Metric fasteners and other curious anomolies

["John Fielding" <johnf@domain.elided>]

A while back (I am behind with my daily reading so attempt to do about a
weeks worth at one session) someone mentioned that 9mm Metric bolts do not
appear in the DIN standard and wondered why Alfa used an odd-ball size for
the prop-shaft bolts.  From memory I would have said these bolts were in
fact 10mm, but it is cold / wet and unpleasant at the present time to crawl
under the Alfetta to confirm this point.

I had intended to answer earlier but needed to check up on a few details at
work from our technical data bank to ensure my information was based on fact
and not what I remembered!

Well,  in ISO 262:1998  the 9mm versions are indeed listed as well as the
alternative fine thread version so popular with motor manufacturers.  The
7mm x 0.75mm thread so beloved by Alfa for lower oil pans on the older cars
is a standard ( although hard to find these days).

The standard "coarse-pitch" for 9mm is 1.25mm whereas the fine pitch
versions are
1.00mm and 0.75mm.  So they are in fact a "standard fastener" although most
sensible folks would prefer to use either 8mm or 10mm instead of 9mm.

The reason why they are not apparently listed in the DIN standard is
interesting!  DIN is a German Industry Standard (Deutche Industrie Norm
sp?) and as such applies only to companies in Germany who manufacture to
German standards.  The ISO standard I referred to is a "European Standard"
and all member countries of the EEC would be expected to adopt this as the
preferred standard.  In fact ISO 262:1998 can be directly traced back to a
British Standard from the BSI (the British equivalent of the DIN) and it was
referred to as BS 3643 /1.  With the advent of a unified Europe and the
Common
Market Directives the majority of European countries belonging to the EEC
would have adopted the new ISO standard and the original ones would have
dropped away.

Alfa Romeo/Fiat/Lancia etc is an Italian company and as Italy is a member of
the EEC one would have expected the Italians to have adopted the new
standard.  BUT  The cars manufactured prior to 1998, when the standard was
adopted, did not have to use the recommended metric threads.  In fact there
are still some Imperial threads being used even today on modern cars because
they are better suited to certain application.  A case in point is NTP and
BSP for sensors for oil and other gases such as A/C switches.  Many of these
are still Imperial and likely to remain so for a long time.  Another point
is that VW/Audi use an Imperial thread on the block end of the cylinder head
studs as it performs better in an ali casting than Metric coarse threads.

I hope this clarifies the situation.

John Fielding
Durban
South Africa

Alfetta 1.8L turbo

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