Crank seals

[D project]

After forgetting which way my crankseal on the clutch side goes in, I'm confused.
Does it really matter which way it's installed? One side of the seal is closed with a flat face the other is open with the wire spring retainer. It's a double lip seal on a 89 RM250. Going by the drawing/diagram in the Suzuki manual it looks to have the open side facing the transmission. Looking at cases for sale on eBay some have the flat side facing out to the clutch, but the majority with the open side out. Flywheel side allways has the spring on the inside. Is it only when the seals are worn that they can suck air and oil in?

[Nathan S]

The open side is generally the oil side.
Crank seals are slightly unusual, in that they need to seal both ways - plenty of genuine main seals are double sided for this reason.

In reality, it matters less than the theory says it does. I've seen plenty of seals fitted both ways, and never noticed a difference. I put the open side facing the gearbox oil.

[John Orchard]

I put the open side facing the gearbox oil.

Same

[tony27]

Quite often it depends on how close to the bearing it sits, if the seal is right up against the bearing I'd put the open side towards it rather than have the face of the seal contacting the bearing race

[pancho]

 Seriously leaking crankcase seals can equal runaway revs.. leading to bruises and distrust of 2/s!
 (memories of sorbent hill} Slightly leaking crankcase seals = lean mixture= siezure and more bruises.

 Interesting experience with worn out Victa lawn mowers- very worn piston = blow-past into crank case resulting in strange muffled backfiring through crankcase.  That is if the thing starts in the first place.

 2/s are wierd, they shouldn't work!
 cheers pancho

[John Orchard]

Quite a coincidence that this subject should come up just as I make a personal discovery on it !

Crank seals must be fitted with the open side (recessed) facing towards the outside of the engine, towards the gearbox oil on the right side, and towards the generator on the left.

This is of more importance the bigger the engine capacity, it is imperative on an open class machine; let me explain ...

I have found the vacuum in the two-stroke crankcase far exceeds the compression, maybe it can be due to effects of the expansion chamber I don't know.  At high rpm the vacuum through the carb is ferocious, now if you close the throttle at high rpm the pumping action is still continuing and drives the vacuum through the roof, it can have a big effect on the crank seals.

Face it, you would rather have some loss of crankcase pressure 'out' through the seal than air being sucked in causing a lean air/fuel ratio right?

With the seal recess facing inwards, the vacuum can lift the seal lip away from the crank, with the recess facing away from the engine, vacuum will cause the seal lip to be sucked tighter onto the crank.

Smaller engine may not have the vacuum strength to lift the seal lip but a big engine will.

I found this on my Honda CR500RE, even with new crank seals, at high rpm & part throttle, when the crankcase vacuum is at its strongest, the seals were leaning-out the air/fuel mixture, and doing it inconsistently. Sometimes the gear oil would partially restrict the leak making me think I am going crazy!

The vacuum leaking through the gearbox seal was also bring in dust through the gearbox breather, making the gear oil dirtier quicker.

I have since rotated the seals (new ones again) and all is well.

[FourstrokeForever]

Makes perfect sense to me John 

[pancho]

 Hadn't thought of that one, interesting John!