Metric threads (was: Re: Spica head-to-manifold studs)

["John Hertzman" <johnhertzman@domain.elided>]

Dana, as is his wont, added both solid information and useful insights on the
fasteners' threads' thread. I particularly appreciated his proposing the
probable (more likely certain) explanation for the two odd nuts on the Spica
intake manifold.

 I had made a cavalier assertion that "There are very few 8 x 1.25 mm nuts on
an Alfa or in an Alfa parts-pile - at my last count about a dozen, with four
different part numbers, on a 2000", and Dana countered with (other than the
engine) "Elsewhere on the car, however, most nuts, bolts and studs use
standard metric threads--1.25 thread pitch for 8 mm fasteners.  This size is
readily available from hardware stores and import-oriented parts vendors.". I
assume he is right, and I undercounted both on the absolute number and the
number of different ones. Skipping one which is probably a typo (listed in
different places as an 8 x 1 and as an 8 x 1.25) I count thirty-nine 8 x 1.25
mm nuts on a 2000 with seven different part numbers - 2120.15046, 2120.15104,
2120.20054, 2120.20149, 2120.92874, 2125.15046, and 2125.20078. One is
probably an elastic stopnut, another is in the exhaust system and may be
corrosion-resistance-oriented, and probably the hardware store generic 8 x
1.25 would do for all in a pinch, but the implication of nuanced design
rationales goads my curiosity. It is probably less rational than the Spica
injector interference case.

 Dana also wrote, in the same section, "In my experience, the external engine
fasteners are about the only ones on the car with a fine thread pitch; 8x1.0
is most common, but the 6 mm studs on the water pump and other assemblies also
have fine threads.   Elsewhere on the car, however, most nuts, bolts and studs
use standard metric threads--1.25 thread pitch for 8 mm fasteners."

 No arguments here, but an observation on what seems to be DIN terminology for
fine and standard. The little information I have is that all the Alfa 6 mm
fasteners are DIN standard, 6 x 1 mm, DIN fine being 0.75 in 6 mm. The 4 mm
and 5 mm fasteners are also counted as DIN standard, 0.7 mm and 0.8 mm
respectively rather than DIN fine 0.5 for both. Otherwise, elsewhere on the
car Alfa fasteners seem to be more commonly DIN fine rather than DIN standard.
The 8 mm certainly come in both 1 mm fine and 1.25 mm standard thread, 10 mm
also in both 1 mm and 1.25 mm thread, 12 mm apparently only in the 1.5 mm fine
thread rather than 1.75 standard, 14 mm and 16 mm also 1.5 fine rather than
2.0 standard, 18 mm and 20 mm also in 1.5 mm.

 And then there is 7 mm, in which the 1 mm thread pitch seems to serve as both
standard and fine, and which must be one of the rarest of Alfa fasteners;
apart from the oil pan there seem to be just two 7 mm nuts on the whole car,
one in the shift lever and one in the hanging pedals.

 My curiosity stems partly from a wrong idea I had in the early sixties when I
first ventured beyond Whitworth and SAE. I culled what I thought would be a
potentially very useful large collection of metric fasteners from a large
local junkyard, and wound up with a surprising number of nuts that don't fit
any bolt I have and bolts that don't fit any nut I have. I suspect that most
of the odd ones are French, but that is probably an unwarranted prejudice.

 John H.

Raleigh, N.C.

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