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MG Midget and Sprite Technical - Speedometer problem

1972 midget

During a recent trip to Norfolk, the speedo behaved normally initially but stopped working after a few miles. After a coffee stop, it again behaved normally initially but again stopped working after a few miles. The same pattern after the second stop and again the renext day for the return trip.

Once home, I removed the cable and identified a slight kink that prevented the easy insertion of the inner so assumed that was the cause.

I purchased a replacement and all was good for a few shortish test drives. Yesterday after 30+ miles it again stopped working. I disconnected the cable at the speedo end to check if the inner was spinning for the return trip. After 10 - 15 miles, it stopped spinning.

I mow assume its either a cable routing issue [causing resistence on the inner] or the g/b output drive. What is the correct routing of the cable? Any other suggestions welcome.
Doug Plumb

When you ran the cable unconnected it may have worked it’s way up a bit and become disconnected from the square in the gearbox drive ?
I had a problem where the Speedo drive shaft started to slip on the plastic gear pinion. Actually I have 2 where this happened over the years, one a new item from Moss. The shaft has a sort of knurling / spline from memory - I don’t know if they are pressed in or moulded. Still looked fine but slipped when you put any torque on it.
Replaced with old stock and ok to date !

You can pull it out in situ but it’s a bit tight.

R.
richard b

It won't solve your problem but when you do make certain the P clip is fitted to the cable and pedal box. If you don't you will get flicker on your speedo.

Jan T
J Targosz

Without touching anything, I did a short test drive today and the inner was spinning normal.

I have a couple of ideas to improve the route of the cable which may be the cause. Then I will try a different g/b out drive gear.

A part of the mystery is why the problem 'clears' after a stop. Is the reason heat build up whilst running causes the inner to expand which then binds against the outer particularly if the routing is not optimum.
Doug Plumb

I've had trouble of this sort. I think the inner cable, having a spiral twist to it, can snag on any damaged, rough or too sharp a bend in the outer. The rotating inner then 'climbs' up the inner like a miniature, probably slipping, worm drive, eventually loosing it's grip on the gear at the bottom, gearbox end. It may still spin when stopped and checked as it slips back down (gravity or manipulation as you undo it) and re-engages at the bottom. I think this is prevalent on some new ones where possibly the inner is slightly short.

I also had a new cable where when measured across the flats of the stamped square section it was undersized compared to the original. It was a loose fit that engaged, but slipped. I think it was from Ahead4Healeys they were good, thanked me for alerting them as it was a supplier problem, and replaced it.
GuyW

Adjusted the route to give a better run and reconnected the speedo. Did a test drive and ok initially but stopped after 21 miles. After a coffee, it was initially ok but stopped after 12 miles. I will now replace the output gear with a s/h one a friend has loaned me.
Doug Plumb

Guy,

I've encountered the AF size issue when I fitted a Ford gearbox in my Sprite, IIRC BMC was 1/8" (3.2mm) AF and Ford was 3mm so the original cable required slight reduction in size to fit the Ford gearbox drive. I could see the wrong inner being supplied might cause issues.
David Billington

Replaced the output gear (no visual issues) with the s/h one and ok during a very short test drive. Will do a longer test drive tomorrow as that is when it stops.
Doug Plumb

Doug,
When you have removed it you can test it by gently holding the end in a vice and try turning the nylon wheel by hand - it probably is worse then the nylon gets warmed up by the oil.

R.
richard b

Found a pic of one of mine that failed.

R.


richard b

Richard,

Have you considered boring that gear out and sleeving it to achieve a better fit on the serration.
David Billington

David,

I have never really done much press fitting and none with plastics, with Nylon type plastics adhesives are unlikely to work - although I did consider trying it with some 'Assembler' grade super glue.

R.
richard b

This thread was discussed between 24/07/2023 and 26/07/2023

MG Midget and Sprite Technical index

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