07-06-2018, 09:18 AM

The stock pump has two scavenging pickups, one for the cam case and one for the sump in the crankcase. It only has one scavenging gerotor which is the pump pulling on both pickups. So if the cam case gets pumped dry the pump will suck air on that side which can cause it to lose prime on the crankcase side which will result in sumping. Try putting two soda straws in your mouth, one shorter than the other. When the liquid gets below the short straw it will be harder or impossible to suck liquid through the other straw as long as you are pulling through both.

The S&S pump has a gerotor or pump for each pickup which makes it more unlikely the sump side will lose its prime. The money invested in it when doing any upgrades or even if it is your only upgrade is well spent in my opinion.

On the TwinCam they managed to counter this by making the sump pickup longer and smaller which also helped counter the variances in crankcase pressure caused by the pistons going up and down. The M8 just has a big hole on the right case where the oil pump connects. Harley has tried making the oil pump pickup smaller, whether that is working I can't say. Also, that's why if you idle it on the side stand there may be some extra oil collect in the sump. The S&S won't keep that from happening, but it should clear it soon after you stand the bike up straight.