The
following technical tip is reproduced by kind permission of Kit Car magazine as
part of Gerrys' Tech Tips series. We shall be
including one 289 relevant tip from Kit Car each issue. If you are impatient,
back issues can be obtained at 25% discount by mentioning that you are a member
of the 289 Register.
If you have a technical tip that would help other members, please consider
forwarding a copy to Kit Car. You never know, you may get it published and
acquire national notoriety!
Although most kits
are now produced in a way that does not require the builder to carry out any
fabrication work, either in GRP or metal, there are occasions when you wish to
include some modifications of your own. In particular, glass fibre is a very straightforward material to work with and
it lends itself easily to fabrication. However, many people still do not
appreciate that it is a simple matter to, say, add an edge to a panel to make
it fit an aperture neatly, or mould a flange to make a good seal. In this
article I'm going to describe how it is possible to add a flange to the raw
edge of a fibreglass panel. In this instance the
panel is a hardtop but the principles involved will serve for any number of
jobs and it will give you an idea of the versatility of GRP and how easy it is
to work with.
We often supply hardtops to people who have cars other
than our own, and other manufactures do the same - it's a case of
mix-and-match. But, no two cars are alike, even those which are copies of the
same original, so to obtain a good watertight fit some work needs to be done.
But before we start, a word on safety. While fibreglass in its cured state is inert, you should take
some precautions when grinding or cutting. Always wear a suitable mask and
protect your hands with gloves and a barrier cream when working with GRP. Some
people react to fibreglass dust and if it gets inside
your clothing it can be uncomfortable. If you think you will be affected use protective
clothing buttoned at the neck and wrists. When laminating, always ensure there
is adequate ventilation.
When people buy our hardtops for other makes of car we recommend they first fit
the hardtop, which may involve altering the rake of the windscreen (usually a
simple nut and bolt job), and then fit the soft-top if there is to be one. This
way the side screens supplied with the hardtop will fit the hood but the hood
will have to be tailored to fit the side screens and the windscreen position.
So it is best to do some forward planning before the build has progressed too
far. But that is by-the-by really.
Fig 1. The narrow footprint of a
conventional rubber seal causes damage to the paint surface and it does not
remain watertight for very long
Here we are more concerned with obtaining a good fit between the hardtop edge and the bodywork. Hardtops for original cars of the type we make were cut back to match the body shape, which was different on every car produced, and the raw edge was fitted with a rubber seal moulding, similar to a boot seal. Because of the small surface area this did not prove to be watertight and the seal quickly wore through the paint surface. We prefer to mould a wide lip to match the body shape, this reduces the damage to the paint and is perfectly watertight.
Fig 2. A wide flange on the hardtop, sealed
by a self-adhesive closed-cell foam strip spreads the load and ensures a
water-tight seal
However,
for cars other than ours we have to supply the hardtop with a single edge,
which has to be shaped to fit the contours of the car to which it is to be
fitted. But rather than revert to the unsatisfactory single edge and rubber
seal-type fitting we recommend moulding on a flange.
Here's a step-by-step guide taken from our construction manual.
1) Mask the car in any area that the paint is likely to be damaged, use
soft cloth padding if necessary.
2) Offer up the hardtop moulding and line it
up with the screen top. Adjust the screen angle until the side window aperture
lines up with the rear of the door line. Clamp the screen in position and check
for satisfactory fit. Check also that the hardtop is central and level. Our
hardtops are manufactured to fit Brasscraft windscreens
and although other makes are very similar, the fit may vary.
3) You will undoubtedly find that the hardtop won't fit the body shape
at the rear, especially on 427 models and it will require trimming - it's not a
difficult job, just work carefully. Scribe a line all round
the lower edge of the hardtop which follows the bodyshape
- you can make up a little jig for this using a block of wood and a pencil.
Using a fine-toothed blade in a coping saw, a jig saw or air saw, trim back to
the line. The body shape usually changes after the door shut line on different
models. Ahead of the shutline it should not be
necessary to remove any material.
4) When you are satisfied that the fit is good, you are ready for the
next stage - move to step 6. If the fit is poor and you have large gaps then
you will need to graft in a section of glass fibre.
For this you will need some release wax, some sheet aluminium,
Acetone, resin and hardener, a quantity of gel-coat and some chopped strand
mat. All of these materials can be purchased at motor factors, some DIY
motoring shops or by post - or from the manufacture of the hardtop. Proceed as
follows: Cut a piece of thin 18/20 gauge aluminium to
follow the body curve and allow it to overlap the hardtop on the outside. Give
the aluminium a couple of polishes with release wax
and clean the edge of the hardtop with Acetone. This will leave the surface
tacky which will assist adhesion of new GRP. Pop rivet the aluminium
to the hardtop.
