus provides the thread for fixing the drive cable outer, or the piece that screws onto that fitting which is part of the cable. It could even be the cable inner or outer that has broken. If it is  a cable problem then a company that supplies speedometer cables for old cars may be able to help.

Edited July 14, 2021 by Tony Brooks

[Tracy D'arth]

Anything like the Austin Healey drive that went on the back of the dynamo? If so it is possibly available from vintage car parts suppliers, Moss?

[Tony Brooks]

10 minutes ago, Tracy D'arth said:

Anything like the Austin Healey drive that went on the back of the dynamo? If so it is possibly available from vintage car parts suppliers, Moss?

I had a feeling the piece that bolts onto the block might look like a typical 1960s speedo gearbox fitting, but what vehicle it might have fitted or if the gear was a special to match the camshaft is anyone's guess nowadays.

 

I think a photo of the area and existing pieces, plus a sketch might help.

 

I also seem to remember the cable is a bit chunky.

Edited July 14, 2021 by Tony Brooks

[SiandSue]

Thanks for all the replies. I am struggling to upload photos for some reason. 

As commented the drive unit is commonly used on vintage diesels so is not hard to source, don't know why previous owner had difficulty. 

What is I think is unusual is that the generator fastens straight on to it using the thread that an outer cable would have used. This means that what you might call the inner cable is about half an inch long and this is what I am missing. 

Currently all my engine gauges are Thornycroft branded hence my preference to keep to original. Once you replace one gauge you end up replacing them all and then your messing with 40 year old wiring so you end up replacing that and then all of a sudden your in yet another n thousand pound hole. You know the story, and then the missus sees whats been spent on the boat and wants a new kitchen/bathroom. 

Any way I'm retired and don't mind a bit of a challenge. 

[David Mack]

1 minute ago, SiandSue said:

I am struggling to upload photos for some reason. 

Maybe your photos are too large for the forum. Try using a graphics program or online image editor to make them smaller.

[SiandSue]

[David Mack]

18 minutes ago, SiandSue said:

What is I think is unusual is that the generator fastens straight on to it using the thread that an outer cable would have used. This means that what you might call the inner cable is about half an inch long and this is what I am missing. 

So all that you need is a short shaft, presumably with ends shaped to fit the matching components in the angle drive and the generator. I imagine you will struggle to buy one off the shelf, but it should be a simple job to make up something to suit.

[Tony Brooks]

Assuming its a short length of square section "cable". I suspect if you can find an old, long-established country general garage they could make up a short length of square speedometer inner that would suit. We need to see down both holes to see the shape of the existing drive parts to be sure. As David says, it may be perfectly possible to file something up.

[Tony Brooks]

Try contacting these people https://speedycables.com/

 

I suspect they will want more info and dimensions about the parts you have and how they join. Much like I suggested  a photo of what is down the two holes. If its two hollow squares, then I am sure they can help you.

[SiandSue]

Hard to see but looks like a square cable fitting in the drive end.

Large slot in the other.

I am thinking I could maybe make something from a brass screw and solder a plate on the end.

[David Mack]

Rather than solder, I would be tempted to make a single piece, filed down from a bit of brass.  Plenty of suppliers on ebay serving model engineering types who will sell suitable small pieces of brass.

[Tony Brooks]

As the shaft and flat piece need to run concentric with each other, I am not sure that I would try to file down from a solid bar because it would be all too easy to make then eccentric. I would probably start with a brass bolt and file the head to form the flat piece.

[SiandSue]

I will have a try and let you know how I get on.

