The Shed – September-October 2019

The repair process

First the damaged piece is forced back

into its original shape. Dennis says that

if this isn’t done before any welding is

attempted the welds won’t hold. The

Jeep’s bumper had suffered major

distortion as well as being holed. Heat

from a heat gun was used to soften the

plastic and then the two of us expended

considerable effort pushing and pulling

it back into its undamaged shape.

The heating had broken a lot of

the chemical bonds holding the

polymer strands, which form the

plastic, together. This meant that the

material was moveable — it was more

‘plastic’ — and able to be restored to its

undamaged form.

Next each of the major splits in the

bumper was addressed separately and

welded up in turn. Dennis tacked the

splits using a hot iron and then reinforced

the join with plastic rod which was

heated almost to melting point using a

plastic welding gun. The tacking process

melts the plastic on both sides of a break

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the two sides together, just like fusion

welding of metals. In plastics this tack

is weaker than the surrounding material

and so must be reinforced with plastic

welding rod of the appropriate type.

Different plastics

Plastic comes in a large number of

different varieties and combinations

and like rod must be used with like

plastic. Dennis has made a serious study

of the different types of plastic used

in cars and, particularly, motorcycles.

There are websites on the internet

which will tell you what Jeep bumpers

are made from. One site that lists

the type of plastic by car make is

If you wanted

something done you

often had no choice

but to do it yourself

The bubble balancer

Before modern wheel balancing
machines arrived, car wheels were
balanced using a jig with a built-
in level called a ‘bubble balancer’.
Dennis has two of these Australian
devices which he has restored. He was
surprised and pleased to be able to
send to Australia for the round plastic

level vials to make them as good as

new. To balance a wheel it is placed

horizontally on the jig. The position

of the bubble in the vial determines

where wheel weights are fastened to

the rim to counteract imbalances in the

rim and tyre so that the wheel spins

without vibration.

Dennis with one of his bubble balancers

Fastening balance weights

to a rim on the balancer