Fig 3. Rivet a correctly shaped piece of aluminium to the hardtop to make a temporary mould
5) Remove the hardtop and turn it over
to achieve a more comfortable working position. Apply a coat of gelcoat (thixotropic resin) mixed
with hardener onto the aluminium and up to the raw
edge of the existing GRP. Allow it to set, but note that it will still feel
tacky even when it is set.
6) Mix some Jay-up resin and hardener and carefully laminate three
layers of 1 oz chopped strand mat on top of the gelcoat
and overlapping the existing hardtop. Make sure the air is worked out with a
brush or roller. Allow it to cure, then drill out the pop
rivets. Trim the edge with a
7) Line up the hardtop perfectly. On the rear of car body panels pick
out the outline of the outside edge of the hardtop with masking tape.
8) Remove the hardtop and mark with tape another line two inches inside
the original outline. If the car is in gelcoat then
you can proceed to give this masked-off two-inch strip 3 or 4
good coats of release wax. If the car has been painted, then apply a
mask of several layers of tape to protect the paint before applying the release
wax. Apply a coat of PVA release agent on top of the wax and allow it to dry.
9) Mix gel-coat with hardener and apply a coat. When this is dry,
laminate about three layers of 1 oz chopped strand mat, cut into two inch
strips, on top. This will form the flange for your hardtop seal and
because you have used the car as a mould, the fit will be perfect.
10) When this has cured, refit the hardtop and if necessary trim it
again to get a good fit and alignment. Laminate two or three stripes of 1 oz
chopped strand mat around the inside corner to join the hardtop to the flange
and allow it to cure.
11) Remove the hardtop, trim the edge of the flange and fill any
imperfections and the rivet holes with filler paste and sand flat. This
technique can be used in any number of ways and if you work carefully, very
professional results can be obtained. The overall appearance of your finished
car depends on getting the detail right and it is worth taking trouble to
achieve a good panel fit. I'll now describe how we fit the front of the hardtop
so that it mates perfectly with the windscreen top because it may be of
interest to people who have trouble obtaining a watertight fit in this area.
12) Position the hardtop back on the car and tape metal corners (U
section channel) in position between the hardtop and the screen. Mark their
position of the hardtop, then remove the hardtop from
the car. Secure the metal corners to the hardtop flange with countersunk 8mm
stainless steel screws, washers and nuts. Just nip the nuts, don't tighten them
fully.
Fig 4. A typical hardtop
to windscreen frame sealing arrangement. If necessary you could mould a
suitable flange to the front of the hardtop just as described here for the rear
section
13) Refit the hardtop to the car and
establish the position of the over-centre catches
(rubber strip will provide the necessary tension when it is fitted later). Fit
the over-centre catches using 3mm pan-head stainless
steel screws and nuts.
14) Remove the hardtop from the car and remove the metal corners. Refit
the metal corners using Sikaflex between them and the
hardtop flange. Refit the hardtop to the car, set the over-centre
catches and allow the Sikaflex to set. When cured,
the Sikaflex holds the metal corners in the correct
position in relation to the screen.
15) When the Sikaflex is cured, remove the
hardtop and fit self-adhesive sponge rubber sealing strip along the front top
flange and inside the metal corners, and around the new flange you have just moulded to fit the rear bodywork. You will now have to sort
out a fixing system to hold the hardtop in place. We make up a simple bracket
that bolts to the hood frame fixing, but each make of replica is slightly
different so it's up to you to devise a fixing to suit your car.
Now you know how it's done, you can carry out all sorts of small modifications
and additions to GRP panels. But please don't get carried away and decide to
move into the kitcar manufacturing business, there's
enough competition as it is. Good Luck.
David Pilbeam
The article below was published in the November 1999 M.G. Car Club Magazine. It
might prove interesting to those of you contemplating using a silicone based
brake fluid instead of that nasty corrosive glycol stuff! I have to declare my
bias in that I have been using the stuff in my original MG based braking system
and now in my Tilton configured system, without any problems, and without
having to worry about replacing every 18 months or being careful not to spill
any on my paint work.
