allowable wear in muffler and exhaust pipe hooks

Hi Alan

The allowable wear is something that you should ask Vans about.  i would guess if it is wearing you should perhaps address what is the cause more importantly.  Harder materials are not a solution.  

The common issue to cause this is of course vibration.  Naturally address gearbox wear, prop dynamic balance and check your engine rubber dampers condition.  Me personally I would replace those at 5 years like radiator hoses as they can get hard and transfer more vibration to the aircraft. To address your spring and hook issue there is a tried and true method.  Check the tension is correct according to the specs.  Install your spring and then run a safety wire from hook to hook with the wire inside the spring and twist it (6 to 8 turns per inch) it is important here to place a screwdriver in one end before you twist so when done the wire can move back and forth about ⅛ to ¼ inch, not tight.  This allows for movement on start and shut down of the pipe at the highest moment in the cycles.  To finish off this is the most important trick.  Run a bead of perhaps ¼ inch, 5 mm, of high temp silicone.  Use the red high temperature stuff and just go from one hook to the other over the top of the spring.  This is important as it will deaden/damp the vibration of the spring that wears your hooks.  This idea was very common with the 2 stroke engines and i know it works.  

When i get some time will dig up a few pictures for you to see. 

Cheers

It's not as common for the steel loop welded on the pipe to wear through, but if it does then time to replace it. I don't think I've ever had a loop wear through. Usually the spring hook takes the punishment.

More damage

Siliconed and safety wired