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

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

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

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.

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.

<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

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

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)