Removing dents from stainless exhaust

[Mick BaileyParticipant @mickbailey28509]

I’m wondering whether to buy a pair of stainless downpipes and ‘H’ crossover section for my Moto Guzzi 850. They’re in nice condition, other than one of the downpipes has a group of dents just below the first bend where the pipe straightens out. I’m open to suggestions regarding removal. The pipes are 38mm OD single-wall, with a thickness of around 1.5mm. It looks like the bike may have been in an accident.

With mild steel I would have annealed the area, welded on a stub in each location and carefully pulled them out with a slide hammer. I don’t have facilities for welding stainless at present, but think this may be a possibility.

The other approach could be to cut out the dented area and make up a matching section from identical tubing and get that TIG welded in place. Even when dressed and polished, my thinking is that the grafted-in piece would discolour where it’s welded due to surface differences inside the tube.

If I can I’ll get some pictures.

 

 

[David Millar 3Participant @davidmillar3]

Hi Mick,

 

I’ve got zero experience of this, but as a viewer of “How’s It’s Made”, could you get a close fitting ball bearing and push it down past the dents using smaller ball bearings, or a wooden mandrel? Perhaps in a press, or by careful hammering?

The ball bearing opening out the dents to reintroduce the original tube profile?

I’ve seen that method used for opening out brass tubing. Perhaps it would work in this instance?

 

David

[bernard towersParticipant @bernardtowers37738]

Big Balls Mick, in the nicest possible way!!!

[Robert Atkinson 2Participant @robertatkinson2]

Wrong tab….

 

[Clive FosterParticipant @clivefoster55965]

In his short novel “Strike While the Iron is Hot” Guy Lautard describes a process for ironing out a dent in a hydraulic cylinder using a short slug of steel having a cone shaped end with a flat machined on it. The slug is welded to a T handle which one protagonist turns whilst the other taps the dented cylinder with a hammer. The process was said to take about 20 minutes or so.

I imagine there is some skill involved in how hard to tap and how hard to twist. Its implied that the dent was taken out a bit at a time. The initial insertion being only so far as resistance is felt with the flat aligned with the dent.

In the story the cylinder is 3″ internal diameter with a half inch deep dent in it. The plug was made 4″ long sized to be a nice sliding fit. The line drawing interrupting the text implies that the flat is approaching 3/4 of the diameter of the plug wide. Which all seems way too deep a dent and way to wide a flat to me. Putting a half inch deep dent in a 3″ hydraulic cylinder without bending it beyond repair sounds less than likely. Allegedly a section of I beam had fallen against it on a construction job.

But Lautard is known to be generally reliable writer. So what do I know.

I’d have no compunctions about trying the ideas on a dent about half the relative size.

The general theory is of course very similar to panel beating where you bonk the depression and push out from the back rather than trying to beat the ‘ump back down.

Pushing a ball through something a stiff as stainless steel seems to have high risk of inducing corrugations in front of the dents. Such dent formation basically involves circumferential stress and distortion so rectification is probably best done in the same direction. As the twist a plug’n tap method does. Pushing a ball through is trying to pull the dent out with longitudinal force so you aren’t operating directly on it. Which doesn’t sound the best way.

As ever with stainless steel you will have work hardening to contend with.

Clive

[Mick BaileyParticipant @mickbailey28509]

If the tube has been mandrel bent it ought to maintain a constant diameter and a ball bearing could be an option. I need to check tomorrow when I’ll get chance to inspect the pipes more closely. Maybe heating the location to red heat may help. The challenge would be to get enough force on a ball without exerting pressure anywhere else. Perhaps a series of bearings could be tapped through. This is where my oxy propane setup would have come in handy if I hadn’t have given up the cylinder rental!

 

 

 

 

 

[Howard LewisParticipant @howardlewis46836]

Once the dent is out, there will be oxidation evidemt (Although if these are exhaust pipes, there will be, anyway, once  the engine is run under any sort of load)

It might be possible to remove the staining by coating the area with oordinary household bleach.

The hypochlorous acid removes the oxide film from stainless, beautifully

Howard

[bernard towersParticipant @bernardtowers37738]

Thats one for the biker boys to remember Howard!

[DC31kParticipant @dc31k]

The trick with the balls is not to be too greedy. Find the smallest ball that will pass and go up in either 0.5 or 1 mm increments.

Have a plan beforehand for removing the ball if it gets stuck.

Clean the inside of the pipe as best you can.

Lubricate well. I think tallow was a traditional product used but maybe some slippery soap product would also do.

[Peter Cook 6Participant @petercook6]

The musical instrument people use an appropriately sized  steel ball inside and a very powerful magnet outside to roll out dents in brass instruments. Might be worth investigating, although the kits are not cheap.

[MacolmParticipant @macolm]

In the event that you choose a method that involves external hammering the pipe when cold, a possible tip is to hammer on an applied film of thick oil. Certainly with chrome plated mild steel, this prevents marking the surface. Stainless may be a different matter as it usually work hardens so easily.

[noel shelleyParticipant @noelshelley55608]

Plug both ends,(by welding) one end has a BSP, say 1/4″ fitting in it and the whole affair is filled with water then pumped up ! Bit like hydro forming ? Worth a try ? Noel.

[duncan webster 1Participant @duncanwebster1]

<p style=”text-align: left;”>I like Noel’s idea, but is there a possibility that the internal pressure could straighten out the bends, like a buordon tube but permanant deflection.</p>
As I understand it the dents are round the first bend, so anything pushed down the pipe has to be not rigid

[CirclipParticipant @circlip]

Problem with ball trick is they might try to escape through the cross pipe. ( I have a Guzzi Lemming so know what the pipes look like) Good luck, Stainless is tough.

Regards  Ian.

[Clive FosterParticipant @clivefoster55965]

Hydro forming to remove the dents is nice idea but the pressures involved would be impractical. I’ve seen expansion chambers, much thinner, hydro formed. The gauge on the hydraulic pump unit went up to “scary silly” before things popped into shape.

Really think you are stuck with cutting the pipe to get at the damage.

Clive

[Nicholas FarrParticipant @nicholasfarr14254]

Hi, stainless steel work hardens, so you may find while removing dents by hammering, adjacent areas may distort. Welding stains on stainless steel, can be removed by what is often referred to as a “Yellow Wheel”, but a good one isn’t cheap, but will last a good while, they won’t produce a gleaming polish though.

Radial Bristle Brush

Regards Nick.

[peak4Participant @peak4]

Never tried either myself, but I’ve seen pulling a ball through mild steel pipes recommended.
Drill the ball and fit a substantial wire rope, then pull it through, possibly with local heating of the dented bit.
The opposite is also recommended on some forums; add water and pop it in the freezer.
A chest freezer might be easier. Some folk just say part fill with water, and others suggest pugging the ends.

Good luck

Bill (LeMans Mk3 owner)

[Mick BaileyParticipant @mickbailey28509]

I took another more detailed look today at the pipes and decided to leave them for the time being. The pipes aren’t mandrel bent, so the bends are necked down in diameter on the bends. The dents are where it begins to straighten out on the full diameter section (between two bends). A ball bearing of the correct diameter couldn’t pass either bend. Pushing the dents out hydraulically has too many possible setbacks for me and perhaps an uncertain outcome. The dents are not uniform and are of different depths so would need constraining.

When I think of the pressure applied by my regular 8-ton press when I’ve used it to press similar dents out of stainless sheet of similar thickness the pressure needed within the tube would be substantial. I think it works out at several tons per square inch.

 

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