[PeterOH]

I also have an issue with a similar tacho. Mine is on a Thorneycroft 154. The sender unit, which bolts to the engine, works fine when in place on the motor and I can see the round cable/brass key holder spinning when the engine runs. This of course is observed with the generator unit removed. The gen unit also works fine and sends a reading to the gauge when I spin it using a drill. The issue arises when I join the two units. It appears that friction somewhere causes the sender unit output to stiffen and stop. With the unit removed from the engine, I can spin the gear end and the output spins smoothly. When I screw the coupling nut over the output nipple, it continues the spin smoothly, but as soon as I nip it up tight, the unit feels like the internal gears are slipping and the output stops turning. It doesn’t feel crunchy or broken, but a nice soft consistent clunk clunk clunk, almost like it’s a safety override. If I ease the coupling nut 1/16 of a turn,  all works again. Bolt everything back together in place,  with the coupling only finger tight, and no signal is registered on the gauge. I’ve tried all sorts of combinations of tight and loose in the two couplings. Any ideas? Can the sender be dismantled? I noticed in the original  photo in comments above that the thread on that sender unit output end is split into two parts. Mine appears to be one solid piece. Appreciate any tips regards the issue. The motor has run less than 500 hours, so I figure the existing parts are more likely to need adjustment or repair, not replacement. Thank you.

Edited June 24 by PeterOH
Replaced .heif file with .png, so hopefully now visible.

[Tony Brooks]

You do not seem to have posted a photo of your sender, which I assume is an electrical one, like the previous poster's. It is also not clear where you think the problem may be, in the electrical sender of the drive gears.

 

If it is the drive unit, then the previous poster's [hot shows the end cover is staked in position. Staked means an up stand in the body is punched over a cover part to secure it, so if you carefully chisel or grind the little rectangular stakes lever with the body and the remove the drive pin from the skew gear, you can probably drive the shaft out of the skew gear and the to push the cover off.

 

If you really do mean the electrical sender, then, without a photo,  I suspect it comes apart in a similar way, except instead to staking they have spun a flange over the top plate that carries the terminals. You might be able to carefully lever up the spun flange holding the top plate in place. I would expect the electrical sender to slightly stiffen and loosen as it rotates at speed as it generates the electrical pulses (be they an AC wave or pulsed DC).

 

Any chance that cleaning the threads up and using a blob of Locktite  to lock the threads in the working position, rather than doing them up tight?

 

I think the electrical sender is similar to the one used in certain Jaguar gearboxes for the speedo from the late 60s, so I don't fancy your luck getting new parts. perhaps the easiest way forward would be to fit an alternator driven rev counter, even if you have to get a phase tap fitted to your alternator.

 

FWIW the number of hours run that you give seem unbelievably low to me for a BMC 2.5 diesel, probably dating from around 1980 unless it is ex government surplus newly purchased.

[PeterOH]

Thank you for the reply. I did post an image and can see it at my end. Perhaps there’s a file-type issue in what I uploaded. It is the same unit as the one posted a couple of years back. The generator is fine. The issue is in the drive unit and happens when the coupling nut is screwed onto it. It happens irrespective of whether it’s uncoupled on the workbench, connected to the generator or bolted to the motor. I’ll have another look at dismantling it using your suggestions. BTW, the motor is in a 40ft ketch I launched 10 years ago. It was purchased new by the chap who started the build a couple of decades earlier. Pleasingly, it started in about three seconds after sitting for so long.

Edited June 24 by PeterOH

[john.k]

Used to be companies that serviced speedos and cables,and such ......do they still exist in the electronic age.?................I know a friend had one of the bulb and tube type temperature senders 'restored' by a specialist ........the cost?  ... $800...thats 400 Quid .

[Tony Brooks]

7 hours ago, john.k said:

Used to be companies that serviced speedos and cables,and such ......do they still exist in the electronic age.?................I know a friend had one of the bulb and tube type temperature senders 'restored' by a specialist ........the cost?  ... $800...thats 400 Quid .

 

Yes, the companies do, and I give the original OP a link.

 

8 hours ago, PeterOH said:

The generator is fine. The issue is in the drive unit and happens when the coupling nut is screwed onto it.

 

In that case, I would suspect one of the gears in the unit is damaged, at least one may be plastic. I described how to get it apart and to reassemble it is the reverse and re-stake between the original stakes that you will have removed.

 

If the cost is prohibitive then, as I said, an alternator driven rev counter is relatively easy and should cost around £100

 

Certainly, give the company I linked to for the original poster a ring. There will be others in the vintage car and old truck/tractor magazines.

