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Changing Sills

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Changing Sills

Postby Zippyrude » Thu 27th Dec 2012, 02:39am

Righty then I was asked to write a technical guide to changing sills. So here we go, first let me state that I am not a professional but I have learnt from some of the best in the business, while I am an amateur I consider myself a good amateur and as such have a large amount of kit which the normal home hobbyist might well not have, however I will show you all how I change sills and hopefully from the images I have put up should provide more of an idea how to do it.

Firstly let's get one thing straight I don't use oversills, they are bad and make the car structurally weak, I also do not go in for the big mistake of cutting out all the rot then fixing it, I do one piece at a time, that way the car stays straight through out the process.

Now here we have our car, it's a 1991 mini which has been owned by my family since new, after building a car for my mum last year, my dad requested one for himself, meaning that the one I'm building for myself gets put on hold again. It's the red one in the background of the pictures.
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As you can see she has been fully stripped, in my opinion the only way to do major bodywork is to remove all interior and fittings and be able to roll it over, I don't go in for this laying on the ground stuff.

The car is then rolled over and supported by 2 axle stands and a piece of wood on the B Pillar; this must be stable and must not move or wobble. Once on its side you can assess the area of interest, on this car the majority of the sill is ok only the front section appears to need attention, I will however replace the entire sill as you never know what is lurking underneath it all.
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A quick word on tools, I have at the moment 2 quality angle grinders, the ability to quickly swap between cutting and grinding discs is invaluable and saves hours of messing about. As far as cutting discs go the thinner the better thin discs cut quicker and create less heat, however they also wear out quicker, for fine work I use 1mm discs but for normal work I use a 1.6mm disc. Grinding discs I use a flap disc normally about 60 grit, these are expensive most shops sell them for about £4 each, I buy mine off eBay for about £1 each.

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Welders as you can see I have a substantial welder, it's industrial quality and pumps out a max of 180amps. For most work I'm using it at 80% max power so that's about 150 amps, the more power you can use safely and without blowing holes the better the quality of the weld will be, however this isn't a guide to welding. Let me say simply that sills are THE main structural component on a mini and if you are in any doubt as to the quality of your work then don't do it.

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The few tools I used here represent an investment of nearly a grand; however they do eventually pay for themselves.

Now where was I, yes its time to remove the old sill, firstly cut along the top line nearest the door, do this as quickly as possible while creating as little heat as possible, then when you can see where the floor joins cut it along there as well, you should then end up with something like the attached picture.
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Now its time to linish off the spot welds, this calls for the grinding disc, firstly grind it back lightly, you should be able to see some dimples, these will be the spot welds, concentrate your grinder on these areas and watch the colour of the metal (hard to explain) but when its going blue then it's thin need to practice this after that I use a wood chisel and prise it off, after some practice you should be able to remove big sections without even touching the metal underneath.

Have a good look at the pictures I have taken they say more then I can here.
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Notice the spot welds can be clearly seen.

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After grinding, look at the blueness of the metal it will now easily break off using a wood chisel and a pair of pliers.

I'd also recommend taking the time to check that the rear subframe securing bolts aren't loose; if they are then simply add a few blobs of weld and that should secure them for the rest of their life. You may have noticed that I have removed the jacking point, this does not need to be done I'm only doing it because this car is having extensive shell modifications and will be having more secure axle stand points incorporated into a seat frame/side impact protection system that I will be building.

Now as you can see this car also had a few holes in the floor, these must be repaired before the sill goes on as they are in the main contact patch, this was done by cutting out all the rot back to solid metal and cutting a piece of fresh metal to go in the hole and fully welding round the edge, don't be tempted to try and continuously seam weld it, lots of short joining pulses are the best way, but the ability to do this depends upon the quality of the welder and your skill.

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Anyway that is now all done and ground back and it's back to the sills.

There is still the remaining lip under the door step to come off, now there is no easy way its hard awkward work as the angle is bad, but it's got to come off or the sill won't be as strong.

Once that is done its time to offer up the sill, as with all genuine sills that I have used the fit is excellent and does not need any modifications, what I now use is a spot weld punch which is driven from my compressor and makes life really easy, its possible to do this with a drill but the finish wont be as good. Punch one hole about every inch as a rough guide, keep offering up the sill to make sure that you are punching the holes in the right place.
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Now another one of my little rants welding is like painting its all in the preparation, the area to be welded must be clean, no paint no grease just pure solid metal, make sure anything flammable behind the area to be welded is removed, I used to set carpets alight many times in my younger days. Make sure your welded has enough gas and wire and that you have a good solid earth. Good preparation leads to good welds, bad preparation leads to bad welds, rust, MOT failure and having to do it all over again, do it once and do it right!

Now it's a case of clamping in position along the top and looking along the bottom, if all has been done right then it should be touching, look along it and find the holes which are closest to the body, push them against the body using something non conductive - I use a piece of wood and weld, do this all the way along (only the bottom line) no specific order is required I just look for the holes which are closest and push and weld them. There is no need to go manic with the welder, for every spot weld I do I'm welding for less then 2 seconds, just enough time to get decent penetration but any longer and I risk blowing a hole. Hopefully your welds should look something like these.
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Now the entire bottom row should be welded its time to move to the top, I start by doing the door area first and use 2 mole grips one either side of the spot to be welded then tap them using a club hammer and a blunt cold chisel make sure they are as close together as possible, you can do these is sequence working from front to back but only in the door area.

Now we come to the rear quarter panel, extreme caution must be taken to avoid excessive heat which will cause distortion, this is why we welded the bottom line first, if we did the top first then the bottom it would all pull and distort badly. Ideally use some form of shield to try and divert as much heat away from the panel as possible, I like to leave time for the welds to cool down a bit in this area.

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After that it's a simple matter of folding the rear heel board round and welding and fixing to the inner wing and it's all secure. Again use your grinding disc to flatten off the welds carefully and clean it all up.

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And that my fellow mini lovers is it, I started this today at 10am and finished at 4pm but this included an hour lunch break and many cups of tea.

So that's 4-5 hours labour per side, £25 in gas, discs, electricity and other consumables, £30 for the genuine sill and a bit more for the extra metal used. If I was to do the job for a customer is would be £150 a side for the sills alone plus extra for any floor repair work.

Its good fun to do and you get a sense of accomplishment but if you are in any doubt as to your skills get a professional to do it.

I hope that this article proves useful and educational. Minis are very simple cars but are very difficult once you start doing major work on them, if you can successfully fit a sill to a high standard then there isn't much work you wont be able to do.

Thanks to Timinichelsea
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Zippyrude
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