Here we have a picture of Simon's displaying his o ring problem........
Now Simon claims this to be caused by movement in the thermostat housing, well I only wished I had seen this picture before replying to his absolutely incorrect diagnosis.
What has caused this failure is simple, this o ring
has acquired compression set, no ifs buts or arguments.
This is caused by the o ring material being in contact with a chemical substance which has altered its structure.
Now I suspect this has been fitted incorrectly, probably lubricated with something containing a substance incompatible with the elastomer, as the only time I've seen an O ring this badly degraded before, it had been fitted to a four cylinder k series upper coolant rail with LM grease
It is
not caused by abrasion, as the o ring would not have failed with only the outer radius of the o ring displaying wear, but in this picture it is clear to see that the elastomer has swollen to fill the entire space available and suffered compression set.
Chemical changes occur to an o ring which result in set, and the cross link density of the elastomer increases.
This results in the o ring losing its elasticity and it's ability to return back to the original shape, and this is a permanent chemical change.
This reduction in cross section results in a lower contact sealing force, which increases the risk of leakage in systems where thermal and pressure cycling occurs.
So there you have it Simon, your o rings have been in contact with a substance that they are not compatible with, it's not as you thought anything whatsoever to do with movement.
Brian
P.S. the reason the grooves in the straight section of pipe are wider then the Herbie clips, is to allow the clips to be removed without damage, which of course is absolutely contrary to your findings, they are completely reusable
