Threadlock – what to use, where to use, and why?
by Had Robinson
updated April 22, 2020
Threadlock is a special type of glue that helps keep threaded parts from loosening. Small 2 stroke ultralight engines experience tremendous vibration because they are made as light as possible. The vibration will loosen fittings after just a few hours of running time. Vibration will also cause various sheet metal parts to flex and eventually break from fatigue and pilots should always be on the lookout for cracks in aluminum parts of their paramotors, including in the frame.
Here is a photo of what can happen when bolts get loose and hit the propeller.
There are three common types of threadlock.
- RED The type is the strongest threadlock. Threaded parts that are assembled with RED can only be disassembled by using heat from a torch in order to dissolve the bond. It should be used on threaded studs that are inserted into an engine case or cylinder. This includes the studs that hold the redrive on and the exhaust port studs that screw into the cylinder.
- BLUE The most common threadlock. Parts that are assembled with BLUE can be disassembled with ordinary tools.
- GREEN This type is known as "wicking grade" because it can be applied AFTER the threaded parts are assembled. It has the strength of BLUE.
How to apply
Threadlock should be applied to absolutely clean threads. Clean ALL parts prior to assembly with ordinary spray-type brake cleaner that is available from any auto parts store. Use compressed air to remove every trace of the brake cleaner before the threadlock is applied to the fastener. If compressed air is not available, let the part air dry or use a hair dryer on it. Apply just enough blue threadlock to cover 2-4 threads of the stud, screw, or bolt. If using red threadlock on studs, cover all of the threads on the stud that go into the stud's threaded hole. Green threadlock is applied sparingly to the joint between the nut and the screw or bolt. Tighten all fittings to the recommended torque value.
Where to apply – and where not to
Miniplane and other manufacturers recommend that most nuts and bolts be assembled using medium strength (BLUE) threadlock. Failure to use threadlock will invariably result in things loosening up at the most inconvenient times.
Note: Important parts of new paramotors may not have threadlock on them but should. Pilots should be certain that redrive and exhaust port studs are screwed into their casings with RED threadlock. Test these fasteners to ensure that they cannot be loosened with reasonable force. If you can easily remove the studs with ordinary tools, they were not assembled with RED threadlock.
DO NOT USE ANY KIND OF THREADLOCK ON:
- screws going into threaded inserts (e.g. the back plate fasteners – see note below)
- spark plugs
- shaft nuts and bolts
- head nuts
- carburetor fasteners
- drain/fill plugs
- starter fasteners (except the center screw in manual starter)
- propeller hub center screw
- threaded holes that are in cavities or spaces that must be sealed e.g. the crankcase and redrive
- locking-type nuts.
If you are unsure what any of these fasteners or parts are, or what they look like, please search the Internet for information and images.
Note: Threaded inserts are commonly used in the frame where they hold the back plate and in plastic moldings. The insert will loosen/fall out the next time the screw is removed if threadlock is used. The threaded insert in the Polini frame pictured below on the back plate became useless because it freely rotates so that the button-head screw cannot be inserted OR removed. Use a lock-washer with inserts, if needed.