444
Modification
to avoid a MGB bonnet catch
failure
Ralph
Coulson describes an interesting modification he has fitted
to his V8 Roadster conversion (Flame Red 4398).
(May
12)
Sometime ago, I was motivated by the many comments regarding
unexplained failure of the bonnet release mechanism and safety
catch assembly. The thought of the bonnet providing my wife
and I with bespoke 'tin hats' did not appeal.
First I investigated cause and effect of the bonnet release
mechanism failure. After a thorough examination of the associated
parts, I realised there were several factors, which when linked
together, would give rise to a bonnet flying open:
> Worn shoulder on Lock Pin.
> Worn Catch Plate.
> Weak Catch Plate Return Spring.
> Seized Safety Catch Pivot.
> Seized or sticky Release Cable.
> Over painting of the moving parts.
> Accumulation of road dirt and grit, exacerbated
by over lubrication without cleaning.
> Poor adjustment.
One other, less obvious factor, is flexing of the body shell.
More likely with a roadster but an 'event' that could lead to
instant personal injury.
Therefore, before embarking on modification, it makes sense
to take a very close look at the existing bonnet release and
safety catch mechanism, paying particular attention to freedom
of action, cleanliness, lubrication, excessive wear on the lock
pin and catch plate and a sticky release cable. If all components
are correctly fitted, adjusted and in good condition, a failure
is very unlikely.
However, a further consideration which could affect our cars;
most now over 35 years old; is the aforementioned body flex,
leading to unintentional release of the bonnet. Just imagine
hitting one of our plentiful 'pot-holes' at 70 mph! Having previously
owned a 40 year old MGB Roadster with a body shell, albeit in
very good condition, best described as 'a little flexible',
I decided to reduce my chances of the bonnet flying open by
fitting a second safety catch.
The photographs show the modification 'carried over' to my V8
Roadster Conversion, even though the Heritage body shell is
noticeably stiffer. A Safety Catch and Ring Bracket are listed
in the Moss catalogue and can be purchased for less than £30:00,
plus a new release cable is only a further £8:95.
Installation is simple, requiring a rectangular aperture in
the slam panel 'mirroring' the existing, then fitting the components
using the original as a guide. Care must be taken when deciding
the position of the new aperture, clearance must be allowed
below for the safety catch. In my case the right hand cooling
fan motor bracket dictated I position the aperture 8.5"
to the right of the bonnet catch, centre line to centre line.
To secure the Ring Bracket to the slam panel, I fabricated a
simple plate to fit under the slam panel as this also has the
advantage of spreading the load. The Safety Catch is secured
using self-tapping screws backed up by a tack weld. |
The
advantages of this modification are:
> Auto engagement when closing the bonnet.
> Raising the bonnet is simple. Just use both hands,
tip the safety catches and 'up she comes'.
> In the event of involuntary release, the bonnet
remains close and parallel to the slam panel thus avoiding side
loading the original safety catch.
> Double the strength of a single safety catch.
> Subtle and unobtrusive.
The disadvantage is:
> Loss of originality. For me, a small price to pay
for peace of mind.
Needless to say, I haven't tested this modification but believe
two safety catches have got to be better than one.
Footnote:
Before using No 10 self tapping screws to fit the safety catch
to the bonnet of my previous MGB, I established a single self
tapping screw, in steel, would support a 400lb load, without
failure. The addition of a 'tack' weld was insurance.
Following a constructive comment from Barrie Jones to use a
5mm Rivnut in place of the self tapper, I viewed the Bollhoff
website, manufacturers of the Rivnut, (Rivnuts come in many
forms). Bollhoff confidently state their Rivnut will out perform
the screw fastener during a fatigue and axial pull test. It
is, therefore, a better/stronger option to use Rivnuts. If an
owner has doubts about the reliability of their bonnet release/safety
catch assembly, or wishes to have a second safety catch fitted
and are within striking distance of Dorset. Mike Rolls of Mike
Rolls Services 4 MGs (01258 820337) is very knowledgeable and
will be able to help.
See our illustrated A4L PDF note online for better copies of
these photos. More |
