Make two parts share 80% of maintenance work.

[nesheim]

Hi

I have a piston and a liner (a replaceable cylinder) and I want to replace these at certain intervals.
If the model is changing piston at a certain time, then 6 hours of maintenance is added.
If the model is changing the liner at a certain time, then 6 hours of maintenance is added.
The liner has approximately 3 times longer lifetime than the piston, and they share the first 5 hours of maintenance in common.
This means that if both liner and piston are changed at the same time, the total amount of hours should be 7 hours, not 12.
How can I make model add 7 hours and not 12 whenever these two are done at the same time?

Also, I want the wear rate to not be fixed, but "last time steps wear rate" +- random value between 0.05-0.075. In addition to this I want the wear to reach a maximum or minimum amount. The idea here is that I can get a trend on change on wear rates.

I am getting better in Vensim, thanks to this forum and some practice, but I still have a long way to go.

Thanks

[nesheim]

I uploaded the model