Spark Exhaust Bent Tube 90° Ø 32 × 1.0 mm Stainless Steel Universal G9128

Thirty-two millimetres through ninety degrees. The hard corner in the first of Spark's thin-wall sizes.

Key Features

• Mandrel-formed 90° bend, AISI 304 stainless steel.
• Ø 32 mm outside, 1.0 mm wall thickness.
• Straight tangents at both ends.
• Matches Spark's 32 mm straight tube, article G9006.
• Spark part number G9128, made in Mantova, Italy.

Around the Downtube

On a small single the header leaves the head forward, and the frame downtube is directly in its way. Getting round it is a right angle followed by another one lower down, and thirty-two millimetres is the size that fits between the tube and the crankcase.

It is the classic small-bike routing problem, and it is why the smallest diameters get the sharpest corners.

What a Right Angle Costs

Pressure loss through a bend rises steeply with angle and falls with radius. A ninety at a production radius costs several times what a thirty does, and about twice what a single forty-five does.

On a small engine that matters less than it sounds — the gas volume is modest and the losses are proportional. On a large one it is worth designing around.

Radius Ratio

What actually governs the loss is the ratio of centreline radius to tube diameter. A bend with a radius of one and a half times the diameter is tight; three times is generous. Production tooling sits near the tight end, because that is what fits on a motorcycle.

Checking a Formed Right Angle

Stand the bend on a flat surface with one tangent down and put a square against the other. A formed ninety is generally within a degree; a hand-bent one is generally not, and the error only becomes obvious once it is welded into a run.

Two Bends Facing Each Other

Two 90° bends with a straight between make a U. At this diameter Spark does not offer a 180° article, so that pair is how a small-bore run doubles back — one extra joint, and full control over how wide the U is.

Leave a Straight Afterwards

Gas leaving a sharp corner is separated from the inside wall and takes several tube diameters to reattach. Anything sitting in that region — another bend, a joint, a sensor boss — works in disturbed flow. Two or three diameters of straight after a ninety costs nothing and recovers most of it.

Ordering

One bend per article. Spark builds 30°, 45° and 90° at thirty-two millimetres.