Silicone brake fluids were developed to solve several problems created by glycol
based brake fluids. Don Harmer, from the Southeastern M.G.T Register, looks at
the background and effects of this benefit to the modern motorist.
Foremost among the problems of conventional glycol fluids is their hygroscopic
properties. When exposed to air, glycols absorb up to 12% water by volume. The
water lowers the boiling point. Under normal driving conditions a car driven
daily, or even weekly, will heat up the brake fluid and evaporate a significant
amount of the absorbed water. At high percentages of water the brake cylinders
corrode rapidly leading to early brake failure (probably due to electrolytic
action as the mixture conducts more). This occurs in cars that have not been
driven for several months. On being driven, the rust or corrosion soon causes
one or more seals to fail.
The other effect, lowered boiling point, does not usually bother the ordinary
driver. It does, however, cause problems under extreme braking conditions; such
as in racing at
The U.S. Army undertook a long-term study of the problems of brake failure,
primarily due to corrosion, extreme heat (desert conditions), and extreme cold
(arctic) conditions. They were experiencing an enormous maintenance problem.
The average army vehicle logs only about 2,000 miles in a year, with most of
the mileage occurring in short bursts followed by long periods of inactivity.
They found that, invariably, vehicles that had been sitting for several months
had brake failures after being driven a few miles. They then evolved an elaborate
scheme of draining, flushing and refilling the brake system with a
non-hygroscopic preservative for storage, and then a similar reverse process
for getting the vehicle ready for use. This was expensive and delayed readiness
but cost less than the continual overhauling of brake systems.
After extensive study they found that silicone fluids (DOT5) could be used
instead. All
Silicones do not attack natural rubbers and most synthetic rubbers and vinyl
and are often used as rubber vinyl preservatives (as in many car care
products). In addition, it does not attack auto paints. The earlier
DOT 5 silicone fluid does not attach British made seals. An exception seems to
be with some German manufactured synthetic seals (such as lately used on
Mercedes). These compounds do react with silicones and more readily with ozone
(the sidewalls of tyres seem to age more rapidly than
those manufactured in other countries).
I have used DOT5 Silicone brake fluid in my 1962 MGB (both clutch and brake)
since 1984 and in my TF1500 since 1986. In both cases I have added fluid only
once since (a slight topping up). I have had no hydraulic problems with either
car. On the other hand, I have had to replace or rebuild cylinders on my other
cars filled with DOT4 glycol based fluid, invariably after the car was laid up
for other reasons.
When I switched over to silicone on the MGB I first attempted to simply flush
out the system by adding silicone and bleeding the brakes. This sounds like it
should work since the two don't mix (are immiscible), and was the method
suggested by the supplier. I found, however, the following:
the old fluid contained a lot of water, which in
contact with the silicone separated into three layers: water, silicone and
glycol. Ordinary bleeding procedures, designed to remove air, left water in the
bottom of the wheel cylinders. A sure invitation to corrosion
and failure in time. However, the brakes work fine at first, since any 'noncompressible' fluid or mixture will work. (See the army
emergency procedures for loss of coolant or hydraulic fluid under combat conditions,
using readily available fluid from every soldier in the unit. Note that a
complete overhaul is then necessary to minimise
corrosion afterwards).
I have found that the best way to switch over is with a complete system
overhaul. The brake lines should be flushed with acetone, which dissolves the
glycol gummy residues and removes any trapped moisture. The lines should be
dried by blowing with air. (Ethyl alcohol can also be used,
it is not as flammable and won't injure paint). All flexible lines and seals
should be replaced so that all rubber that has been exposed to the glycol is
removed. I think that the problems some have had with silicone may arise from
not doing this. It may be that the problems come from the interaction of the
different swelling agents in the silicone with those used in the glycol based
fluid.
Some time ago, an article appeared about the dangers of using silicone. It was
written by (I believe) a Sales Engineer for a company who markets DOT4 type
fluid. The myths created by this have been widely quoted since. Among the myths
is the fact that silicone is 3 times as compressible as glycol (TRUE) and that
this leads to excessive pedal travel, such that the pedal will travel as much
as 3cm further (FALSE). While it is 3 times as compressible, the
compressibility is still a very small number. With the volume of our brake
systems, the additional compressibility would at most add 0.1mm to the pedal
travel. Most of the problems with a 'soft' pedal arise from air bubbles
entrapped, and poor bleeding. One should take the precaution of pouring
carefully so as not to introduce air bubbles. Wait for 10-15 minutes for any
bubbles to escape, and then bleed the brakes.
