Don't worry about the timer motor. It is actually quite strong, I managed to almost shear off one of those little plastic nipples that some timers use just because it got stuck somewhere. I am more concerned about the plunger bending or breaking when the rubber gasket dries out and doesn't move as well anymore.
I am planning to use them as timers too... Again, I don't think it would be a problem for the motor. You can see that I removed the "timer function" from the unit pictured on the left, but it isn't necessary.
(BTW... If you use the timer that I pictured, and a wooden extension like I used, you need to open up the timer and remove the blue override wheel, really easy to do.)