A comment was recently made about avoiding any pre-compression issues, and
reducing the initial purchase expense, by simply mixing new and old gear
cushions. I have been mentally debating whether to comment on that posting,
out of concern that my input might come across as 'sour grapes'. I have
decided to weigh in on it anyway.

There are two key reasons not to take this approach. You can weigh them as
you see fit.

First, the IPC (and some maintenance instructions) caution against mixing
donuts. While that is a legality you could easily ignore, they don't say
'why'.

The second reason, the 'Why', is the part that matters. This type of
rubber compound does not get into the needed 'elasticity range' until it is
loaded. Once loaded (pre-compressed or compressed in service), it actually
becomes more 'jelly-like'. An uncompressed/unloaded cushion will seem very
hard indeed, having almost no 'give' at all. If you mix new and old
cushions, particularly in the retract size (thicker and larger in diameter),
what happens is that the old donuts will take virtually all the compression
forces. It will take a little while before they flatten and harden, and the
loads get transferred to the newer cushions, therefore 'softening them up' a
bit. But at that point you only have part of the intended cushioning
available, since the older cushions have now flattened out; and you are
faced with re-doing the job. You will probably also have the sides of the
old cushions squeezed out and riding on the inside of the gear housing, due
to the 'overload' being applied to them. In most cases, this will happen
soon enough that the newest additions will be more closely matched to the
ones that were recently installed, so the side effects won't be so
pronounced; but it will still be visible to the knowing eye. Part of the
cushion stack will have more 'swell' in it than the other.

No need to ask me how I know this..


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