Bolts
Gerry Hawkridge
We recently had to do some work on a kit previously built by someone else. Part
of the work involved removing the axle. In so doing it was necessary to remove
the prop-shaft and we were very surprised to discover that stainless steel
bolts and nuts had been used
Whilst it is a jolly good idea to use stainless steel
fixings for certain items, we certainly do not recommend their use on items
subject to high load or where strength is required. If bolts on a prop-shaft
were to fail, particularly at speed, then the prop-shaft would flail around and
destroy most things in its path, usually your bum and anything within two feet
of it! (of course you could consider fitting a propshaft hoop - Ed)
I always like to be safe and prefer to use cap head steel bolts on items such
as prop-shafts wherever possible. These are generally of 12.9 grade, compared to the 8.8 grade of the normal high tensile
bolt. In order to be able to give our members sound advice I tried to get
comparison figures from a few stainless steel bolt suppliers but they all
appear to work to a different system, which is difficult to compare.
I discovered that there are far more different specifications of stainless
steel than I had imagined. Some obviously quite strong, but
many which are probably unsuitable for our uses, other than for the attachment
of decorative parts. In fact, tensile strength figures of some stainless
steel fasteners appear lower than those of the normal mild steel versions.
One must also be careful when using components of different materials in close
proximity to one another. In the 1960s there was a craze of attaching
'sacrificial zinc' anodes to the bodywork or chassis of your car. The idea
being that they would corrode instead of the steel bodywork of the car, due to
their relative position in the electrochemical series. Many vehicle
manufacturers apparently were fitting these as standard, but wrongly referring
to them in the vehicle manual as door handles!
I was chatting to Ian Hopley, from STATUS, and he
apparently was also concerned about the increasing use and possible misuse of
different materials used for fixings on our kit cars. Ian was, in fact, just
completing a very similar brief piece on stainless fasteners, sharing many of
my concerns, but with help of the metallurgists and boffins
at
Please note, however, that the table of safe working loads provided is for
guidance only and Kit Car Magazine, STATUS and the 289 Register can take no
responsibility for any problems arising. You MUST check with your bolt supplier
for any safety critical specifications.
Stainless
Fasteners
Ian Hopley
I have recently noticed an increase in the popularity of stainless steel nuts,
bolts, self tapping screws and other fasteners among kit car constructors. I
feel that a few thoughts on the subject interspersed with a few 'pearls of
wisdom' from my metallurgist colleagues in the department may be of some use to
use to the readership.
Firstly, there's stainless steel and there's stainless steel! It is available
in various grades and each has its own advantages and disadvantages. The two
most common types are grade 304 stainless and grade 316. 304 is
also commonly called 'A2' or EN 58 E or class 70 (18/8). 316 is often called
'A4' and, doubtless a few other things besides! All of this makes finding out
what you're buying somewhat complicated. However, the story does not end there!
Both A2 and A4 are available in at least three different material conditions
and the properties vary dramatically. Interestingly, there is no difference
between the mechanical properties of the two grades. Secondly, while the
corrosion resistance of stainless steels in general is not in question, please
be assured that it is NOT immune from corrosion. This is where the difference
between the two grades becomes more important. A4 is generally regarded as
having better corrosion resistance (in most environments) than A2. This is
because it contains slightly more nickel than A2 and also some molybdenum. It
is also slightly more expensive and less suitable for high temperature (above
300°C) applications.
Another thing worth remembering is that when placed in electrical contact with
a dissimilar metal (mild steel is quite dissimilar enough!) and kept in damp
conditions, there is the potential for electrolytic corrosion to take place.
This is a process whereby whichever metal is most likely to lose electrons to
oxygen attack does so sacrificially to the benefit of the other. There is a
well established 'league table' of metals in this respect and it is known as
the 'electrochemical series'. Metals high up in the table (like Gold) are very
good at hanging on to their spare electrons whereas metals low down in the
table, like zinc, are very bad at it and corrode easily. When a metal from high
up in the table is put in electrical contact with one lower down in the table
and an electrolyte like water is present, the lower one corrodes and protects
the one higher up by donating its electrons. This technique is well known and
is used to protect ships and oil rigs from attack. The further apart in the
table the two metals are, the more vigorous the reaction is likely to be.
