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MG Midget and Sprite Technical - Half elliptic spring repair panel - advice

Hi I am going to fit a half elliptic spring repair panel to my Sprite and wonder if anyone who has done this job has any advice to offer. There is only a couple of rot holes on the edge of where the spring hanger mounting plate bolts up but it does appear there is a large repair patch welded onto the floor panel in this area. The inner heelboard isn't that old (relatively speaking and while the whole floor is a lot older it isn't the original. The inner and outer sills both sides are good and recent.

I don't plan on fitting the whole of the repair panel and plan on shortening the width side to side as it sits in the car and at the front. The panel also has a round punched hole in it that doesn't exist on my car, any idea what it's for? I can shorten the panel width to exclude it.

My plan is to completely remove the existing patch repair and then cut out what remains of that part of the panel where I plan to weld in the new panel.

I'm also interested in whether to have an overlapping joint (if so by how much) or butt to butt joint. Any comments or advice.

Obviously until I remove the old patch I can't be certain what there is underneath it (thin and rotten) but generally the car is solid. I know I could make a couple of small patches and weld them in but would rather do a neater job.
Daniel

Hi Daniel, I take it that the panel you have is in effect the rear section of the floor. The rear part of that forms the underside of the box beam section behind the heelboard. The round hole was introduced on later cars, I think to provide drainage and air circulation to that part of the beam cavity. Its useful for spraying Dinitrol through to preserve the box.

I think more investigation is needed before deciding exactly what you will replace and certainly before trimming down your repair panel.
The spring mounting plate is bolted up to the floor with 4 bolts through the area you are repairing and will need to be removed first. The rear pair go into captive nuts inside the box section and can be fun to remove. You can then assess the extent of rot and judge how good previous repairs are and whether that needs cutting out too.

The box section has 3 (or is it 4?) 'A' shaped webs inside which rot from the base up. There will be a pair of these either side of the spring mounting plate and typically these are the first to go where they meet the floor panel you are replacing. From what you say this is the area rot is visible so I suspect it is worse than it looks. It always is in that area! The webs may need replacing which can result in a much bigger job on the rear heelboard box beam. Or you may be lucky and able to just cut out and patch the lower inch or two but its working in a confined space. It all depends on exactly what you find once that spring plate is removed!
GuyW

Having written all that and then re-reading your original - as the inner heelboard has been replaced the A shaped webs inside may well have been sorted at that time, when access was easy. It may not be as bad as I envisaged. First task is still to unbolt the spring hanger and pivot it clear so you can cut out whatever needs removing of the floor, and fit your repair panel.

As an aside, on my car I shimmed the bolt that takes the spring front metalastic bush to reduce side play in the spring. Something you might want to do whilst its accessible. Though I suspect you have a panhard anyway.
GuyW

These days you also have to consider what would be acceptable to an MOT tester, from MOT inspection manual:

"Repairs to structural components must be properly carried out and appear to be as strong as the original structure. This requires the use of suitable materials and any plating or welding extends to a sound part of a load-bearing member.

You can only pass spot welded repairs if the original panel was spot welded and the original panel or section has been removed. Stitch or plug welding can be used instead of spot welding.

In all other circumstances, patch repairs must be continuously seam welded."

Which I take as meaning if you don't use a complete repair panel to a structural join then you will have to seam weld - also depends on your skill level, butt weld is much nicer, neater and less likely to trap moisture but must be done very competently. Overlap, preferably joddled and seam sealed afterwards was easier for amateurs like me when I did mine many years ago.
AdrianR

Thanks for the replies. The spring hanger bracket was fitted last year and was in part made by myself so as to pick up the anti tramp bar without having a separate bracket bolted to it. I also use ARP stainless bolts which are considerably stronger than grade S and 'ordinary' stainless and are ok to remove.

Yes, it is the rear section of the floor (Moss part number MS24R). Looking at the Moss body panel drawing I see what you mean about the A shaped webs. They might be ok but might not and until I cut out the rotten section I won't be able to see what I have.

I think I will get a joddle/joggle tool and open to advice if I need the £100 from Sealey or get a cheaper one?
Daniel

Daniel, I am very much an amateur and used to do mostly joggled lap joints. But a butt joint is certainly neater and avoids issues of rusting within the lap. The key seems to be to cut a close fitting butt joint and then tack and then seam weld in short sections with plenty of cooling time.
GuyW

Have you considered weld thru primer for use on your joddled joints?
Alan Anstead

Weld through primer helps, but its not a 100% solution as it still burns off in the vicinity of any plug or spot welds. The best protection is a good seam sealer.
GuyW

Overlapped (joggle) joints are a thing of the past. A good close fitting butt joint is the go. The closer to zero gap the better. The bigger the gap the more shrinkage you'll get, so taking the time to get a real good tight fit really pays off.
William Revit

Hi Willy, when butt welding, don't you need a bit of a gap for 'thicker' metal, to ensure complete penetration?

Or do you just butt as close as possible, and up the amps to get that?


anamnesis

Adjust the amps to get the best penetration---but up to a point. Anything over around 3mm the joint needs V ing to get the penetration right into the joint, but again the the joint needs to be as tight as possible to help prevent shrinkage.
William Revit

Ah right, make a v shape. Cheers Willy.
anamnesis

I guess you are using MIG? One of my welding regrets is that I didn't learn TIG when I was younger and could still see close up without needing glasses, however the units were very big and expensive then.

