The BSA A7-A10 Forum

Technical => A7 & A10 Engine => Topic started by: iansoady on 18.10. 2010 17:08

Title: Chronometric trip rebuild
Post by: iansoady on 18.10. 2010 17:08
Has anyone attempted this? I've managed to strip it and get it back together (note to self: the second time is easier!). I had to strip the speedo anyway as some previous owner had managed to almost fill it with horrible grease - hence reading up to about 130 mph! I know A10s are fast, but...

Anyway, I perhaps foolishly took the trip apart so that when I squirted contact cleaner around the place the numbers wouldn't come off (as I later discovered they seem to be proof against the cleaner). On reassembly, which isn't exactly an easy matter, as it's full of little springs and spacers, all the drums seem to turn together when the mechanism is spun by an electric drill, ie it starts at 000.0 then reads 111.1, 222.2 etc etc.

I know I should give it to someone who knows what they're doing but I like to try myself. So, any pointers would be useful before I give up.
Title: Re: Chronometric trip rebuild
Post by: a10gf on 18.10. 2010 17:35
(I'm no expert on this)... but obviously sounds like the 4 wheels (dials?) are glued\stuck\pressed together (it's only the small tabs that should make the next LH dial\wheel click one notch up pr. revolution. Maybe something is too tight, or residues of the grease ?
Title: Re: Chronometric trip rebuild
Post by: Stu55Flash on 19.10. 2010 00:08
The counters, washers and drivers are probably in the wrong order. See this document for the correct sequence:

If you search the internet for Repairing Jaeger and Smith Speedometers by Anthony Rhodes this gives the best guide. I don't know how to upload it here,

Title: Re: Chronometric trip rebuild
Post by: iansoady on 19.10. 2010 11:05
Thanks both. Mine doesn't look quite like the drawing, although the principle is obviously similar. Ageing eyes and consequent long sight don't help matters!

I'll have a look for the guide you mention Stu.
Title: Re: Chronometric trip rebuild
Post by: iansoady on 19.10. 2010 11:11

Found the guide (which is excellent, although referring to magnetic instruments rather than chronometric). I think the key lies in this section:

The ?old? style odometers work by friction trying to turn all the odometer wheels and then a restraining clip underneath the wheel prevents motion except at certain times.  The drive gear is keyed to the shaft and there are keyed washers between each of the wheels. The wheels themselves are not keyed and can turn freely.  As the drive gear turns, it turns the shaft.  The shaft turns the washers between the wheels.  By friction, the wheels try to turn, but the clips prevent turning.  The restraining clips underlie two adjacent wheels, so one wheel can disengage the clip under the next wheel to the left.  The left and right edge of each wheel have a thin metal edge with notches.  These notches engage the clips.  On the right side of each wheel the edge has ten notches.  The left side of each wheel has one notch.  When looking at two wheels, as the right wheel turns one entire revolution, it disengages the clip under the left wheel once.  The right wheel moves ahead by one notch, then the clip re-engages and prevents further forward motion until again disengaged.

So I need to look at how the clips are engaging and disengaging.

Fascinating stuff!
Title: Re: Chronometric trip rebuild
Post by: iansoady on 19.10. 2010 14:32
Having pulled it to bits again I have found the problem - the little finger leaf springs weren't touching the drums - probably I'd bent them by being ham-fisted when I dismantled the thing. It does seem to be working fine now - at least the tenths drum is going round without moving the next one.

That article is excellent for understanding what should be happening and I commend it to others. It's here (
Title: Re: Chronometric trip rebuild
Post by: a10gf on 19.10. 2010 20:00
Good to read you nailed the problem, and thanks for all the xtra info.