I picked up a nice little mechanical rev-counter on ebay recently (I collect
these in various forms) and it arrived yesterday morning.
It is pretty much complete, although damaged.
Like all early mechanical rev-counters it has a series of wheels which are
driven off a worm which itself is the input shaft normally. The better quality
units had two or even three reduction stages with engraved scales on each stage,
so you could go up to quite high rpm. A stopwatch was a required accessory in
The worm shaft has had a decent knock which has bent the shaft. The worm is OK
though and I can straighten that out myself. The problem is with two threaded
parts, one is a shouldered bush with male thread on one end and a female thread
t'other end, and the remaining item is a round headed screw or shouldered bush,
but I haven't got that as it broke off before I received the item! It has to
act as a retainer for part of the framework that moves under spring pressure to
keep the gears engaged, releasing them allows the counter scales to be reset.
The spring I can source out of a biro.
I have put some pictures up on our website:
First is the collection of fasteners. The body and broken-off thread on the left
is the shouldered part. The little taper pin is about 3/16" long.
Second picture is the bent worm shaft.
Third picture is the complete counter. There are two nickel-plated frames, one
behind the other. The fixed frame has the holding handle, the moving frame is
the one that holds the gear that is driven off the worm.
I haven't got anything small enough in the way of machinery to tackle these
bits, the threads are 8BA or something Metric.
If anyone can assist (paid for of course!) could they contact me off-list. I
will do a proper drawing of the parts before manufacture.
Peter & Rita Forbes
Engine pages for preservation info:
18 years ago