How much pressure is the sight glass under?

I would hazard to say that it would be too low to measure with a pressure gauge. Probably get an indication using a clear "U" tube filled with water...I would say that at max you would have 1/2" water...

Don't forget that the crankcase is vented into the airbox.......

Pull the oil fill plug when the engine is running and put your thumb over the open hole. You will get an idea of the pressure exerted in the crank case.

Remember that the motor crank/engine case is free breathing through the upper vent hose on the cam cover, however crankcase pulses (pressure and Vacuum) can and will be felt at any opening such as the oil fill hole. Also remember that when the piston is moving towared TDC its creating a vacuum in the crank case. Keeping all this air flow neutral allows the motor to operate with only mechanical drag and in some cases oil windage, which in our machines is somewhat lessened by the semi dry sump oil system.

Ive seen KTM sight glasses fail. Even a factory KTM bike at the Vegas to Reno. I poured extra oil in the bike at the second pit, just to keep it going.

Always seems like a rock hits it.

Its not common, i agree with that.

I just got some 6mm thick clear vinyl from Dual Star for my front headlight-to protect from rocks. I will put a piece on my 610's sight glass and see if it takes the heat this weekend. Should be an easy fix.

The sight glass isn't under any pressure. Most all sight glass have a baffle that either remove oil during operation or the angle of the oil passages are not pointed directly at the window. As with Hondas, oil cycles throught the sight glass portage with a type of trbulent flow that is not aimed directl at the sight glass. Pressure is minimal. Your sightglass will never fail. The base and head gaskets are under far more pressure than the sight glass will ever see.

By the way, I concur with Mr. Kay. The onlytime I hav evere seen a sight glass fail was when A rock struck it directl. It's really about a one in a millloin shot. The sightglass I saw crack was on a quad that was not running nerf bars.

hey I won the lottery,,,and you guys are correct,,,I also call it a non issue ,,,and no I did not take photos of it,,,,,,but I do have witnesses... guys its just one of those freak deals like winning the lottery!!! My lottery came in the fact that it was on a 1 lap sighting lap, not during the 5 lap race/ride in which case $$$$$$$$ would have been flying out along with the oil. R roost is your enemy pass the rooster asap!!! enjoy the ride!!
PS,when i do reinstall the glass i will have some kind of xtra/outer cover installed over the whole cast in boss.

And get a suddne bath in oil ???

Dean-

Ideally there would be a very slight vacum drawn in the crankcase below the piston to aid in ring sealing and control blowby.

Crankcase clearing is a very important thing!
This is why the beather is routed into the fresh air intake tract, well- that and emissions control. But it does aid in crankcase scavenging, none the less.
That is why I don't recomend attatching a K/N filter to the breather line (making a road draft out of the system) and leaving it stock.

I doubt the amount of crankcase pressure on any single is enough to pop it out, esp from the inside- there's other places for that pressure to go first, like the breather.

I'd be more concerned with it getting pushed/punched in or vibrating loose.

4 stroke singles don't work that way, if you are refering to crankcase pressure.

The Base Gasket is not under any pressure. The sight glass will see more pressure than the BG.

Head Gasket is only under pressure from the coooling system and the radiator cap is the safety valve.

If you attempt to do this only cover the hole. Do not stick your finger in a running motor or you could be less a finger.

No, I did not loose a finger. I am just saying....