Edited June 25 by Tony Brooks

[john.k]

Icknield still make all the high end marine tachos ,drives, gauges and stuff ..........Ive got a full set of Icknield instruments for a  6L3 Gardner ,never fitted .

[PeterOH]

Finally found time to dismantle the drive unit and investigate what’s causing the problem. Pics attached to explain and for future reference for forum users.
The first section which includes the skew gear looked fine. However, the nylon gear in the second section was able to spin on the spline/shaft because the groves in the gear’s hub have been stripped.

I also found something strange which likely caused the gear to strip. The shaft is able to clunk back and forward about 2mm. It doesn’t feel right and in the forward position the shaft stiffens and barely turns. See pic of the output end where the key/cable slots in. You can see the air-gap here as the shaft is extended the 2mm.

So, I’m assuming the shaft is clunking into the extended position, stiffening up, and causing the spline to strip the gear’s inner hub. Not sure why the shaft is moving. Just guessing…but it’s possible when the generator unit is connected that the brass key/ cable should sit tight at both ends, which would prevent the shaft moving? My key/cable doesn’t look like it’s been shortened through wear. Perhaps that is the role of the coupling nut, to adjust and ensure both ends are tight and the shaft is therefore pushed back and spinning free.

Welcome any ideas. And yes, will look to change to alternator style tacho in future when I need to replace the existing alt. Thank you

[Tony Brooks]

It seems that the end float in the shaft is set by shims (washers) that you seem to have found between the case and the metal skew gear. If you look at the hole for the shaft in the third photo, it looks as if a degree of wear has taken place around that hole. Unfortunately we can't see the adjacent surface of the plastic gear, but if they have been running in contact with insufficient lubrication than there may well be enough fiction to cause the stiffness/locking. Skew gears do produce end thrust, so the load of the cable and rev counter will tend to drive the gear and shaft either against the top disk that you have removed or down against the place that looks worn. I suspect the plastic gear is being driven onto the face around the hole.  It is possible the ends of the teeth were bidding on the other gear.

 

I think that I would thoroughly degrease the gear and shaft and try to Araldite them back together. Then cut/make some brass shims (or file down brass washers) to put under the plastic gear to minimise the end float. Keep the shims that go under the metal gear. I would use Moly grease on the plastic gears and plenty on the new shims.

 

I am far from sure Araldite would be successful, so to minimise the load I would lubricate the cable if you can pull the inner out.

 

FWIW It is relatively simple to solder a fly lead to the alternator phase to get pulses for a rev counter. Any autoelectrical specialist should be able to do it for you.

[Tony Brooks]

Further thought. If you were really careful, I think you could drill right through the middle of the gear and shaft, drive in a small roll pin (sometimes called spring pin) that is short enough to sit below the base of the teeth, and dress the edges of any burred teeth.

[PeterOH]

Thank you Tony. I had a quick look at the second gear today and its outward pointing face looks in very good condition and there’s a single shim between it and the metal face. So if I slide the gear up to within a whisker of the shim and glue it in place there, that should stop the clunking back and forward. That also appears to centre the gear in the housing, well aligned for the first gear. Next week I’ll follow up your tips regards the force created by the first gear and if it is properly aligned or needs extra shims inside. It also had one inside, and yes, three outside. In the third photo, what you’re seeing is the inside shim held slightly ajar by oil, before I picked it out. That’s why it looks worn on one side. You’re seeing the casting underneath. Presumably the three outer shins help align the skew gear? Anyway, I’m feeling hopeful and will let you know how it goes next week. Regards Pete

[PeterOH]

Hi @Tony Brooks I used epoxy glue on the spline for the second gear and put a little on the first gear just for good measure. Everything is running smoothly, although only with my fingers spinning the skew gear. You suggested moly grease, but I was wondering if half filling the unit with synthetic oil would be a better option. The lowest positioned gear will splash it around and keep the  unit good for years?? I worry a smear of grease will quickly work its way off the gears and sit in the corners. Or do I pack it full of grease? I also wonder if the engine’s oil will work its way down the skew gear shaft and into the unit anyway. There’s a helical groove in the shaft.  Appreciate your advice on this before i pop the second end cap into place and bolt it back onto the motor.