Spark Exhaust Bent Tube 30° Ø 40 × 1.0 mm Stainless Steel Universal G9101

A thirty degree elbow in the diameter most aftermarket link pipes turn out to be. If a run needs a gentle lean rather than a corner, this is the part.

Key Features

• Mandrel-formed 30° bend, AISI 304 stainless steel.
• Ø 40 mm outside, 1.0 mm wall thickness.
• Straight tangents at both ends.
• Matches Spark's 40 mm straight tube, article G9001.
• Spark part number G9101, made in Mantova, Italy.

The Default Diameter

Forty millimetres carries a large twin or a middleweight four without slowing the gas, and routes around a shock and a swingarm without fouling. It is also the internal bore of several Spark db killers — the wide Force and Moto GP inserts and the Force Evo and Konix Evo plug.

Those parts and this bend are sized for the same class of machine, which is why a Spark-based build tends to settle on forty and stay there.

Bend Radius and What It Costs

Every bend has a centreline radius, and a tighter one costs more flow than a generous one at the same angle. A production bend is made at the tightest radius the tooling will hold without distorting the section — which is a good deal tighter than anything achievable by hand.

At thirty degrees the loss is small in any case. This is the cheapest change of direction there is.

Stacking Bends

Losses add. Three 30° bends in a row cost noticeably more than one 90° at the same radius, because each transition disturbs the flow again before it has settled.

Where a single larger bend will do the job, use it. Where it will not, put a short straight between the bends and let the gas recover.

Test-Fitting

Aluminium tape holds a bend to a straight well enough to hang the assembly on the motorcycle. It is worth doing every time: the difference between a run that looks deliberate and one that looks bodged is usually a few degrees of clocking, and you cannot see that on a bench.

Deburring the Cut

Trimming a bend leaves a burr on a curved edge, which is more awkward to reach than one on a straight. At forty millimetres that burr sits squarely in the gas path and stops a slip joint seating properly. A half-round file or a rotary burr clears it; a thumbnail run round the bore is the test.

Ordering

One bend per article. Spark builds 30°, 45° and 90° at this diameter, plus a 500 mm straight.