Rear bumper 2

The next step was to make a method of attaching the bumper to the alloy tube. I cut some plates from 3.2 mm aluminium with a large 9mm  centre hole and four 4 mm countersunk holes.

I put an M6 rivnut through the centre hole and fixed in place using the special tool which collapses the rivnut and forms a collar which locks it in place.

I enlarged the holes in the ply, which had previously held the screws which temporarily fixed the ply to the alloy tube, and pushed the protruding ends of the rivnuts through. I then fixed the plate to the ply with four 4 mm wood screws.

I screwed in some studding to keep the rivnuts’ threads from getting blocked and then sealed any gap between the ply and the back of the bumper with expanding polyurethane foam.

I then bonded the centre part of the ply to the back of the bumper with two layers of chopped strand mat which also locked the rivnut plates into position. I left the ends of the ply un-fibre-glassed until I had glued the ends of the bumper to the centre (remember, these were temporarily held in place by M3 nuts and bolts).

The next step was to screw the bumper to the tube and bolt it to the car. I checked that the end pieces of the bumper fitted nicely to the car’s bodywork. I then removed them and prepared some “glue” made up from body filler thinned with a little laminating resin, tinted with a little black stain. I coated both sides opf the joint, slid the parts together and substituted pop-rivets for the M3 nuts and bolts (I can drill these out later and fill the holes).

When the glue had set, I removed the bumper and filled the void at the end of the ply with expanding foam. When that had gone off and been trimmed to shape, I fixed the end pieces of the bumper permanently in place with two layers of chopped-strand mat.

I then filled all the holes and gave the whole thing a good rub down with 220 grade wet and dry paper. I degreased the surface with white spirit and put on several coats of satin black. Next day, I tempered this with a thin coat of black trim wax.

Left: the tube has been screwed to the bumper ready for re-fitting to the car’s body.

Left: the old and new compared. The old bumper weighs in at 21 kilos whereas the new tips the scales at 8 kilos.

Left: the new bumper fitted to the car - just the fog light to be re-connected.

Collar formed on rivnut.

Left: the finished fittings.

Foam.