You sure it was a mechanic he asked and not a cop?
I heard one a while back
A guy was driving down the road well in excesss of the speed limit and passed a cop with his radar on
Of course the cop came out and stopped him
The cop asks the guy why he is in a big hurry.
The guy says I have to get to work.
The cop says, what do you do?
He says I am a rectum stretcher.
The cop says I never heard of that, what do you do in that job.
So the guy tells him, first I work 1 finger in, then 2, then 3, then 4 then my hand, then the other and, then I keep stretching till it's 6 feet around.
The cop says what the hell do you do with a 6' *******
The guy says we give him a badge and a radar gun
Fine $5000
Jail time 15 days
Look on cops face: priceless