I just finished installing a under dash vintage a/c on my 67 convertible with a 200 CI, I6. Everything is good but I can't stop the belts from squeaking when the compressor is turned on. At first it was fine but then started squealing. I have checked the tension on the belts that appears to be good.

 

Make sure the belt is wide enough. If the belt is too narrow for the pulleys, the belt will ride on the bottom of the pulleys and not on the edges. When this is the case, you can't get a belt tight enough to not slip. I am thinking this might be your issue. This is a particular problem when running a modern compressor on a classic car. Often when this is the case the sheaves are wider on the compressor than they are on the engine, thus a stock belt (to the car) wont work. You can get around this issue (usually) by running a bigger belt that fits the compressor. The belt may "ride high" on the engine, but it usually will work.

 

We used a breaker bar between the block and compressor to tighten the belt enough to stop it from squealing

 

Thanks. I will check the wide and tighten it again. Thanks again