Having now got a cheap inverter TIG I find it leaves a much softer weld which can be hammered flush in a butt joint almost to the point of invisibility - probably irrelevant for a floorpan though unless you are trying to win concours events.

I also spot welded my sills with a hired clamp on unit, I recall that as being really quick and easy, just like stapling the metal together, again probably irrelevant for floorpan as unlikely to be enough access.

AdrianR

I have a spot welder that is nice to use - very quick and with clean metal it produces good consistent welds. But it was surprising on the frog body how little of the weld lines were accessible even with the extra long welding arms that I made for it. And anyway at present it is fubar as the secondary coil needs rewinding. Result, I think, of welding rusty metal on a neighbour's trailer.
GuyW

Why do you get more shrinkage with a bigger gap and is that shrinkage the metal on either side of the joint kind of pulling away from the joint on cooling?

The advice I had was to "go slow" but if you want a good seam weld don't you have to keep moving along the joint rather than doing a bit, stopping, bit more etc?
Bill B

I found this explanation Bill.

"Why does shrinkage happen?

The thing you need to keep in mind here is that welding processes involve heat to form strong bonds. But, this will result in a heat affected zone (HAZ). Here there will be some level of thermal expansion. When the metal cools following the welding, it will contract and shrink back to normal. This can cause tensile stress that could result in flaws.
What flaws can occur?

The expansion and contraction of the metal can cause two different types of shrinkage; longitudinal and transverse. The first will run parallel to the weld and heat affected zone. The latter run perpendicular to the weld and HAZ.

In addition, there may be non-uniform contraction in some cases. This occurs because of the thickness of the metal. What happens here is there can be angular distortion as well as the two shrinkages.

Other problems can include bowing and dishing, buckling, and twisting. All of them will cause problems with the final products, potentially making them unstable."

https://www.formatndt.co.uk/think-about-metal-shrinkage-after-welding/


Loads more explanations in this google search.

https://bitly.cx/klrS

anamnesis

As I said:
The key seems to be to cut a close fitting butt joint and then tack and then seam weld in short sections with plenty of cooling time.

😁
GuyW

Thanks Anam & Guy. I still find that welding is rather like paint spraying in that when you pull the trigger you're not totally sure how it will turn out despite trying to replicate a previous successful session.
Bill B

I have on a number of occasions done butt welds I would describe as hot and fast, welding with a much higher amperage than normal for the thickness and at a high travel speed to compensate and not burn through. Tight fit up of the joint and sometimes a backing bar but not always. I've only done this on straight welds, horizontal welds on vertical panels and horizontal panels. I've used a guide bar to make sure the tip follows the joint due to the travel speed. The idea behind this was to get the weld done quickly so it's largely all at the same temperature to minimise distortion and that was achieved. The longest I've done to date was floor pan replacement sides where I welded the joint from just behind the gearbox crossmember to the rear inner bulkhead panel in one quick run where the start was still glowing when I finished the weld. This was done from above, the joint at each end was then done from underneath as I didn't have a rotisserie to flip the car.

GuyW,

How is it you need to rewind the secondary of your spot welder? IIRC they're normally a few of turns of heavy copper flat bar.

David Billington

Hi David.
Whoops! Senior moment, I got the nomenclature wrong! I am of course referring to the thin wire that forms the Primary input coil that operates at 240 volts in this case. The weld tips are of course connected to the secondary which in this case is about 10 layers of 40mm wide flat copper ribbon. That hasn't burnt out!
GuyW

David
Now I'm confused! I thought you had to go slowly with cooling intervals on a seam/ butt welds to avoid shrinkage and distortion? It does seem to be a dark art (despite me having done a lot of MIG welding).
Bill B

Not how I do it Bill,- not skilled enough. I can see if you do a continuous weld moving along rapidly, it has less time at any one point for the heat to spread out into the adjoining panel. If the heat expansion remains close to the weld then there is going to be less distortion and possible rippling in the larger panel.
GuyW

I did check the date after posting and 31/3/2025 not 1/4/2025, it was a serious comment. Definitely a matter of having confidence in ones skills and the welder as it needs to perform without skipping a beat. I've only tried it with my 160A Eland MIG bought new in about 1985 and still going strong. If you try it I would advise some practice pieces first.

GuyW,

Did you rewind a drill or similar in the past? I expect the spot welder is a round tuit. I have one that'll do 2mm + 2mm and have used it on spridgets before, and many other things, so it'll be coming out when I get back on to mine as it makes quick clean work of joins and looks OE and it's what weld through sealer was made for.
David Billington

Quite right David. I am no electrician but With a scant memory of ancient A level physics plus patience I can work out and rewind stuff. I've done several motors, a Spridget heater motor, a coil wound flasher unit and this welder. It sounds similar to yours with capacity up to 2+2mm mild steel.

I did rewind it once before which fixed it for a couple of years before it failed again. As I mentioned,I think if the metal is rusted it upsets the resistance across the weld tips and this impacts on the back emf in the primary, which then overloads. Or maybe I just got my calculation of the length of wire needed a bit out!
GuyW

This thread was discussed between 27/03/2025 and 01/04/2025

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