This is a pre-stretched custom chain from Stihl. If you are running a Makita UC3530A or UC4030A with the original bar (CSP-C1-9-50-SD) you will need a 39 Driver chain. If you are running a Makita UC3551A or an older Makita with a Newer Bar (CSD-SP-10-50) you will need a 40 Driver chain. These chains are a 3/8 Low Profile 50 gauge chain.