start motor makes a tick sound when I hit the ignition

last time this happened, replaced the inside contacts and I was good to go again for the last 2-3 years

ran fine last night when I went to fill up
come this morning I go to start the car and get the dreaded tick again
pull the starter motor out and clean all contacts again, all seems okay

put it all back together, no go
hook the battery up to another car, battery terminals hit 13.8v and I try to start again, no go

with the car off, battery reads 12.6v
when trying to crank, dips down to 11.4v
with the car hooked up to life support, it stabilises at 13.8v
when trying to crank while on life support, it dips down to 12.5v

checked the connection from the start motor to the battery and I'm getting the same readings as if I were probing the terminals themselves so the connections solid

anyone have any more ideas I can try? getting pretty desperate here