Exhaust system springs

by Had Robinson


Like the rubber engine mounts, the springs on the exhaust system serve an important purpose.  The vibration of two cycle paramotors is so severe that if it were not for the springs holding the exhaust system together, the vibration of the engine would quickly destroy important parts of the engine.

When the engine is running, it undergoes axial vibration at the same frequency as the revolution of the engine.  The rotational movement of the engine can be 4mm-6mm near the engine mounts.  This high frequency movement is the source of the destructive forces that affect all parts of the paramotor, including the secondary wire and the paramotor frame.  This movement is similar to the action of a hammer-drill used to cut through concrete.  The further the distance engine parts are located from the center of the crankshaft, the greater the movement.  It is easy to see that the exhaust system (the cylinder exhaust port, the tuned pipe, and the muffler) is located the furthest from the crankshaft center.  Consequently, it will undergo the greatest amount of movement.

This is why there must be springs connecting things together.  There must be the ability of the parts to easily move with respect to one another.

A. Spring installation

Use a loop of paraglider line to stretch the springs for replacement/removal.  Do NOT use Vise-Grips or pliers to stretch the springs because such tools will nick the tempered surface of the spring connection point and weaken it.  Here is a video on how to properly do it.

B. Spring failure

Correctly manufactured springs that have been properly installed will last the life of the exhaust system.  However, Miniplane and their Italian suppliers have significant quality control issues that I and other pilots have long observed.  As a result, we occasionally see batches of defective springs (and other engine parts) installed on new engines that have a very short life.

Below is a defective muffler spring from an almost new Top 80.  It is easy to see that the manufacturer did not properly temper the spring.  Miniplane, unfortunately, denies everything but to experienced machine shop guys, it is obvious who is to blame.  Pilots that experience muffler spring failure on engines with less than 300 hours, should contact their dealer and ask for replacements.  Unfortunately, there was a stretch of time when all of the Miniplane springs had problems.  Just keep getting replacement springs until you get ones that are correctly made.  You will be content to know that it is nothing the pilots are doing that is causing failure of the springs.

defective Miniplane exhaust spring on the Top 80