I would ask to see a compressor tear down report on the compressors that were installed and failed. In order for the compressor re-manufacturer to give your contractor a credit on the failed compressors, they will tear them down to see if it was an error by the contractor that led to the failures. Believe me if they can prove contractor error, which the report will show, they will so they are not giving away new rebuilt compressor to a "parts changer" for free. They report will show what the failure was, where the failure began in the compressor and that will give an idea of what to look for in the system that keeps causing the compressors to fail. Rebuilt compressors can fail though, I had a tech just replace a 50 ton compressor with a rebuild. We have a set procedure for our techs to follow when replacing compressors so I know it was done right (tech could skip steps with out me knowing but most likely not). The compressor grounded out after running for two minutes. The customer being upset that they had no air for several day (1st rebuild was out of stock and took two days to come in plus the time it took for us to do the replacement). I went out that evening with another compressor and another tech and we replaced that compressor. After 30 minutes of run time the second rebuilt compressor grounded out. The next morning I picked up a third rebuilt compressor and replaced the second failed compressor. We made no changes to the system in between the failed rebuit compressor replacements other then new refrigerant filter driers, new refrigerant and contactors. The third compressor has been running for a week. By the way I have paid for all three compressors, supply house was quick to get me the invoices. The company I work for won't get the money back or a credit till a tear down on the compressors has been done.