Is it possible to register a car and bike in my own name without a visa? I only ever stay in Thailand for 4 weeks at a time as I work offshore (4 weeks on, 4 weeks off). I am fed up renting and want to buy a car. I have a letter from Immigation confirming my address (have lived at same condo for 2 years plus).

You will need the letter from the Embassy for your address or from Immigration. You have the letter

from Immigration so that is ok. I needed a visa to buy my car in my name, i have a 1 year O Visa, i am not 100% which Visa you need. Try doing a search in the motor forum.