Thanks very much for the video answer. I've worked a bit o 2 strokes (not aircraft) and about to do my first Rotax twin bottom end. Great video. I learned something. My guess is that one of the benefits is NOT having to split the crank to replace rubber seals. Again THANKS.
Glad you enjoyed the video! It is a really different type of seal. There is a canal lift lock near by that I believe uses the same labyrinth seals since opening in 1904.
Brilliant! I thought about this. Why is a crankseal leak so serious on a two stroke? As we all know the engine will suck in false air from the leaking seal (s), and the engine will run lean and therefore be prone to piston seizing. And even hard to idle and hard to start...like my 503 was. Why would a leak between the cylinders not be that critical? Maybe because there are gas/air mix in both chambers? Just my thoughts. Thanks for sharing! Torbjorn.
A crankcase seal leak is very serious indeed. A two stroke engines primary compression is produced in the crankcase. The secondary compression is that area above the piston. Totally different than a four stroke engine. Visualize the piston on the upstroke creating a low pressure in the crankcase therefore allowing atmospheric pressure to push the fuel mixture from the carburetor into the crankcase. On the piston upstroke the atmospheric pressure would enter through the path of least resistance, the open carburetor and not much through a small leak in a crankshaft seal. Air is lazy and will that the path of least resistance. Now for the piston downstroke, this downward movement must now compress all that fuel mixture that has entered the crankcase. This pressurized mixture is then forced through the transfer ports into the cylinder above the piston. Visualize how much mixture under pressure would escape past a leaking crankshaft seal. A very large amount of fuel mixture, that contains oil will be lost to the atmosphere. This lack of fuel and oil available to enter the cylinder is greatly reduced resulting in a lean fuel mixture and a lean oil mixture. So not only is it lean of fuel it’s starved for oil too. Seized or sticking piston ring will produce the same result. Hope this makes sense to you. Best regards John
I've been thinking about this some more. How does one check a labyrinth seal is still good? Are they normally changed when the crank is rebuilt and the bearings changed? Thanks. I'm glad your back feeling better.
Good question! I’ve never had to replace a labyrinth seal. I inspected it when changing bearings for physical damage. Remember the outer surface is captured by the crankcase and doesn’t rotate and the inside diameter doesn’t touch the rotating crankshaft so I don’t see anyway of the labyrinth seal “wearing” out.
I absolutely pronounced labyrinth incorrectly! When I realized that fact it was too late for a video do over. My high school shop teacher pronounced it incorrectly and I’ve followed that for 50 years. I’ll correct my pronunciation moving forward.
