High quality black Viton Primary gear seal.
The Primary gear seal should be replaced each time you build a motor or replace a clutch. The primary gear does come out through the bellhousing hole when replacing the clutch.
Oil leaks occur for many reasons including too much wear in the bushes (about 3.5 thou is recommended), a good surface on the gear for the seal to run on (linishing with fine tape held steady is good), and not too much end float on the gear (.004-.006" is recommended), and some oil on the gear to lubricate the new seal TIP masking tape wrapped around the gear can assist in not damaging the seal as you slide it onto the gear.
