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Advice On Floor/sills/heelboard Replacement.

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#1 CooperSport99

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Posted 31 January 2017 - 09:09 PM

Evening all,

 

Im into ownership of my second mini now after purchasing it as a project in august - with an MOT! I drove the car around for a few hundred miles to see what it was like and was fairly surprised, doing little things to make it better etc. However I know there was some rust up the front end so I bagan ripping it apart in october time... Since I've welding mainly the front end (bulkhead, flitch etc) and have completely stripped the car back to a bare shell except the rear subframe. 

 

I'm hitting a point now, and am not sure how to play it so any advice would be great. I bought a new crossmember as the current is rotten and patched up down the side. The O/S front floor has been replaced to a reasonable standard but not pretty. Both outer sills are a combination of original , patched and oversills that are rotten in places. The inner sills are bad and holed in multiple places too. The icing on the cake happened today whenI removed the rear subframe to find the rear heel board totally shot both sides. The rear of the floors are also patched up badly. 

 

So really the only thing thats ok here is the tunnel itself. I've been wondering wether to try and patch the floor bits up, plate the inner sills, replace the heel board at each ends and then to put new outer sills on. But quite frankly I'm not sure il ever be happy with it so i might be looking at a full floor section. More cash of course, but less faff. What are peoples opinions on these? I know minispares sell a Genuine one with outer sills and heel board all on for about £650, and M Machine do one for around £715. Heard goo things about the M-Macine one too! 

 

Any advice is appreciated. Cheers. 



#2 Daz1968

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Posted 31 January 2017 - 10:12 PM

The heritage one should be fine for later cars, think it's the mpi pressing, m machine is far better for earlier cars especially with the remote change tunnel as its only correct panel available. I fitted full m machine floor to my mk2, pretty easy to do but budget on changing more than you think, full boot floor is also easier than patching old one.
the other advantage is that it's dimensionally correct

#3 Ben_O

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Posted 31 January 2017 - 10:30 PM

fit an entire floor and don't look back



#4 CooperSport99

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Posted 31 January 2017 - 10:31 PM

Thanks for that - Should have mentioned my car is a a 1978 Mk4. So perhaps the M-Machine panel would be best?



#5 tiger99

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Posted 01 February 2017 - 12:10 PM

Doesn't matter right now unless you really care about originality. The later Heritage panel has extra pressed grooves for additional fuel lines etc but is structurally compatible and may even be useful if you eventually upgrade to some modern engine.

#6 Van13

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Posted 01 February 2017 - 03:06 PM

fit an entire floor and don't look back

+1 will never do half floors again bit more money but far far less work

#7 Ben_O

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Posted 01 February 2017 - 06:07 PM

 

fit an entire floor and don't look back

+1 will never do half floors again bit more money but far far less work

 

Plus if you do it right, protect it well and keep it all clean/maintained, you will have original looking floors for years to come instead of seam welds and patches all over the place.



#8 tiger99

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Posted 01 February 2017 - 08:03 PM

If you are doing half or quarter floors, which I do not recommend unless part is really in good condition, the way to do the long butt welds is to use a copper (don't try any other metal, they don't work, except silver) backing bar. You will get less distortion and no burn-through and if you use a bar with a small groove, the welds will look really good on whichever side is the back. I would suggest welding from the top, as far as possible, to impress the MOT tester when he sees an impeccable seam underneath. If the crossmember is out, it is simple to do all the welds, but if it is in, there will be areas that can only be welded in the normal way from one side.



#9 Van13

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Posted 03 February 2017 - 07:41 AM

If you are doing half or quarter floors, which I do not recommend unless part is really in good condition, the way to do the long butt welds is to use a copper (don't try any other metal, they don't work, except silver) backing bar. You will get less distortion and no burn-through and if you use a bar with a small groove, the welds will look really good on whichever side is the back. I would suggest welding from the top, as far as possible, to impress the MOT tester when he sees an impeccable seam underneath. If the crossmember is out, it is simple to do all the welds, but if it is in, there will be areas that can only be welded in the normal way from one side.

hi could you go into more detail in your explanation as I do not understand I only class my self as a mediocre welder thanks

#10 tiger99

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Posted 03 February 2017 - 05:27 PM

I recommend getting a good book on welding from the library. That is where I learned of backing bars, many years ago. There may be a few more tricks that you will find useful. But I will try to give some explanation on the use of a backing bar.

 

Molten steel does not stick to copper or silver. Both of these metals conduct heat so well that they do not melt if used behind a MIG seam. Seam welds suffer from burnthrough because the heat buildup is excessive if more than a very short run is attempted. Putting all these facts together, a fairly thick copper bar can be put in tight contact with the back of the joint. It will add support to the molten metal, to prevent it collapsing, conduct heat away, again with the same effect, and form the back of the seam the way you want it to look. I suggest a shallow rounded groove, depth less than the radius otherwise the copper will mechanically lock on the seam as it cools. A rectangular groove is also usable, maybe 4mm wide by 2mm deep. The groove runs the length of the bar. Some use a flat bar and get a flat back to the weld, but a slightly raised bead is stronger and in my opinion looks more professional.

 

You can hold the bar against the joint by something like a jack from below, weld from above. Or put a harrow slot in both ends and use a pair of extended Intergrips, as you might use for clamping the panel edges together. Oh, and you will want a gap of the metal thickness, approximately.

 

The bar can be some electrical bus bar, maybe an inch wide by a quarter thick, larger if you like, and a bit longer than the amount of weld you will want to do at once. Some people actually just flatten old copper water pipe. but a fresh piece of bar is preferable. Ideally groove it on a milling machine, but a burr in a Black and Decker on a stand with a guide will do, copper is not hard. I suspect that a woodworker's router with a carbide cutter might also be up to the job, if extreme care is taken.

 

If your bar is 300mm long, plus a bit at each end, you might want to weld 0-75mm, 150mm-225mm, 75mm to 150mm, 225 to 300mm to spread the heat as is usual with all forms of welding rather than a continuous 300mm run. And I am not kidding, that much continuous weld in 0.9mm sheet is possible with a backing bar.

 

There is little heat spread away from the seam, so not much thermal expansion and subsequent contraction, except longitudinally in and very close to the weld bead. To avoid distortion you may need to stretch the seam slightly. Drop the copper bar off as soon as the weld is done (HOT!) and hammer gently along the seam only, against a dolly. It is easier when the metal is still hot.

 

There is a different trick where one side of the weld will not be accessible after it is done, and it is of major structural importance, such that full weld penetration, which you can't see, is essential. You use a permanent narrow steel backing bar, and leave a gap at the joint so you can see that the weld metal is wetting all 3 surfaces. You fill the joint up flush, plus a little bit. I would be inclined to use that method to fit a new end section to the main bulkhead crossmember, if I had to. A highly skilled welder does not of course have to do that, but most of us here are only occasional welders.

 

Those with adequate machining facilities may care to think about making a water cooled backing bar. These are used in production of quite large and heavy structures sometimes. The use on a Mini would be so that you could do a length of weld and not have to wait for ages for it to cool before moving to the next length.



#11 Daz1968

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Posted 03 February 2017 - 05:38 PM

You should join mig welding forum, I use backing when plugging holes, but find butt welding easier without, I use the clamps which slot in between panels and tack at various points before welding, it all comes with practice and minis give plenty





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