I don't think this a job area that gets gratuity consideration.
With some jobs employers pay workers with the expectation that tips will make up the difference. A good automotive or motorcycle shop that is going be successful should be paying a wage that rewards the skill of each employee. I should not be expected to make up their deficit in pay (they shouldn't have a deficit in pay if it is a well run shop).
Like BUBBIE said, tell them and their management that the job was well done.
About as far as I might go would be like HDDon, Skootch and Jack said, bring them some pizza, bbq or chicken for lunch. In my opinion this would still be a rare reward for me to give a well paid mechanic to do what I expect him to do.
I have turned down tips @ work in the past only to find some money shoved in an envelope placed on my tool box, these were long time customers, that would be offended if I did not take the money, Tho i always did the best I could for them and Always did a bit extra for them, They would some times come in when I was off and refuse service when I was not there, This always made me feel EXTRA good:s