There is a special sealant you should use also, can't remember the name right off the bat, but someone will know what it is... Mine was installed by the engine builder without sealant on the gasket and eventually it began to leak a little. Dripped from the cowling at the back of the engine, so it was hard to find. Finally traced the oil to the front of the engine but there wasn't much. I was definitely not convinced it was actually the nose seal since the area behind the flywheel was hardly damp at all. Pulled the prop and flywheel and the nose oil seal just pulled right out.
Put in a solid one (MUCH less expensive than a split ring) by heating the seal in oil and using a couple of large screwdrivers to lever it on. Installed the ring spring behind it, lathered on some of the sealant (whatever the heck it is called!) and pressed it in place. Hasn't leaked a drop since. Most surprising was how little the original leaked even with nothing holding it in place at all.
Regards,
Lee...