The silver spanner will repair up to 66% of the maximum durability of the component currently being repaired, repairing up to one full point of durability per use. For example, if your engine has a maximum durability of 4, and it has 0 durability left, using the silver spanner once will repair it up to 1/4. Using it again a second time will repair it up to 2/4. Using it once more will repair it up to 2.64/4 (which is rounded to 3/4 when the game displays it, keeping the decimal values hidden). Since 2.64/4 is 66% of the maximum durability, the silver spanner will refuse to recover the remaining lost durability, requiring the use of a better spanner or a Repair Kit.
It can be found included with the Tool Rack Lvl 2.