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Repairing structural rust

September 7, 2011 at 3:35 pm
filed under Rust
Tagged Automobile, Casting Molding Machining, Corrosion, Rust, Welding

Quite a small amount of corrosion in an important part of a vehicle structure can certainly render the vehicle unsafe. This is especially so where the corrosion seriously weakens a load bearing structure. But replacement parts can often be bought and welded in neatly using equipment from a hire shop.

Cars can have very different load bearing structures de-pending on the design of their body and suspension system. For example, the sill on one car may be purely a cosmetic panel just there to enhance the looks. On another make, it may well be an important part of the vehicle construction.

How to diagnose

Many older cars fail road worthiness tests because of structural weaknesses caused by rust, so it makes sense to check your car regularly.

An official examiner will obviously be aware of the load bearing structures of your particular car and will examine them accordingly. This will involve pressing any suspect areas hard and noting the amount of flexibility that results. It is not necessary to poke or subject the suspect chassis member to heavy impact blows. The suspect area will be examined more thoroughly if the particular member seems to have been ‘dressed up’ or camouflaged with very thin metal, which is illegal and dangerous. Any corroded area should be repaired correctly to official test standards.

How to repair

The repair should be carried out so that any plating or welding extends to a sound part of the load bearing component and should be such that it is clear that the strength of the repair is the same as that of the original component. In some cases, it may be that the only solution is to replace the complete load bearing structure.

And the way to cut the cost of a repair is to carry it out yourself. With a little know-how and the pattern part , the job can be relatively easy.

Of course, the skill required here is welding, but by using the right equipment and a little practice initially, there are few jobs that cannot be dealt with. The easiest form of welding for the uninitiated is are welding. It is fast, economical and produces only localised heat.

The welding tool is essentially an insulated clamp which holds a rod or electrode, which in turn generates the continuous spark or are required to melt the metals to be joined and also provides a suitable source of metal for completing the join. The necessary safe, low voltage power is supplied by a heavy duty transformer matched to the welding tool.

are welding equipment is not designed for welding thin gauge metal such as car body panels, boot lids or bonnets, but is ideal for chassis repairs, mounting plates, brackets and so on.

A special are welder which allows the welder to produce a much smaller are ideal for thin metal is also available. This is called a solenoid controlled welder.

Basic techniques

Every are welding set should have a set of instructions listing which particular electrode size to use on a particular thickness of metal. In many cases, there will be a variable setting on the transformer and this will have to be set according to the application.

For general use it is worth bearing in mind that an electrode size of EMS 10 is suitable for 3 mm thick steel and above. A typical welding current of between 90-120 amps would be required. An EMS 12 electrode would be recommended for slightly lighter steel and a typical welding current would be 70-95 amps. An electrode used for thinner metals should be EMS 14 with a welding current of around 45-75 amps.

It cannot be over-emphasised that certain safety pre-cautions must be taken when using are welding equipment and these are referred to in the accompanying panel. Always wear protective clothing, particularly the protective head shield with dark vision glass to protect the eyes. You can hire this with the welding set. Avoid inhaling fumes which can prove harmful and work in a well-ventilated area.

Regardless of any particular application on the car ii. Is essential that the battery is disconnected before connecting your welding tool.

First of all one lead from the welder is attached to the area to be welded. The lead clamp must make a good electrical contact and where necessary rust, sealant or paint will have to be thoroughly scraped off to expose bright metal. The other lead from the welder is attached to the welding gun which is then fitted with the appropriate size of welding electrode.

Using pieces of mild steel of similar thickness to the projected job, practice ‘striking’ the first are as this can be

the tricky part of the operation. Clamp two of the pieces of metal together and gently apply the electrode to the joint with a slow, sliding motion to start the are burning. If the electrode is applied too hard it will stick to the work area and will have to be twisted clear again to be restarted.

The striking technique is rather similar to that of striking a match – a gentle scraping action for a short distance. Once the are has occurred the two metals will melt very quickly. As soon as this happens the electrode is moved along the path required and its point gradually moved nearer the weld to compensate for the loss in length from burning.

Once the ‘strike’ is mastered then it is simply a matter of moving the electrode along the joint at the correct rate of travel and at the same time moving it inwards to keep the electrode always at the same distance from the molten metal.

The flux that covers the electrode melts with the metal and hardens as ‘slag’.

If you happen to lose the are, chip away the slag before ‘striking’ again. Chip away again after the welding has been completed and underneath should be a good strong weld.

Typical problems

Certain cars have their ‘Achilles heels’ as far as corrosion is concerned and an official examiner will certainly know where to look for trouble on a particular car. These are common problems regardless of any particular make. Bear in mind that ‘pattern’ replacement sections specially designed for repairing rust areas are usually available for the more popular cars. Lists of parts available can be found in most motor accessory shops. The new part will have an overlap of extra metal to allow the welder to attach the new metal to good metal rather than simply covering the existing corrosion.

A suspect point on many cars are the rear spring hangers, which often need replacement. The spring would have to be removed and the offending member cut off with a bolster chisel. A new ‘pattern’ part will have a special overlap area so that the new part will weld to good metal. It is obviously important that the new section is aligned correctly so that the rear spring hangs correctly.

MacPhcrson struts as used on early Fords and Chrysler cars will sometimes develop a corrosion problem around the load bearing area at the top of the strut. It is simply not good enough to lay a new plate on top. The strut will have to be removed and the bearing area re-plated both under the area and on top. Again matching ‘pattern’ plates are available for most cars suitable for welding into position.

