Welding car chassis tunnel

  1. DrGonzoPOW Member

    Posts: 41
    Hi, new to the forum and am seeking some advice on my car project.
    I have recently taken an engine out of an old bmw and found a couple of nasties once i cleaned all the crap away.
    It appears that its been in a bump somewhere in its life which has resulted in the front left chassis tunnel (just infront of the front sub frame) being slighty bent and with a large crack in it.
    I was given a Clarke turbo weld 130te a fair while back and have never got round to using it but i think with this, its probably about time i learn how to mig weld! Im thinking of using around 1.2mm mild steel with 0.8mm wire and argon gas shield. Would it be best to cut out the setcion and butt joint a cut panel in or could i get away with just placing a patch over the area and do an overlap weld ?
    This is a crap pic of the section in question. Ill post a better one up later
    I also found a pic of the same car with the same problem that someone sorted but i dont know how the went about it. I would like to get away with not having to drop the front sub frame off but that is probably going to have to be done
    Any help would be massively appreciated. And you'll have to talk to me like an idiot cuz ive never welded before. Cheers

  2. Hello Dave,

    definitely not the sort of project to start to learn to weld. I make no apologies for being negative, but the structure of a car is critical. Practice on other things until you are certain you know how to do the job.

  3. rikrobson

    rikrobson Forum Supporter

    Clean all the rust off first as the hole is gonna be bigger thanit first appears.
    you ideally want bright clean metal around the hole 10-15mm will do.
    make a template larger than the holw and smaller than the clean area from card.
    cut some metal the same thickness as the chassis rail.
    weld the seam all the way round.
    you would probably be better using the stitch method .
    it should then look similar to the second photo.
    then paint it and waxoyl the inside of the member.
    Good luck
    it must be fully seam welded to pass MOT.
  4. DrGonzoPOW Member

    Posts: 41
    I wont just jump straight onto the car mate. Ill practice alot on some scrap pieces of metal first. Just wanting to know what the best thing to do with something like this would be. Cheers
  5. DrGonzoPOW Member

    Posts: 41
    Well this is what im dealing with
    What do you guys rekon ?
  6. Hello Dave,

    that picture is better, that car hasn't had a bump, there's no sign of distortion. It looks like two layers of metal, which have corroded where the inner faces lie together. It's difficult to see if the inner layer is much thinner than it should be. You need to cut a larger patch out of the outer layer and evaluate how sound the inner layer is.
    You may need to cut out some of the inner layer and let in a patch then a second patch in the outer layer then plug weld the two together.
    Not a job for a beginner I would say?

  7. cresad67 Member


    I agree with above , 2 peices with rust in between pushing them apart , seen it loads of times on our old toyotas.

    You'll probably find that the shiney peice in the middle is basically a shiney peice of rust and if you give it a knock it'll go. It needs to be cut bigger and replaced. Is anything showing on the underneath of that box section ? ( behind the bolt )

  8. keith19 Forum Supporter

    Hi Dave,

    Alec's dead right, that is a tricky one. Looks like corrosion is right down to the bottom edge of the member, so most likely you won't be able to get away with just a face plate. Once you start cutting you may find you have to drop the subframe in order to weld an angle plate in place. If you've plenty of time for the job, make sure you practice lots of welding first. When you get good then do more practice from the sort of position you'll be welding in when you do the job. If you don't have the time, get an experienced car welder in to look at the job; if you remove the subframe yourself and prepare the job first it won't be nearly so expensive.
  9. rikrobson

    rikrobson Forum Supporter

    the sub frame is gonna have to come off. no question.
    you will need to cut back the outer skin until there is no rust or just enough to get a thin enough layer which can be chemically treated.
    Then cut back the inner layer until no rust. then weld the inner and gring flush and then weld outer skin.
    It's possibly gonna end up wrapping round the corner underneath.
  10. DrGonzoPOW Member

    Posts: 41
    Wish id have just left it mucky and never uncovered it :(
    Will have to start taking things to bits then eh. Cheers for the advice
  11. DrGonzoPOW Member

    Posts: 41
    Bottom of tunnel beneath bolt looks solid from the outside
  12. DrGonzoPOW Member

    Posts: 41
    Regarding cutting out the section and then but welding panels in. Would i try and chop the out skin seperate to the inner or just lop the whole bit out in one?? Cheers again for advice so far
  13. keith19 Forum Supporter

    Personally I'd be inclined to grind back the top layer around the corrosion first, just to see the extent of the damage underneath. The bottom layer will be the thickest and most important, and what you'll be welding to.

    To all intents and purposes the top layer, if that's what it is, is purely cosmetic now. When you have decided where to cut the bottom layer, grind the top layer back so it doesn't get in the way of welding; it might be quite difficult to blend it into the strong welds needed on the repair section.

    I find it difficult to see any engineering advantage in having two layers of steel on a frame member, the top layer might be just a coating of some kind.
  14. DrGonzoPOW Member

    Posts: 41
    Update on this....
    Grinded back the outer layer and its not actually too bad. The inner layer is very solid. Im really struggling to find any mobile welders in my area to come and do it so it looks like i may have to at least attempt to do it myself.
    I know the proper way to do it would be to chop out the section and weld a new one in but im no way going to be confident enough for that, so its going to be a patch job. I may attempt to start chopping bits somewhere down the line when i can hopefully get half decent at welding.
    What approach would you guys use to just go for a basic overlapping patch ? Ive already used some rust eater and caked it in u-pol weld thru primer. Im planning on using 1.5mm thick steel or there abouts for the pacth
  15. malcolm

    malcolm Bob the builder

    Posts: 8,386
    Bedford UK
    That looks quite promising. The rust tends to be formed by corrosion between double skins of steel, and often the difficult one on the inside is still good even when the outer skin has a hole.

    You'll probably find the rust extends way beyond the hole in the outer skin - if you tried to weld to the edges of the hole the rust behind would make the weld a mess.

    Cutting the outer skin back to beyond where it is deformed by rust might make it easier to weld. Small overlap or butt weld and a few plug welds back to the inner sheet would have things dandy.

    Worth a look underneath the chassis member - presumably the story will be much the same or worse under there. Rust doesn't tend to form on the sides of double skinned members - it normally starts underneath.
    • DSC00520.jpg
  16. Faaluke Member

    Posts: 82
    New Zealand
    You need to cut back the outer metal until you can't find any rust between the layers. If you don't do so you may as well just bog over the whole thing as anything less than a proper job is just delaying the rust coming back. I should know, i'm into old Toyotas that sometimes have THREE layers with rust inbetween..... not fun! As mentioned earlier, I would drop the crossmember for sure. Sure its more work, but if you don't fix it properly now, you'll have a bigger mess later. So remove it, grind the surrounding area back, under the crossmember etc and see what you're looking at.

    As for trying this with no welding experience.... not a good idea. I practised for ages before I attempted anything proper.

    Apart from that, good luck haha.
  17. DrGonzoPOW Member

    Posts: 41
    Another update on this. Dont shout at me but i went for the easy 'patch' route as this project has been put on hold for ages now and im just not experienced enough to be chopping bits out just yet. Anyway here's how it turned out
    Had to clean it up a fair old bit but....
    Then just used some o'that UPOL weld thru primer. Its great stuff!
    If it lasts a couple of years then hopefully ill have learned enough to do a proper job next time. Gonna try get the engine in on the weekend now!! :clapping:

    Thanks for the advice! Sure ill be needing much more in the near future
  18. stuboy Member

    Posts: 48
    whack with a hammer and see if falls off...
  19. cresad67 Member

    That's not that bad , give it a couple of years and you'll being doing things like this


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