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Fooked 750K0 gauge possible repair.

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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:

--- Quote from: JamesH on June 15, 2023, 12:01:46 PM ---Ash - if you need spares drop me a Pm.

--- End quote ---

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.

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