BTR LS Piston Ring Compressor Tool 4.000 bore, ALUMINUM 6.0 LS2 L76 L99 LQ4 LQ9 One piece aluminum tapered ring compressors have been around for some time, but BTR has made a significant improvement to the design. Rather than relying on the piston skirt to locate the pistons as they're being inserted into the block like one piece tapered ring compressors currently on the market, the new BTR ring compressors have a small but very important reverse chamfer at the bottom that fits into the chamfer at the top of the block which serves two functions. First, it locates the ring compressor exactly over the center of the hole. And, secondly, it reduces the chance of the oil ring getting caught between the ring compressor and the top of the block. The laser etching on a black background is easy to read and looks good even after lots of uses. In our beta testing against competitors tapered ring compressors, the BTR was easier to use and never caught a oil ring between the compressor and the block.