The first URL is a good hackers guide, best I've seen. The second more conventional. Notice the reference to isobutane mixtures in both (sources listed in first). http://www.vettenet.org/acfaq.txt R-12 replacement FAQ http://www.startext.net/homes/ggoodspe/ACCONVRT.HTM R-12 to R-134a Refrigerant Conversion