The Electrochemical Series
Platinum
Gold
Titanium
Copper
Brass
Lead
Stainless Steel
Cast Iron
Mild Steel
Cadmium
Aluminium
Zinc
Magnesium
This is where the good old bright zinc plated (or zinc passivated)
carbon steel bolt is our friend. For as long as the zinc coating lasts, neither
the bolt nor the chassis will corrode in the vicinity of the bolt. Once the
zinc has gone, at least we have the comfort of knowing that the chassis and
bolt are made of the same stuff and one will not corrode at the expense of the
other. There is some argument that says that the chassis will be powder coated
and therefore not in electrical contact with the bolt. While this could well be
true, I think most people would agree that in reality, it is unlikely to be so
perfectly coated as for this to be the case. If one considers the case of a
self-tapping screw into a chassis, the idea of electrical insulation between
the dissimilar metals evaporates completely. It is worth remembering that
because stainless steel is higher up the tree than mild steel. It will be the
mild steel that suffers!
We should also consider the other material properties carefully. Stainless
steels are generally not as strong as their carbon steel counterparts and can
have inferior fatigue strength. Where a bolt is used in constant tension or
shear, this shouldn't be a problem but where a load case involving repeated
bending loads exists, great care should be taken in the selection of the bolt.
The tables below give various properties of bolts commonly used and may serve
as a useful reference guide.
Before the tables mean anything, an explanatory note about the terminology is
required.
Firstly, the column entitled 'Grade' refers to the grade of the bolt. The first
three (8.8, 10.9 and 12.9) are all commonly available grades of 'high tensile'
carbon steel bolts. The grade is usually stamped on the bolt head and means
something. The first digit (say '8') means that the approximate 'Ultimate
Tensile Strength' (the stress at which it breaks) is about 800 N/mm2
(or about 50 tons per square inch). The second number represents the percentage
of the first number at which the bolt starts to yield (permanently stretch). So, for example, an 8.8 bolt would snap when the
stress got to about 800 N/mm2 and it would start to stretch when the
stress got to about 80% of that - 640 N/mm2 (or 40 tons per square
inch).
Unfortunately, stainless fasteners don't have the same classification system.
The categories I have chosen are A2, A4, A2SH and A4 SH. A2 figures represent
values for A2 stainless steel in its softest state and A2SH refers to
properties for the same material in its strongest 'strain-hardened' state. A4
figures follow the same convention. As one can see, there is a vast difference
in each case and the only way to know what you are buying is either to test it
(we can do this at minimal cost) or obtain some kind off guarantee ( a certificate of conformity) from your supplier.
One final note. All the loads I have quoted in tonnes are theoretical loads applied in pure tension (like
a cylinder head bolt). They take no account of the stress concentration at the
root of the thread so a real life failure load could be considerably lower.
Similarly, if there are any bending loads on the bolt (and most cars have
something loaded in bending!) the carrying capacity of the bolt will be further
substantially reduced.
Tensile Breaking Loads in Tonnes
|
Grade |
M6 |
M8 |
M10 |
M12 |
|
8.8 |
1.6 |
3.0 |
4.7 |
6.9 |
|
10.9 |
2.1 |
3.9 |
6.1 |
8.9 |
|
12.9 |
2.5 |
4.5 |
7.2 |
10.5 |
|
A2 |
1.1 |
1.9 |
3.1 |
4.5 |
|
A4 |
1.1 |
1.9 |
3.1 |
4.5 |
|
A2 SH |
1.8 |
3.2 |
5.1 |
7.4 |
|
A4 SH |
1.8 |
3.2 |
5.1 |
7.4 |
The second table, showing maximum working loads also refers to bolts in pure
tension. It is based on the bolt bearing a stress equivalent to 70% of its
yield stress (the stress at which it starts to stretch). These are the kinds of
loads that one might be regarded as foolhardy to exceed in service. Obviously,
the application and any likely overloads encountered in service need to be
considered very carefully if you use the bolt in shear or bending, please make
further allowances for this!
Max. Working Loads in Tonnes (Tension)
|
Grade |
M6 |
M8 |
M10 |
M12 |
|
8.8 |
0.9 |
1.7 |
2.6 |
3.8 |
|
10.9 |
1.3 |
2.4 |
3.7 |
5.4 |
|
12.9 |
1.5 |
2.8 |
4.5 |
6.5 |
|
A2 |
0.3 |
0.5 |
0.9 |
1.2 |
|
A4 |
0.3 |
0.5 |
0.9 |
1.2 |
|
A2 SH |
1.0 |
1.8 |
2.9 |
4.2 |
|
A4 SH |
1.0 |
1.8 |
2.9 |
4.2 |