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Discovery Sill Replacement Write-up


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This is a write up of the sill replacement job I just did on my 300tdi Disco. I got an advisory for rust on the nearside sill back in December, but I didn't fancy doing this job in the snow, so I waited until the weather improved. I was a little apprehensive about taking on the job, but in the end it really wasn't that difficult, it just took time. I managed to do the job by myself in the back alley behind my house using not much more than an angle grinder and a crappy Clarke 110A mig welder that I borrowed from a mate. I gained some confidence through reading a few other forum write-ups on this which were very helpful in giving me an idea of what I was dealing with and how to go about fixing it:

Write up 1

Write up 2

Write up 3

The last link shows you how to achieve a factory finish. However, it is probably the most difficult and expensive method, and won't have the strength of the 'box section' approach.

So to start with, a picture of my old, nasty, crusty nearside sill, a familiar sight for any disco owner:


Luckily for me, the rust had only really set in to the outer sill, and the floor/body mounts/inner sills were still solid. This makes the job considerably easier as you can get away without supporting the bodywork/taking the doors off etc. If you have a garage to work in and can leave the motor in bits overnight then you're ok, but I wouldn't fancy leaving my door-less pride and joy overnight in one of the less leafy suburbs of Newcastle.



I found that the easiest way to get the old sill out was to drill the spot welds along the bottom lip and take an angle grinder to the rest. If the inner sill had been rotten too I wouldn't have bothered trying to release the welds and cut the whole lot out.

If you don't have a spot weld drill, go out and get one immediately, miles better than trying to use a standard 8mm bit. I bought a cheapo one off ebay and it did the trick, not sure how long it will last but it's still sharp after doing this job.


If at all possible, leave enough of a lip from the old sill so that the new box section has something to attach to:


After a few more sparks have flown you are left with this, along with mixed emotions over the butchery you have just inflicted on your (t)rusty steed:


Clean out all the mud and cavity wax to have a really good look at the inside of the old sills:


A fair amount of pitting at the bottom edge of the inner sill became apparent, so a bit more slicing ensued, followed by a coating of weldable primer on the inside of the old sill, as this will become inaccessible once the box section is in place.


I chose to use 120 x 60 x 3.6mm box section mild steel. The only critical dimension is the width (60mm) as the box section needs to fit properly in the space left by the old sills. A greater wall thickness can be used, but 3.6mm is bloody strong as it is, far stronger than the originals and considerably cheaper than say 5mm. The box section needs to be at least 1400mm long, I chose to leave mine as 1500mm as it came from the supplier but if it had been any longer I would have cut it down.

As I am not mates with any fabricators, I found it difficult to get hold of a small quantity of steel and this lot were very helpful and gave me a considerable discount when I informed them their list price was rather steep! If there isn't an outlet near you there is a fair amount of stuff on ebay, which surprisingly isn't too expensive even when postage is factored in.

Test fitting the box section:


Using 60 x 120 box the new sill extends down to where the lip of the original was:


I decided to cap the ends of the box section to stop them filling with carp and rusting from the inside out:


I found a piece of angle iron makes a very good cutting guide if like me, you are not a freehand grinding master:


Box section all welded in. The welds are not exactly textbook, but they'll do the trick, and considering that there is about 10 feet of continuous weld holding the new sill in, the odd dodgy spot isn't going to cause major problems. Don't tell the MOT man that though. After doing this job I have a lot of respect for anyone that can make good, tidy welds whilst lying upside down trying to join steel of dissimilar thickness. Especially so if they have to wait for the intermittent gusts of wind to stop blowing all the shielding gas away!


The next task was to cut the rot out of the passenger footwell:


And then use your finest pigeon-sh*t welding to attach some new sheet steel. At least this can be done standing up rather than lying on your back, but again welding thin sheet using MIG is quite difficult, and I ended up blowing a few holes even on the lowest power setting. I don't want to taint this forum's reputation by giving out bad advice, but primer, paint and seam sealer are a dodgy welder's best friend.


I had to remove the outer wing to weld up the floor, and decided to give that a good coating of underseal whist I had access to it.


Having capped off the ends of the new box section, it seemed pointless to drill a load of holes in it to reattach the rockslider. I could have left it off given the strength of the new sill, but it protects the doors, has hi-lift jacking points and makes a useful step for my shorter passengers, so I decided to weld it back on after cutting down the mounts a bit.


The black stuff is Tiger Seal which I have used as a seam sealer, but also makes a bloody useful adhesive/general sealant as well (similar to sikaflex). Very good for sealing leaky sunroofs/alpine windows etc.


Testing for strength, doesn't look like it's going anywhere soon:


Everything was then basted in zinc primer and two coats of Dinitrol underseal:


Just for laughs, this is what happens when you are a student and your bedroom is also your living room and garage :hysterical::


A crate of engine oil, paint, EP90, white spirit and the like was removed not long before the photo was taken as apparently it was a fire hazard :o.

I'm pretty pleased with the outcome, took about four days to do but I reckon I could do the other side in half the time now. A lot of time was spent thinking about how to tackle the problem, and a lot of time wasted packing up the tools and getting them out every day, waiting for the rain to stop etc.

A few other random bits of advice I can offer having done the job:

  • Buy loads of 1mm slitting discs they are brilliant, but don't last that long (I used 6 or 7).

  • Similarly a few flap wheels and wire brush will be needed.

  • Two jacks are very useful to get the box section in place before tack welding, particularly if you don't have an assistant.

Boot floor is next on the list, hoping to get hold of a big CO2 cylinder from a bar manager mate of mine first though as I went through 3 large disposable canisters doing this job (£40 from Machine Mart!).


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After looking at what you had to deal with it makes the thought of dealing with my sills a really simple operation, of course unless I find more holes once I start cutting . Good idea using box section rather than the usual repair sections.

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unless I find more holes once I start cutting

Rust is virtually always worse than you first think, just to prepare you! My sills didn't look that bad until I started giving them a good prodding with a screwdriver.

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  • 4 weeks later...

Well done. Replaced ours recently and agree with the discs, eBay and classic car rallies seem to be the cheapest places for them. Not easy working in the street and stopping for passers by but shows it can be done.

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  • 1 year later...

just copied what you've done on this thread although I used 1400mm long and 3mm thick box.Took ages cutting out all the old sill and removing rust,,,,,however well pleased with results..thanks for posting an idiots guide on how to!

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No worries. I think I'm going to tackle the offside some time this summer.

I've since moved into a house with a double garage but the disco doesn't fit under the bloody door!

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No worries. I think I'm going to tackle the offside some time this summer.

I've since moved into a house with a double garage but the disco doesn't fit under the bloody door!

My scrapheap Disco only sort of fits in the garage (well half of it does) if you let the tyres down.... I did consider fitting some 15 inch rims (currently residing in the garden with slick mud terrains fitted) to alls more manoeuvrability, but never got round to it. Worth a try tho.

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  • 10 months later...

After reading this I had a go, and yes the amount of cr#p that had to be removed was incredible :)

though I decided to add tree sliders to mine.
this is the guide line measurements I used as the metal came in legths of 1.5m for the sills and 2m for the sliders.
Cut out the wedge at each end to form the angles, I tried to keep one side complete rather than cut if completely
All ready for welding together,
Just a quick trial fit
All Welded up, I then blanked the ends and paint them with some undercoat, mostly for the parts I'm never going to see again
All welded in just need to file down the sh*t welds and paint them with a final coat of undercoat then I'll paint them black and add some anti slip tape which comes in 25mm strips which is the width of the sliders :)
When I first built them I was slightly worried about the extra weight, until i cut out the rust to fit them ;)
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