Shaping tube

Making the shapes you want

by Julian Edgar

Click on pics to view larger images

At a glance...

  • Using preformed bends
  • Making your own bends
  • Ovalising tube
  • Use of sand-filling

Often when you’re working on cars you have a need to form tubes with bends, depressions or other shapes within them. You might be working on an exhaust pipe, intercooler plumbing, a cylindrical airbox, or even heater or turbo oil drain plumbing. So how do you form these bent bits of tube?

Using preformed bends

If you are making something like an exhaust or intercooler plumbing, buying pre-formed bends and then joining these is the easiest way to go.

The bends – best are mandrel bends where the internal diameter remains constant – are available in steel, stainless steel or aluminium. An exhaust should use the steel or stainless bends, while intercooler plumbing can use any of the three types of metal.

Joining of the bends can be carried out by welding – MIG, TIG or gas welding in the case of the two steels, or TIG in the case of the aluminium bends.

If you don’t have a welder, tape the bends together after placing ‘witness marks’ (where you have used a marker to run a line along the pipe and across the join, showing how the bits line up) and then take the assembly to a welder.

Mandrel bends are available in a range of angles (eg 15, 45, 90 180 degrees) and diameters from about 1.5 inch to 4 inches.

When making plumbing using these bends, ensure that you:

  • Use a friction saw with a large diameter blade to cut the bends to length. Don’t try using a hacksaw – it is nearly impossible to make a cut that is sufficiently straight it can be easily matched to another bend.

  • Try not to cut the bends anywhere except where they are straight – cutting on the bend itself will reveal a wall thickness thinner than the unbent tube (because the wall has been stretched) and so the weld is more likely to intrude and the join will be weaker than if it were made where the tube is straight.

  • Always debur the inner of the bend after cutting it. You can use a special tool or just a round or half-round file to do this.

If you are using mild steel bends to form intercooler plumbing, the final result can be blasted, undercoated and then powder-coated for a durable and professional end result. Stainless steel or aluminium can be polished.

Making your own bends

You can also form your own bends from straight tube.

The benefit here is that you can make the bend the precise required angle, rather than being limited to the angles in which preformed bends are available. The downside is that unless you happen to have a very expensive mandrel bender sitting in your home workshop, the bends will have a degree of crush and you may have some wastage before you get a bend you’re completely happy with.

In general it’s not worth trying to form your own bends in large diameter tube. A typical ‘pipe bender’ that uses a hydraulic jack and curved tooling is designed for heavy-wall pipe and will give poor bends in thin-wall tube. (However, in an emergency you may be able to get away with sand-filling the thin-wall tube – see later.)

However, small diameter tube can be successfully bent with a hand bender like this one. It comes with dies to suit 3/8 inch, ½ inch, 9/16 inch, 5/8 inch, ¾ inch and 7/8 inch (most tube sizes are imperial).

Here is a piece of 5/8 inch diameter steel tube bent with a hand bender like that shown above. It is an oil drain pipe for a turbo.

Ovalising tube

If you need to gain some clearance, it is possible to ‘ovalise’ round tube – even when working with a preformed bend.

The trick is to fill the tube with a coarse sand before starting to reshape it. The coarse sand has lots of voids between the grains that will progressively close-up as the tube is squashed. The presence of the sand resists the change in shape, giving the tube more support and so preventing deformation in the wrong directions.

This 2.5-inch mandrel bend was used as a turbo dump pipe – it’s shown here after being ovalised. It was ovalised for two reasons – firstly, the oval shape matched the exhaust housing of the turbo, and secondly, the oval shape needed to continue along the tube to provide clearance to the alternator and steering tie rod (shown here in its worse position of maximum suspension droop and full right-hand lock).

The bend was first filled with coarse river sand. Note that if you intend heating the tube (eg with an oxy) the sand must be absolutely dry. Here the sand is shown in a cast iron baking tray drying out over a wood stove.

After being filled with sand, the ends of the tube were capped with aluminium foil and tape. Contrary to first thoughts, the end caps aren’t under a lot of pressure - the sand doesn’t flow along the pipe that easily.

The sand-filled pipe was then placed in a hydraulic press. Two hefty pieces of flat timber were placed above and below the pipe, with a steel plate placed under the press’s ram. A clamp was used to stop the arms of the bend spreading as the ovalisation occurred. In this case the work was done without the tube being heated.

The pipe will try to form a figure-8 cross-sectional shape as it is being compressed; the outer edges can be pressed separately (as is occurring here) to lower their height as required. Note the use of the timber block – this deforms a little and spreads the load. Use of a metal plate straight on the tube will tend to dent the tube.

Here is the tube after the pressing. Note the left-hand side is oval and this gradually changes to round by the other end of the bend.

Ensure that you check the sand level during the pressing process – as the grains are crushed together, the level can drop.

Placing a depression in tube

If you have a need for clearance at only one spot, you can place a depression in the wall of the tube. As was described above, best results occur if the tube is first filled with sand.

This 150mm diameter tubular airbox needed a dent placed in its wall to provide adequate clearance to a starter motor solenoid. The dent was placed in the tube (ex truck exhaust tube) as the first step after the tube was cut to length.

The depression was formed in the following way:

  • Discs that were a tight fit in the ends of the tube were cut from chipboard

  • The tube was fully filled with sand, kept in place by the chipboard discs and some temporary tape

  • A 50mm tow ball (one with a flat top) and a hydraulic press were used to form the depression in the tube wall

Note that this approach gave a far neater result than using a ball-pein hammer and forming the depression by traditional panel beating techniques.

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