Oil Way

[frankfast]

Can someone describe the oil way at the mating surface of the cylinder and crankcase. On a 350 W/C is it just a hole surrounded by the paper gasket at the base of the cylinder or is it a hollow stud or rubber o-ring? I found mine to be just a hole in the cylinder and crankcase sealed by the paper gasket. Is that correct?

[Bevel bob]

yup.

[frankfast]

So, if one finds oil in the combustion chamber, the two most common means would be by the rings or down the valve guides. Correct? I tore the top end apart when I lost compression and found a little oil laying on top of the piston. The piston and rings are very new and the valve guide seals look good.The valves seem to be seating correctly. I blew air in the ports and felt nothing in the chamber.The only thing that I found was that the valve clearances were tighter than they should have been. There was very little clearance at TDC. However, I don't understand why that would be the cause of oil entering the combustion chamber. Any thing else I should be looking for?

[graeme]

Rings, valve guide seal or loose valve guide?
Oil can't get into the cylinder from anywhere else unless there is a crack in the head around the guide?
You said it lost compression? Suggests rings aren't sealing? Or valves aren't closing?
Was the tight valve clearance preventing them sealing?

Graeme

[frankfast]

I'll check the guides but I replaced them a while ago although there is very little mileage since then. They were a nice snug fit. The clearance on the valves was minimal but the valves did close. The head was a mess when I found the bike. The cam was completely worn down due to a broken oil pump. Replaced it with a cam from a 250 Scrambler and the rockers (using shims) were replaced by rockers with screw adjusters. The piston and rings are new and the rings are in good shape. There is really no obvious cause for the lack of compression or oil in the combustion chamber. Tight valve clearances were the only fault. I didn't cut the valve seats but only lapped the valves when I put it together.

[Nick]

With the head off, pour some gasoline down the intake tract and see if you have any gas leaking past the intake valve on the combustion chamber side; repeat for the exhaust.
Any leakage and the valves aren't sealing.

[graeme]

But,,,,, as long as the valve / rocker clearance isn't overly tight and keeping the valves from seating the compression and springs will close them enough to run.
If you have lost compression (even with new rings) check the cylinder for marks or sticking rings.
But oil in the cylinder above the piston can only come from the guides or the rings.
Unless you have a crack somewhere?

[Bevel bob]

Is it possible that the engine was overfilled with oil, or some oil put into the cylinder to protect and you forgot about it?. suspect poor sealing between valve guide and head bore on inlet side . valves do settle in and clearance closes up but would not affect oil situation. If head ported guides can rock.

[kmev]

Following a complete rebuild, my 250 NC was a smoker. The piston's valve pockets had pooled oil when I pulled the head off. I spent a year looking at everything until I found that I had put the top piston ring in upside down.

If it smokes immediately on start up and then tapers off, I would look to the valves. If it is clear on start up and the begins smoking after it has been running a short while (smoking really bad at high RPM), then take a close look at your piston rings.

I used this to determine it was upside down: https://www.hastingsmfg.com/ServiceTips ... lation.htm

[frankfast]

I used Nicks method of pouring gas down the ports and found the inlet valve seeping gas. The exhaust valve was tight. I confirmed the seepage with soapy water and blew air down the port. Since I don't have the means of cutting the seats, It looks like I have to send it out for a valve job. Unless the valve head is bent slightly and since the guides are new, it would mean just renewing the seats and lapping the valves. I used to get stuff from Syds in Florida and I know he used to perform machine shop work. It's been a while since I contacted him. Don't know if he is still in business.