R/H oil seal

[Benelliracer]

When installing crankshaft oil seal does flat side with numbers go next to bearing or next to lobe on crankshaft? This is to a 1973 100 motor.

[taber hodaka]

Remember I have a old memory, but the right hand seal the large one, the flat side is next to the crankshaft lobe. ----Clarence

[Benelliracer]

Thanks, general oil seal sites say number side faces out unless manufacturers say different.

[Bill2001]

And generally the Lip Side (non-flat) faces the pressure side, in this case, the crankcase side, so at first glance, the flat-side-to-the-crank sounds backwards. Not debating, just sayin'.

[Bullfrog]

But, specifically, for the Hodaka main seal on the clutch side . . . the "smooth" face is "in" (toward the crankshaft lobes).
Ed

[CelticDude]

Hi Ed,
I see that in the parts microfiche but why is that different with Hodaka? With most seal applications of this type, the "smooth" face is the low pressure side and the convex side faces the high pressure side. This is due to the fact that the increase in pressure on the convex side tends to help the seal work more efficiently.

Thanks!

Doug

[BrianZ]

The crankcase pressure is constantly changing from high pressure when the piston is descending to to low pressure when it is rising. This is why the seals are double lip type.

Brian

[Bullfrog]

Good answer Brian - but here is a bit more on the topic.
Defining which side of the seal is the "high pressure" side depends entirely on which exact instant during the engine rotation you are talking about. The crankcase seals experience periods when the pressure inside the crankcase is higher than ambient air pressure - and periods when the pressure inside the crankcase is lower than ambient air pressure. So the seal must operate properly "in both directions". A single lip seal will not work as a crankshaft seal in a Hodaka engine - double lip seals are required.

[Bert44]

Would the ‘wrong’ direction of the seal also prevent gear/clutch oil being sucked into the crankcase during the low pressure cycle?