I have had my 4 door A35 (AS5) for some 16 years and I fitted static seat belts to it early on.
I removed the trafficator blanking plates & drilled a 29/64th hole in the door pillar about 6" down from the "guttering".
I then took a piece of 1 1/4" by ?" strip mild steel about 7" long, and offered one up inside the pillar to enable me to mark the hole, which was then drilled 15/32". A 7/16"UNF nut was welded over the hole in the plate.
A 3/16" hole was then drilled in the pillar above the large hole, marked against the steel plate, which was itself drilled (number 20 drill) & tapped 2BA. The plate was then fitted inside the pillar, the upper seat belt fitting secured to it and the trafficator blanking plate refitted.
At the lower end on the door sill a similar procedure was adopted, but the plate was welded to the sill, which had been drilled to accommodate the nut, which then did not protrude into the car.
The centre fitting, in the transmission tunnel, were similarly constructed, except that the plate was not welded, simply secured (under the car, of course) by a 2BA screw to facilitate single-handed fitting.
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(c) H Daniels, 1998. Whilst great care is taken to ensure that the technical
information and advice offered is of the highest standard, neither the author,
nor The Austin A30/A35 Owners Club, or the Officers of the Austin
A30/A35 Club accept any liability at law for any death, injury or loss whatsoever
arising from the use of advice contained in this article.