Welding an exhaust back box?

[1969Mk3Spitfire]

My other car still has its original, 35 year old, exhaust system. The back box (the only box as it’s a turbo) is starting to rust, per the picture.

Replacements are not available, only “boy racer”, noisy versions in stainless steel. I want to maintain as close to originality as possible.

Talking to guys on the owners club stand at NEC, they suggested repairing it.

Any experience of welding an exhaust back box, or other suggestions, please?

Once upon a time I was quite competent with oxy-acetylene and electric arc but that was during my apprenticeship in the 70s. Wouldn’t know where to start today and I subscribe to the automotive Hippocratic Oath of “don’t make it worse”

Appreciate the wisdom of this forum.

 

[Clive]

You could attempt a repair. No different to bodywork, cut out the rusty section, and weld in fresh metal.

However, teh internals may well be rotten too.

I have had success sourcing stainless steel boxes off ebay from companies who make their own exhaust parts, in one case contacting them and asking for a bespoke length (which was no problem at all) 

You may want to find out what the internal layout of the box is, so you can get a sound that is very similar. 

[JohnD]

Silencers typically rot from the inside out, as they are (usually) galvanised, but face constant hot water in exhaust gases.     The metal will be very thinned all around the holed area, so that you will need to cut back forward, back and around, to find sound metal - if you can!       To buy time, while you find a proper replacement, you can seal up the hole with an "exhaust bandage".    See the Holt's products, widely available. Gun Gum Repair Bandage | Holts (holtsauto.com)

Then, for a 30+ year old vehicle, you will not find original parts.    A decent exhaust workshop should be able to cut out the rotten silencer and put in a perfectly good, and quiet, one form generic stock.     Consider having a new system built - those pipes must be rotting too! - and exhaust guys can bend pipes to suit!

John

[johny]

Ive seen silencers like that just wrapped in a new outer skin which is welded in place. Certainly look ok and although of course the corrosion will continue inside unless youre going to replace the damaged internals youre no worse off.

The back box which runs cooler and whose slower gas flow rate accumulates condensate is always the first to go - the pipes themselves should last years longer👍

[68vitesse]

The exhaust system on my Mk1 2L Vitesse is the same one on the car when I bought it in the mid nineties, had a stainless one on for a short while but put the original back on as the stainless was a terrible fit. Front section when bolted to manifold to tight on bell housing and middle section to short.

Regards

Paul 

[1969Mk3Spitfire]

Many thanks for the responses thus far, much appreciated.

I’ve Googled “welding repair exhaust system near me” and have a couple of leads to follow.

DIY is always of interest. There seems to be an incomprehensibly bewildering array of electric welders available, not sure where I would start 😩. My preference would be oxyacetylene as I imagine I’d have greater control. Is it possible to buy, beg, steal or borrow a gas set for use in a home garage?

[Stratton Jimmer]

1 hour ago, 1969Mk3Spitfire said:

oxyacetylene

Oxyacetylene is a reducing flame and takes oxygen out of the metal being welded. It may therefore help if you have any rusty bits to weld. It's easy to blow holes in metal if you misuse it though. I watched my dad welding all sorts of things over many years. He was a coded welder qualified on pressure vessels although that was only part of his job.

[Pete Lewis]

welding without any practice or guidance gives Blow holes or Bird sh1t  

welding old rust metal ...use gas and old wire coat hangers   they melt so easy 

Pete

[PeteH]

Part of my Remit, back when, was to check the quality of weld samples, witness testing, certify results, and issue certification to welders working on Gas and Oil lines for the North Sea Fieids, Shipyards and Boiler Makers and Repairers. I knew all the codes and methods, X-ray`d samples etc; BUT my own welds are still "a lot to be desired". I can lay pidgeon poop with the best. Welding is 20% knowledge and 80% practice, practice...................😁. I had a great deal of respect for those pipe line guys, they performed in conditions many would balk at. They mechanised it in the later stages.

I fabricated a new shell for an American R-V silencer, using the old internals. lasted over 5 years. Then I replaced it with a Stainless One custom made by a Company Near Bolton, £300 just for the silencer, £200 cheaper that getting one from the USA!!..

Pete

[Mjit]

As JohnD said the inside of the silencer gets warm in use which, especially in the colder months, results in a lot of internal condenation forming and slowly rotting both the internals and the outer skin from the inside. So even if you can weld the normally very thing gauge metal it's still likely to be shot internally.

Find a decent exhaust centre and they should be able to find a stock back box that's the same size/bore/has a 'stock' noise profile rather than 'performance' one.

[johny]

well its not so much getting warm as not getting hot enough so that the moisture in the exhaust gasses from combustion can condense in the silencer instead of passing out the tail pipe. Its made worse by the fact that even with low sulphur fuels theres some which then combines with the condensate to produce sulphuric acid with obvious consequences. The car needs long runs at higher speeds to get the exhaust hot enough all the way along and is why I dont like starting mine while its laid up....

[dougbgt6]

16 minutes ago, Mjit said:

slowly rotting both the internals and the outer skin from the inside

I had to replace my stainless exhaust because the internal baffles were shot and some of the external welds had started to go. Only 40 years old! 

Doug

[Colin Lindsay]

For a cheap repair to that sort of damage, take the label off a suitably-sized tin can, remove the bottom, split the seam and open out to a flat piece of metal. This can be set against the hole with a good quantity of exhaust putty to seal the edges then a good bandage wound round it, John's Holt stuff, will keep it all in place. I've seen them last for years that way.

[Josef]

Ensure you have removed the contents of the can before taking the bottom off through

[Colin Lindsay]

16 hours ago, Josef said:

Ensure you have removed the contents of the can before taking the bottom off through

an exhaust smelling of baked beans can't be the worst, surely?

[dougbgt6]

No, tuna takes that prize. 

db

[1969Mk3Spitfire]

Update.

Thanks for the suggestions but the thought of tuna and beens aromatics just as the car reaches temperature was not convincing.

A local, “old school” garage thinks that they can make a sympathetic repair. Still dependant upon “what we find when we cut it open” so fingers crossed.

The garage only wanted the exhaust, not the car, so could work on it at will without taking up much space. The one piece exhaust hadn’t moved in 35 years and put up a bit of a fight but I eventually won. I put in on roof bars on my A3 and glad that I didn’t have to travel very far.

I’ll report back later………..