srw
It's a bit more complicated than that...
This is a really good idea. Many jobs, even in big companies, come through personal contact. And a part-time job is often a precursor to a full-time one.Many years ago I worked for Halfords as a car mechanic back when they first started doing the cars and you had to be a fully qualified light vehicle technician with City & Guilds qualifications. That sadly has changed over the years but the thing I learned in the 3 years I was there was that they still work by the small shop ideal they started with. If you go in when it is quiet and ask to see the manager, put your case and offer to do a few hours for free as a way of showing what you can do they will love the enthusiasm and most likely count that as the interview and you will be in if you can do the job. It is many years since I worked for them but I keep in contact with some old friends and things don`t change much. Halfords started with bikes and small shops and head office still like to remember that, many of the management there started as shop boys before they left school.
One more thought - if the particular shop takes standards seriously you may well find that what they're really interested in isn't technical competence (which can be taught or learned fairly easily) but the other stuff - Health and Safety in the workshop, speed of working, cleanliness and professionalism, customer care. If you've got good stories to tell about those you'll be one step ahead of others.