Fooked 750K0 gauge possible repair.

[AshimotoK0]

As you can imagine I was totally gutted when my newly rebuilt 750K0 tacho fell off a shelf and the shaft that the needle fits into snapped off  . When this happens, I believe even pro. gauge restorers write off the alloy cup/shaft/hairspring assembly. However, I am determined to come up with a fix. I stripped down a late model, large diameter N.D. 550 tatty but working tacho that I got for £2 from an auto-jumble. I sussed that the top bearing is neat on this tacho as it is combined with the brass damper tube and is screwed into the top steel bridge and has a tiny locknut. The main section of the 550 shaft is the same as the 750 but,  even though the needle is very similar on both, the needle is slightly thinner and the tapered shaft is sightly smaller in diameter. Even so, I think that you would have to be a hawk eyed, super nerdy anorak (like me) to notice the difference.
So my plan is the tap the top bridge of a 750K0  tacho (I have a spare one to experiment with and I reckon the thread is M4x0.5mm) and fit the 550 damper/bearing tube. Then I will carefully  part off both the 750 broken shaft and the 550 shaft so that I fit the top section of the 550 shaft onto the 750 one, using the top bearing of the 550. I have found a certain size of electrical bootlace ferrule has a  bore diameter very slightly smaller that the tacho shaft diameter but if you take a sharp scalpel you can cut down the sidewall and allowing it to 'snake' a little as you cut, you get a perfect coupler to join the two shafts together. It's probably tight enough but once assembled and tested could be epoxied /Loctited onto the shafts. Luckily the increased diameter of the shaft still fits into the brass damper tube without fouling.
Just waiting for my M4x0.5 taps  to arrive but basically I have nothing to lose and all of the modified parts will be from another 750 gauge I got scrap with the same problem. Or is this all just a pipe-dream 

I did email Peter H about it but the email keeps bouncing back.

[JamesH]

Ash - if you need spares drop me a Pm.

[AshimotoK0]

Ash - if you need spares drop me a Pm.

Cheers James appreciate that ... just going to try the bodge first, as I reckon there a quite a few gauges out there that have broken needle pins and that assembly is incredibly rate. As far as I( know, even Peter H doesn't have a fix. But if it all goes t*ts up then yes please !

[AshimotoK0]

Lobo/Simon .. I accidentally erased your comment .. sorry please re-post ... here is reply anyway

Peter Horton is really your man Simon. But I guess it could be debris on the magnet catching on the alloy cup  .. so that instead of the induced eddy currents driving the mechanism, there is effectively a direct drive from speedo cable to the dial shaft. Worth stripping and cleaning because a broken or over-stressed hair spring is not good news. At least you don't have the stainless crimp ring to cope with on the sandcast/K0 plastic gauges.

[Lobo]

Thanks Ash, hadn’t considered that as to my knowledge it’s a new unit, tho’ did appreciate the needle is effectively being dragged by the cup (assumed congealed oil etc). Will open it up; have done a few gauges myself and indeed grateful of no crimp ring.

[Lobo]

… just outta interest Ash had it all apart today, and found no sign of solid debris within. The cup certainly dragged on the magnet, but in a fluid sort of way, ie my gut feeling was congealed oil or suchlike. Wasn’t too keen on a major strip down @ this early stage, and so simply flooded the cup with WD40 and span it up with a battery drill to mid-range for a couple of mins. All seemed freed up & ok now - so reassembled & back on the bike.

It sure was a delight opening it up / resealing versus the 400F & 750K2 crimp rings…

[AshimotoK0]

It's great that you checked it out Simon 'cos sometimes chips of the magnet can get between the magnet and the alloy cup and cause real damage to the mechanism, that is almost impossible to fix if the hairspring gets damaged I asked Peter Horton about this and he says he also sees a few gauges with worn bushes that cause the magnet to contact the alloy cup but I guess yours is fairly new, so at a loss to suggest anything else. Maybe Peter will chip in
 and suggest another cause but hopefully, yours is OK now.

[AshimotoK0]

Update: Experimented with a few methods of concentrically joining repair shaft to broken gauge shaft and settled on reaming out an electrical boot lace ferrule  with a 1.82mm reamer.
These tiny precision reamers are typically expensive but this one was only £6 off eBay. Result ... bl@@dy perfect,  tight  push-on fit. Looking like it will be a decent repair method .. will post update later.

Phew! .. possibly saved another mega-rare gauge! 

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[Johnny4428]

Nice bit of micro engineering and improvisation there Ash!

[JamesH]

Brilliant work Ash, brilliant.

[McCabe-Thiele (Ted)]

With that level of dexterity/fine motor skills  you should have been a brain surgeon - unless you already are.

[Trigger]

Nice work for a scientist 

[JamesH]

Nice work for a scientist 

Nice comment for a career wind-up merchant :-).

[Lobo]

Cunning, Ash; nice job, esp that an old instrument gets saved.
I’ve an assortment of these bootlaces, and rarely use ‘em as intended - not that I’d have figured your great fix. They make neat swaged ends for SS cables, or ‘protection’ around such where a grub screw anchors said cable. (Kombi chews accel cables - horrible job, … change it annually to save roadside dramas)