Main shaft seals

[Puffs]

Well, a worn seal on the gearbox side results in gearbox oil (which has all sorts of dopes) entering the crankshaft cavity, generally resulting in lots of smoke with a distinctive smell. On the ignition side it's more difficult to detect, but you'll probably see some signs of leaking, also in the form of oil there.

I wouldn't overestimate the effectiveness of your "inertial air block", from memory the SG of air is 1.35kg/m³, and air is known to vibrate at even higher frequencies (ask any violin player). But I do agree that the amount of air (or mixture) passing past a worn seal on the ignition side is small; the intake allows for far more effective breathing. Anyway, I'll now worry a bit more about that...

[nice2day]

Hi Puffs A 2 stroke is more likely to draw oil through the crankshaft-seals when the engine is on the "over-run" where the engine is being driven by the movement of the bike but the throttle is shut. This condition is more likely to create a very low pressure (vacuum) in the crankcase area and thus sucks in the oil...Obviously some oil can get in at any time but I am describing the most likely scenario....

As regards the violin and sound: There is not an actual flow of air. Sound is a high frequency vibration that is transmitted through the media to the ear but there is no actual gas flow occurring. You can have a room with deafening noise inside but there is no air escaping from that room.

Les

[Puffs]

While no air normally escapes earth either, air has been demonstrated to move in & out of the violin sound box through the f-holes. With both the Helmholtz resonance frequency as well as that of the overtones as occurring in top & back plate. Yes, if you wait a bit it goes back in, but still it flows.

If you make the enclosing volume big enough, and the time long enough, the average (net) flow is always 0. But vibrations in air do exist, and they do constitute movement of air. In fact, resonances in both intake & exhaust tract play a major role in 2T tuning for speed.

[nice2day]

You are giving examples of instruments and noise generators in free open air....Not moving air and reversing it at a rate of 25 times a second with a wavelength of 14 metres, though an air gap that is in the order of square micrometres... maybe the diameter of a hair....I contest that in trying to do the above task, it is near physically impossible and has no correlation with sound generation. It is a matter of simple physics as the massive energy that would be required to override this resistance is not available in the engine set up. This resistance is caused by the air's inertia and it's flow resistance, something I did not mention earlier. IE: It does not happen

Les

[Puffs]

Well, any cavity with a hole in it has a Helmholtz frequency (https://en.wikipedia.org/wiki/Helmholtz_resonance), not just a tone generator (or loudspeaker), and sound consists of air moving to & fro in the direction of propagation (https://www.physicsclassroom.com/class/sound/u11l1c.cfm). Trust me, sound is actual flow of air.

It would seem the positions are about the effectiveness of this "inertial air block", which you think is effective in preventing flow, yet I think it is quite insignificant. While we seem to agree that worn seals can cause gearbox oil to enter into the crankshaft cavity from the gearbox side, opinions diverge on the other side. In my opinion, on the ignition side the impact of worn seals is more difficult to detect, but it may result in that bearing missing part of it's lubrication, as well as a marginally leaner mixture (clean air moving in @ -'ve ΔP, and mixture moving out @ +'ve ΔP). Your opinion on that may be different, but I think we both replace both seals when we have the engine apart, so in action taken we would agree. Hence lets not quibble over this.

[AlanJ]

Hi Guys,
Just to add to the mix, I have a TS 250/1 and 2 Saxon 301's and the seals on all three are clutch side facing out against the gearbox oil and the right side
against crankcase pressure facing in. Could it be that it was found by the MZ engineers that this was the best solution and maybe, and I can only guess at this,
that the pressure build-up in the clutch side requires the lip against the oil in the gearbox. If this was the case then I can understand and as Puffs has said, put
the new ones as the original. You may have read my post, way back, when I got the Saxon 301 going, after buying it the clutch side seal (after standing 13 yrs)
was stuck to the crankshaft and was spinning in the housing. The Saxon started but you have never seen so much smoke in all your life, my wife and next door came running, thinking I was on fire, to be honest, I couldn't stop laughing.
Wish you well guys Alan

[dave47]

If Alanj is right about the TS250/1 seals facing the same way as on the ETZ, and I think he is, either MZ changed them at some point in the production run, or the published drawings were in error. I will edit my earlier posts to reflect the uncertainty.

[Puffs]

Yes it's been a bit of an eye-opener to me that the MZ documentation is not always correct... Or they adapt, depending on experiences - without updating the documentation?
And that is a remarkable story: gearbox side seal spinning in it's housing, hence leaking oil into the crankcase!

With the pressure inside the crankshaft cavity fluctuating up & down, I guess it is difficult to decide which orientation of the seal (with a spring on just 1 side) is optimal. It depends on what your priority is. On the gearbox side, if your highest priority is to prevent gearbox oil entering the crankshaft cavity, you'd place the springed lip pointing outwards to the gearbox side. On the ignition side they want to prevent lube oil leaking out more than a marginal bit of air entering, hence there that lip points inwards.

[AlanJ]

Hi Guys,
As I said earlier, I have changed all my main seals with a 251 to go but it always is a surprise to me when you read posts of guys buying bikes that have
been standing (often in adverse conditions and not checking) and as you know the clutch is often the worse for the amount of rust either from condensation or water running
down the clutch cable. All my 3 bikes have had something similar to sort out and to be honest while one is looking at the clutch you might as well do the seals
after all they don't cost a bomb and (if you use anaerobic multi gasket you can take your time with the cover) I thought I wuld get that one in.
Anyway guys wish you well Alan.