125-200cc Question that makes me sound nooby

Trying to figure this out with animated vbideos of two stroke engines but can't. In 2 stroke engine, there is a bottom end, two bearings atached to the conrod right? And that takes the power to the gearbox. Well when I change my gearbox oil that is, take out the drain plug underneath the bike and fill it up again at the side by the kickstart, am I changing the oil to lubricate the gearbox or does it lube the bottom end bearings too? If not, is the bottom end lubed by the oil that goes in with the fuel? If it's the oil in the fuel mix, that baffles me as it seems such a small amount, I always imagined the 730 ml of oil bathed both the bottom end and the gearbox for smooth operating?

Yes the mix oil in the fuel lubricates the crankshaft main and rod bearings as well as the piston and rings.

thankyou for clearing that up. How often would you say the gearbox oil needs changing then? I ride on road only, quite hard revs too, currently I change it every 800 miles or so, or about every 30 hours

Really? That baffles me.

How does that baffle you cosmo?

I am baffled because in my head I see the piston rings in the cylinder, and they stop any of the oil and petrol on top of it from going down past the conrod and onto the bearings, how does it get past there? I just don't wana blow my bottom end one day so I'm trying to find out how many miles to change gearbox oil ast, but if it relies on the premix oil then I have no idea

Hours or miles are not so critical on the 2t machines as on the 4t engine when it comes to oil changes ... The hour number you have above is probably 1/2 as much as I put on my 08 WR250 before I change oil and that is only a guess as no meter on my bike ...

The piston rings on a 2 stroke are there to hold compression, there is no oil ring like on a 4-stroke. The charged air enters the crankcase and then is pulled into the cylinder. At any given time, there is charged air on both sides of the cylinder.

As for the change interval, the service manual for my 250 says every 8 hours for ms riding and 16 hours for enduro riding. I don't really follow this because the main thing that contaminates oil in a healthy engine is the clutch fibers from clutch wear.

lets start here 2t crank case, intake to exhaust is closed system......lets begin here piston goes up creates vacuum(suction) in the crankcase, air fuel mix is sucked past reed vavle into the crankcase, oil (fuel) lubs the cyl wall,rod and bottom end bearings (all no oil pressure bearings). Piston comes down pressurized crankcase air fuel mix is pushed up throgh transfer ports (reed valve is check valve to stop mix from going back to carbby. air fuel (and oil) is pushed and stops into combustion chamber, piston goes up compression of mixture then light off, as piston goes down past exhaust ports mix is expelled out exhaust all these cycles are all ahppening at the same time. that is the basic deal. your gearbox is driven from the end of the crankshaft which is projected out of cases on both ends through crank seals, one dry ignition side and one wet gearbox(clutch side. Thats about as simple as I can say, without getting into ports expansion chamber and all the details. Your gearbox is a separate case from the crankcase.

And then, and then..... oh, it's handled....

But we didnt mention the expansion chamber as the pulse of the combustion forces burnt mix out of the cylinder it resonates/ bounces around the expansion chamber this reverbaration holds the fresh mixture and some old mixture against the exhaust port as the piston is trapping the mix to go all over again.
Tuning an expansion chamber alters a whole lot of power characteristics.
Think that covers most of the basics.

Oh. After seeing Kelly's posted video, I get it. That's not at all how I thought it worked. I thought it was more like a 4 stoke. It's actually pretty ingenious.

I sparked a big response! So in that animation, that red gas stuff from the left, imitates petrol in gas form coming from the carb plus premixed oil for lube, then as the spark at the plug ignites the mixture is forced out the exhaust port to the right, pushing piston down, meanwhile the rod, bottom end is all lubed by the oil coming in as well? That is not how I imagined it worked, my mind says surely you'd need way more than 30 to one ratio for lubing all of that but apparently not

My KTM manual says I should run it at 60:1 which seems insane. I'm going with 40:1

This is also why you should never run crappy fuel with ethanol that is potentially corrosive when exposed to the moisture in humid air.

Ethinol eats steal fuel tanks and perishes alot of conmon rubber parts, i hate the stuff i use premuim fuel from shell 99ron less ethinol in that as the cheap stuff alegedly has 10% mix here :s not good for motors or my peace of mind.
I mix 50:1 always have with my 360
Used to blast around on my 125 at about 80:1 cus i just put 100ml in a 9l tank she ran for years no signs of wear.