Practical MIG welding - How to fit new sills

Distortion is your enemy when replacing sills or floors. It's really easy to end up with doors that don't fit for many reasons. I'll try to cover all of them on this page.

Before you start, bear in mind that the car will loose it's torsional stiffness during sill replacement, and also you'll need to take steps to prevent the door gaps from closing/opening up. Leave the doors in place where possible so that you can keep an eye on the door gaps throughout the sill replacement.

Setting up: The red wine level gauge

I use a plastic tube full of red wine to set levels. Water doesn't flow to to the correct level in a thin tube. Tape one end of the tube to one corner of the car and the other to another corner. Mark off the height of the wine on a bit of masking tape on each side.

Check the car side to side and corner to corner and mark the levels. Then check regularly throughout sill replacement to make sure nothing has moved.

If you level the car using bits of wood/metal under the axle stands you will also be able to use the red wine level gauge to position bits of chassis and suspension mounts. The accuracy is within 1mm, which is better than body manufacturers can guarantee.

Red wine level gauge

Maintaining panel gaps

Once you've recovered from the hangover from drinking the rest of the red wine you'll need to add some temporary structure to support the car while the sills are off.

I tend to weld a steel bar between the A-pillar and B-pillar before removing the sills. This will retain the door gap. You can grind the bar off when you've finished. It is also possible to buy special bars that bolt onto the door seal flanges.

A saloon car or coupe will have some strength in the roof, so a single bar just above the sill will be enough to retain the shape (as in the photo). A convertible would need bars at the top and bottom of the door aperture.

Sill rust

Cutting out the old sills

The old sills can be roughly cut off using a gas cutting torch, and the remains can be removed using an angle grinder.

I've angle ground the remains of the sills from the chassis members and floors leaving the original flanges on the floor in place.

As always, any metal you need to MIG weld to has to be spotless and shiny. I needed to make a few repairs to various chassis members before I got to this stage.

A good trick is to clean up every bit of metal at the earliest opportunity. Otherwise you could find the new sill gets in the way of your angle grinder.

Sills removed

Plug welds instead of spot welds

The sill was originally spot welded in place. Actually, being an old Aston Martin, the sill was spot welded then comprehensively seam welded.

I positioned the sill on the car and used a marker pen to mark the position of the floor flanges. The sill came off and was drilled for plug welding.

The plug welding holes are 7.5mm in diameter and spaced at 1.5 inches. See the plug welding technique page for more details about this type of weld.

Plug welding holes

Clamping in place

It takes a long time to align the new sill. Here the new inner sill is clamped in place. I used metre long steel rulers, a square, and long steel bars to ensure the sill was straight.

Mini G clamps cost £1 each and are wonderful for clamping flanges together. I've also recently invested in a set of three welding clamps for just £10. These are mole grips with fancy bits welded on the end to improve their reach and spread the load.

Sill trial fit

Tack Welding and Plug Welding

This is where you have to be really patient. If you get any heat into the sill before it's very much in place it will distort. Start off by tacking the ends in position. Then put a few small tacks on the main structural joints. Do a couple of plug welds, then take a break to let everything cool down.

Put in some very short seam welds (10mm or so) the first at one end of the car, the next at the other end. Then take a break to let everything cool down. Patience is the name of the game. It's much faster to weld very slowly than to correct the distortion after a long seam weld.

With the plug welding completed the rest of the sill would be fitted prior to seam welding.

Sill tack welded in place

The outer sill was also plug welded in place with the plug welds spaced by about 1.5 inches. This completed the box section and the sill could be seam welded to the adjoining box sections. Even at this stage there is a risk of distortion so the seam welding needs to be carried out in small pieces to avoid getting excessive heat into the sill.

With the sill structure the various other panels could be welded back in. I tend to put the panel joints in the original locations. It's nice when the finished work looks like the original.

Outer sill

There were plenty of other bits to do before the sills were completed. This car had a further sill section which was seam welded to the main box section, and acts as an abutment for the A and B pillars.

Sill structure can be complicated (especially in this Aston). Probably the most important part is the planning stage at the start where you'd figure out what you need to remove and what has to stay temporarily to act as a reference for the new sill. Take lots of photographs and measurements, and do the sills one at a time so you can refer to the other side. There is nothing worse than cutting everything out then wondering exactly how the new part is fitted.

Sill welding completed

The one thing I should mention is that despite your best efforts the car will pop when you remove the reinforcing bar. Expect the door pillars to move closer together by about 1mm. You can adjust for this by using an adjustable reinforcement bar, but for me the gaps are about 2mm too wide on this side of the car so I'll accept the movement.

Most of these photos are from my Aston Martin V8 Restoration.

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