Re: Stromberg diaphragm retaining screw tool

[Mike Wattam <MikeWattam@domain.elided>]

Hi Glenn & all

Pozidrive screws have completely taken over from Philips-headed screws
(incidentally these are now being partially superceded themselves by 'Torx'
screws).

The difference is that Pozidrive screw heads in addition to the one main
'cross' in common with 'Philips' heads, also have another smaller secondary
'cross' spaced between the main 'cross' and this can be seen on the screw
head.  Thus, Pozidrive screwdrivers have an additional set of 4 small
blades to fit in the secondary 'cross' and these can be seen on proper
Pozidrive screwdrivers (only with a magnifying glass on the small ones!). 
The idea is that the additional area helps improve tightening torque. 
However, as the secondary 'cross' sides are angled, a substantial amount of
down-force is needed to prevent screwdriver slippage.

Thus, a Philips screwdriver can be used on a Pozidrive screw, but not the
other way round as the Pozidrive screwdriver with the correct main cross
wall thickness usually cannot be correctly located in the screw due to the
secondary cross projections.  Also, a Philips screwdriver can be used in a
Pozidrive screw, but there is a danger that if a very high torque is
applied, the walls of the main 'cross' could crumble into the secondary
cross.

Your colleague has probably had a change of screws by the DPO as the parts
book only shows one part number for these screws.  Or they were fitted by
Stromberg or Triumph on production without being documented as an
anti-tamper measure for the USA?

Although Pozidrive was introduced during the Stag production run (1972 I
believe?), I think it is unlikely that any Pozidrive screw would be found
on the Stag unless many of our DPO's have fitted non-OE screws.

Isn't life complicated?  I have now found that not only has the Stag got
some Metric threads in addition to UNF & UNC, it also has BA & BSF/BSW.  If
anybody needs any info on screw identification and screw head spanner
sizes, please let me know.

Mike Wattam
Chairman - Triumph Stag Register