At a recent Auto-jumble I was lucky to find a Rist horn complete and in
working order but rather rusty and dented, needing refurbishment. Tests proved
that the horn worked well but in order to return it to its original condition I
decided to strip it completely and to repaint it. What I learnt during this
exercise may be of interest to other Austin 7 owners.
On stripping the horn I found that it consisted of five main constituent
parts, the Trumpet (1), the electromagnetic mechanism on mounting plate (2), a
spring steel diaphragm (3) and the end cover (4) and mounting bracket.
When stripping I found I had to be careful with the fibre insulating washers
on the horn mounting bracket and the terminal mounting assembly and in fact in
the end I resorted to making new ones. The whole horn is held together with
through 4BA screw and nut system to clamp the trumpet body, diaphragm and
electromagnetic assembly together.
The electromechanical assembly was checked out visually and the contact
points carefully cleaned with 320 wet/dry paper.
The other metal components were paint stripped, rotary wire brushed and
treated with Jenolite. All dents in the trumpet and end cover were easily tapped
out and then, owing to the heavily corrosion pitted surface, spray painted with
high build primer prior to satin finish black.
During assembly it was important to remake three gaskets which sandwich
between the horn body, diaphragm and electromagnetic mechanism mounting plate (
see diagram ) These were made from 0.030” thick paper gasket and this
dimension is important to maintain the correct distance between the actuating
pin of the magnetic mechanism against the spring steel diaphragm which gives the
Rist its characteristic sound.
Owing to the corroded nature of the original screw fixings I replaced then
with new 4BA nuts and screws supplied by Surplus Supplies [see parts source]. It
was remarkable that they could supply exact replacements down to screw head
profile and the square dished self locking nuts.
It is important to ensure that the 6v supply terminals are well insulated
from contact with the horn assembly and that the 4 horn mounting bracket screws
are insulated from the horn with fibre washers and rubber sleeving Test for
continuity with a test meter before finally linking up to 6v supply.
Operation of the horn may initially give a muted sound due to the actuating
pin being out of adjustment with the diaphragm owing to dimensional deviation of
the assembly or compression of the gaskets etc. There is a locknut and
adjustment screw on the top of the magnetic actuating mechanism. [See Photo
above] This will need only slight adjustment to give the plunger suitable
clearance to vibrate against the diaphragm. Experimentation will achieve the
best setting for optimum sound.
The final detail is to fit a new Rist badge to the horn trumpet and the whole
assembly looks and sounds as it did 75 years ago when first delivered to
Longbridge.
