Rich; of course, lab and aircraft quality gages have jeweled bearings as
well as designs which are to pricey for magnetic gages prior to approx 1972
when the Fed Govt mandated that ALL vehicles sold in the USA must tell the
truth within the "legal" speed limits of the entire country. This was
brought about mainly by the deliberate abuses caused by hertz, Avis etc.
who rented cars with odometers reading too high, as well as speedos reading
high. They argued in congress this was justified by making people think
they were going too fast and would slow them down....etc. They kept quiet
when asked about the excess fees collected on false mileage renters were
billed for. Tamper proof drive cables as well as odo's which were NOT
easily wound in reverse (they will break ) was also a result of these new laws.
I respect your opinions re. speedo shop experience but will state that I
too have hands on experience with several top level speedo shops . They
always say...we can calibrate an OLD stle unit to read spot-on at say 20-50
mph and by the time it gets to 60-70 it will be only 1-2mph off, and this
is good enough to keep drivers from tickets.
Beyond that mph...anything can be seen and it won't "likely" be near the truth.
Non-belted bias ply tires "grow" in dia. as rpm increases. And, all tires
roll differently when they are well worn vs being new. Speedo shops cannot
allow for this as they are spinning the speedo either in the car or on the bench.
Driven and driving gears are NOT an exact match for all the tire sizes the
car is sold with. They come very close on most but there is still errors.
Even the selections of driving gears does not always make the speedo
perfect..but damn close.
Police get those "certification" lables put on as a means to convict you in
court. Common folk don't know how to contest a LEO saying his speedo has
been calibrated.
I was, and still am....doing court work as what is called a "recognized
expert". When I was ever called to testify re. a CLOSE CALL speeding case,
I NEVER failed to obtain a court ruling to the affect....The speedometer in
the police car may be in error as to the speed which it measured against
the defendents car..! This does not always get the guy off free because of
other factors or case circumstances, but it takes away the charge of
speeding..as claimed by the LEO !
These firms which fix, test and label LEO car speedos, do not do a perfect
job anyhow and most times, they just slap on a new dated sticker and
collect their fee.
These days...it is rare to have a LEO use his speedo as evidence in court.
Too many past rulings have assured that if you get a good lawyer who knows
about this part of the law, you will beat the charge. Judges know this full
well.
These days its ALL radar, laser or the other one which times you on a
measured section (I forgot it;s name).
I also have been called on radar cases and with the help of the Cinncinatti
Microwave Co defense package, the defendent asks for a jury trial. Then
before his peers, we show how inaccurate radar CA be and how useless the so
called tuning fork records and certifications etc. are.
We explain how the LEO can show the driver a MPH reading locked on the
radar gun but that there is NO WAY to assure it is what the defendents was
doing. Etc. etc. Jurys are highly acceptent of such logical evidence which
overturns the literally "Biblical inerrency" of LEO speed measurements on
public roads.
I AM a firm supporter of all LEO people but when they are given equipment
known to be inaccurate and then they use it to convict otherwise safe
drivers, (sometimes to meet a ticket quota as well) then I am happy to be
part of any team which beats that scheme.
------------------
On your nut/bolt query: US inch size nuts and bolts AND metric size nuts
and bolts are described by stating the nominal dia. of the thread size.
Example: a 1/2" bolt has a thread "approx. 1/2" in outside dia. and the
matching nut has a hole approx. the same size (thread depth causes a slight
variation). The size across the hex head of the bolt or nut will NOT be
1/2" but will be a larger figure such as perhaps 3/4" and this size socket
or wrench will be needed to turn the 1/2" nut or bolt.
Now.. Whitworth is another matter ! This English size system uses NOT the
hex head as an indentifyer, but uses the actual outside dia of the threaded
portion of the bolt. Therefore we see some sockets and wrenches markeds as
being say 1/4"W but which are much larger openings because they are made to
fit the hex head the 1/4"W requires. This system carries on from 3/16"W to
1/2"W . I believe this goes on to larger sizes as well.
There is another thread system found on Rootes cars which is for ONLY
smaller sized threads. It is like the Whitworth but is not called by that
name, it is called " B.A. Thread". These BA sizes run from approx 1/8" dia
up to about 1/4" dia. and are called BA-0,-1,-2,-3,-4,-5,-6 (seven sizes).
They also use a hex head which is proportionate to the thread dia but their
hex is of some larger (unknown to me ) sizes. Such small sizes are of
course found in instruments and other places where such sizes and mild
clamping forces allows them to used.
By general agreement...the UK industry agreed to cease use of Whitworth and
BA sizes approx. in 1955. However, where tooling and carryover of a common
part was needed, we find that many parts of a UK car have both these
systems still employed. Example: The jet adjuster nut on SU carbs never
changed on those models using the hex nut. Jaguar oil drain plug remained
Whitworth the next 20 years or more. Some internal parts not expected to
require servicing were also allowed to remain Whitworth (such as internal
rear axle parts). I believe we have no vehicle using these two thread
systems in any location, since perhaps 1975??
I have sevral Whitworth and BA sets of sockets and wrenches (Craftsman as
well as Snap-On, Britool and German brand. It is a mess keeping things
straight. Because I did restorations on Rolls, Jag, Rootes and other UK
cars I had to have this stuff,as well as the taps and dies to match. Don't
use them much these days as I no longer earn a living wrenchin, but who
knows when I might get my hands on some UK bicycle, motorcycle , car or gun???
------------
Chris; aside from opening up the diff. and counting teeth, or seeing a tag
on the housing, the only other way to measure ratio is to mark the crank
pulley or set at zero deg. Then with plugs removed..and car in 4th gear,
mark the tire wall, then push car STRAIGHT forward slowly, counting engine
turns until tire makes one full turn. You should see 3.89 engine turns if
your rear axle is this ratio.
NOTE......it is hard and perhaps impossible, to do this by jacking up one
rear wheel and turning it by hand, due to the "differential action" of the
spider gears which will give you a halved reading of engine turns. Even
this is not trusted because they may slide a bit and give an inaccuracy.
Floor pushing works and is totally accurate. Because the OD won't be
engaged with engine OFF, you cannot do this test for the OD final ratio. We
don't need to anyhow, we KNOW what the OD will do when engaged.
Dick T.
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