Other points to look for on cars are load bearing areas where a major mechanical component is attached. An early Mini engine mounting bracket may show signs of corrosion and both brackets will have to be replaced completely. Another suspect on the Mini would be the rear subframe. Usually when a subframe starts to deteriorate it is advisable to replace the complete assembly rather than try to repair it.

On the Triumph Herald it is worth carefully checking the outriggers. They are in a vulnerable position to catch mud and water and can deteriorate quite rapidly. Again there are ‘pattern’ parts available. The outrigger should be replaced if the corrosion is extensive.

The rear gearbox/engine support areas along the outer chassis members can be suspect too – here a Vauxhall Viva is highlighted. You shouldn’t wait for a test failure before repairing the damage, because you may discover it for yourself at a very inconvenient moment – for example, when jacking up the car after having a puncture.

The wishbone area up front on some cars like the Hillman Imp will sometimes need re-placing. Certainly a hazard and not a particularly easy job because of all the necessary stripping down.

The subframc on the.Leyland 1100 and 1300 range should be checked; even if it has recently been replaced it is worth checking where it is attached. Sometimes the support areas will rot long before the actual subframe. Also lift carpets and check the security of the floor support.

Carrying out the job

With the correct ‘pattern’ part or enough good metal of the correct gauge to carry out an efficient repair, you will also need all the required tools to strip down any particular mechanical components to get at the corroded section of your car.

If working below the car, ensure that the vehicle is well supported on axle stands before venturing under-neath – don’t rely on jacks alone, milk crates or bricks. Also ensure that the axle stands are positioned under a ‘sound’ area of the car.

Remember that your are welder will be producing sparks and plenty of heat and consequently you must be very wary of petrol tanks, fuel lines and trim areas possibly positioned on the other side of your welding area. If in any doubt at all then remove all possible hazards first.

If the petrol tank is positioned near the particular repair, the fuel has to be drained and the tank removed. In some cases it may be possible to protect an area using an asbestos sheet – especially when guarding trim or wiring is positioned on the other side of the metal to be repaired. Easy access to the repair is all-important for carrying out a first-class job.

When all the surrounding area is cleared of mechanical parts a thorough inspection will have to be carried out to see how far the corrosion has spread. This means cleaning the area thoroughly with a clean, stiff wire brush, making sure that all underbody sealant, paint and dirt are removed. Cut away enough metal so that the ‘pattern’ replacement will fit easily. It is not necessary to remove all the existing spot welds and, in many cases, the old chassis member joining lips can stay in place. A lot will depend on how easily you can chisel the old part away. If only a small section of the member or panel is corroded, then a hacksaw can be used to cut away the area neatly. Then the ‘pattern’ part can be cut accordingly to fit the missing area. The important thing to remember is that the weld must take place around good, clean metal.

When using a bolster chisel with its sharp, V-shaped cutting edge make certain that you protect your hands and eyes – flying sparks and pieces of metal are dangerous. Place it firmly in position and hit it with the hammer, moving the chisel along smoothly and away from yourself. There should be no need to hit it very hard; let the chisel do the work rather than the hammer. Once most of the old metal is cut away then you will be able to sec if corrosion has taken place beneath the old metal – if so, now is the time to put this right also.

Once you have established that all the remaining metal is strong and in good condition you can clean the area up with a wire brush ready for welding. If the pattern part has been painted for protection then you should remove the paint just around the edges or where you intend to weld.

If the other side of the ‘good’ metal is in the boot, it is obviously essential to remove all the contents and check that wiring, carpet or undcrfelt is moved out of the way. For further protection it would also be useful to place a sheet of asbestos – not asbestos cement sheeting – on the other side to insulate against the heat.

Now the new section has to be positioned accurately and held in position. This can be achieved several ways and will largely depend on the positioning of the particular repair that you are carrying out. Molegrips are especially useful as they can grip two pieces of metal together and retain their own grip. You could even use pop rivets in each corner to hold the new piece in place while you start welding. Or a jack can be used to jack the part into position. Once you are completely satisfied with the positioning of the new section or plate then you can ‘tack’ weld the component.

Connect one welder lead to bare metal near the repair and check that you have the correct electrode in your gun. Also ensure that you have observed all safety precautions before starting to weld. Tack welding involves just carrying out a small weld at each corner to hold the new section or plate in position while you carry out further welding – a ‘tack’ on each corner and side will prevent the new metal from buckling or distorting if you happen to develop too much heat in one area. A tack weld is easily achieved by switching on the welder and creating a ‘strike’ and literally moving die electrode along just 7 mm or so. It is best to tack at each end first and then place further tack welds in between. Once the metal is firmly in position then the molcgrips or jack can be removed and the positioning of the new section again checked.

Having now established that everything is in the right position then the welding can take place to secure the new section in position per-manently. If the original section was spot-welded then it will not be necessary to weld around the edges of the new piece completely – 50 mm welds with 50 mm gaps will be more than adequate. If, however, you are installing just part of a new section then it is necessary to weld completely around the perimeter. It is essential that the new section is as strong as the original.

Once the welding has been completed the ‘slag’ will have to be chipped away and the welding examined very carefully. It is worthwhile protecting the repair from further corrosion by using a rust inhibitor, cspeciaily inside box sections. A coat of bituminous paint or undcrseal will also help to prolong its life.

When dealing with sill panels visible from the exterior, in most cases it will be worthwhile fitting the complete panel as a cut in section will require a lot of ‘dressing’ and filling to achieve a perfect contour.

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