decksetter 50 #1 Posted October 2, 2013 Question and a theory... Out of all the onan p series that have thrown a valve seat, how many have had recent valve adjustments? I ask because I have been inside 3 onans in the past year to adjust valves and in one case re set an intake seat. All 3 were fairly high hours (one 750, other two around 1200) and in all 3 cases the intake valve had no clearance, it was tight enough that you could spin the valve around with your fingers, so wouldn't hold compression. If the spring can't hold the valve tight against the seat when it's closed, then when the engine heats up and the steel seat and the aluminum block expand at different rates/temperatures, then it eventually pops the seat loose. Soooo... My theory is, onan valve seat troubles don't have anything to do with the rear cylinder overheating, I think the intake valve on the rear cylinder is for some reason prone to tightening up and holding open a little bit. Then the valve seat loosens and either 1) pops out and/or 2) creates enough off center pressure when it fires that the rod effectually breaks. Thoughts? Is it plausible? Share this post Link to post Share on other sites
zieg72 209 #2 Posted October 2, 2013 I also think it gets hotter but I think it fails mainly due to the oil breaking down due to excessive heat. I would venture a guess the oil is the problem more than anything else. Share this post Link to post Share on other sites
JackC 616 #3 Posted October 2, 2013 Sounds plausible. I bought a 416H with a P216 ONAN with about 1,000 hours on it and a broken rod on the rear cylinder. The valve seats appear to be OK. I will have to check the valve clearance on the rear cylinder valves to see if the valves are too tight. 1 Share this post Link to post Share on other sites
sorekiwi 761 #4 Posted October 2, 2013 The theory sounds plausible to me. Most of my background is automotive (water cooled) so I tend to think of a tight valve clearance leading to a burnt valve, but maybe in an aircooled engine the seat might be just as abused. Not so sure about your "off center pressure when it fires that the rod effectually breaks." theory. I need to think about that some more, its something I have never heard of, or considered. I bought my Onan with a broken rod, but I am reasonably sure that it was low oil level that did that one in. However I must admit that I didn't check the valve clearances when I pulled the motor down. Share this post Link to post Share on other sites
decksetter 50 #5 Posted October 2, 2013 (edited) Not so sure about your "off center pressure when it fires that the rod effectually breaks." theory. I need to think about that some more, its something I have never heard of, or considered. I'm not sure about it either, just threw it out there as a possibility since the broken rod problem always seems to be the rear cylinder too. Intake valve holding open would also contribute to the carbon buildup problem these commonly have. Intake valve not shut all the way, so exhaust goes back in the intake. Maybe the carbon buildup pops the rod somehow? Edited October 2, 2013 by decksetter Share this post Link to post Share on other sites
Red-Bovine 289 #6 Posted October 2, 2013 (edited) I did notice when my P220 threw the seat that the valve was loose and could spin. Appeared not to fully close and held open by the lifter. I don't understand how the valve train could get longer preventing the valve from closing. Wear would tend to make the lifters loose, thus the valve would close, but not open fully. What am I missing? I did not ever adjust the valves on my P220 unless repairing the valves. Didn't know I was supposed to. Red Edited October 2, 2013 by Red-Bovine Share this post Link to post Share on other sites
sorekiwi 761 #7 Posted October 3, 2013 Wear on the valve seat and the matching contact face on the valve will close up the clearance between the lifter and the end of the valve. 1 Share this post Link to post Share on other sites
Red-Bovine 289 #8 Posted October 3, 2013 I guess I never considered the wear would be that great as to hold the valve open. I see very clearly now that the open valve could cause the seat to become hot enough that it could loosen. Good point and maybe the lack of periodic adjustment could be the root cause of valve seat failure. I don't recall ever seeing any mention for the need to adjust valves on the Onan engines in the consumer literature. Did I miss that, too? Red Share this post Link to post Share on other sites
decksetter 50 #9 Posted October 3, 2013 I don't know how well wheel horse pushed the maintenance issue on these things. Also I don't know if there is something different about our p series engines or if companies that built generators/welders/other tractors with them just were better about pushing maintenance. I have heard some say 500 hours was the recommended interval to adjust valves and decarbon them, anyone know for sure? I think I heard someone say 200 hours would be better, just to be safe. Share this post Link to post Share on other sites
decksetter 50 #10 Posted October 3, 2013 Wear on the valve seat and the matching contact face on the valve will close up the clearance between the lifter and the end of the valve. That makes sense. Spec for the intake valve is .005". That's paper thin. If you figure you're running around 3600rpm, in 500 hours at full speed that engine has turned 1.8 million revolutions. I could see it wearing a little. Also, who knows how picky they were setting them from the factory. I just adjusted the valves on my 750 hour p220, I don't know if they were ever adjusted before because I haven't had it that long. The front cylinder was right at spec for the intake side but the rear was being held open just a touch. Share this post Link to post Share on other sites
JackC 616 #11 Posted October 4, 2013 This is from a seller on eBay that sells de-carbon kits. ONAN VALVE GRIND- DECARBON GASKET SET IF YOU WANT TO EXTEND THE LIFE OF YOUR BF B43 B48 P-216-218-220 ONAN ENGINE YOU NEED TO REMOVE THE CARBON AND CLEAN THE COOLING FINS AND ADJUST THE VALVES EVERY 200 HOURS OR SO .ALMOST EVERY TIME WE TEAR DOWN AN ONAN ENGINE THAT BLOWS A ROD THEY HAVE BURNT OR OUT OF ADJUSTMENT VALVES ON THE OTHER PISTON THEY LOOSE SOME POWER SO PEOPLE JUST TURN UP THE THROTTLE AND THE OTHER SIDE CARRIES THE LOAD FOR A WHILE THEN THEY TAKE OUT THE ROD,SO THEY REPLACE THE ROD AND DO THE SAME THING AGAIN ,THEN THE ENGINE IS JUST A PIECE OF JUNK IF YOU WANT TO STOP THIS CYCLE A LITTLE PREVENTIVE MAINTINECE GOES A LONG WAY,THIS IS ALL YOU NEED. 1 1 Share this post Link to post Share on other sites
JackC 616 #12 Posted October 4, 2013 From the ONAN P Series service manual. IMPORTANT: Do not lap valves. The sharp seating surface between the valve and seat will be removed resulting in shorter valve life. 1 Share this post Link to post Share on other sites
sorekiwi 761 #13 Posted October 4, 2013 Its been a while Jack so I might be a little wrong, but I remember reading a little about this when I fixed my engine. From what I remember the valves are made from something a little special and lapping the valves will cut through the hard surface on the valve. I am fairly sure that I left the valves alone and recut the seats. Then I just used some Bearing Blue to check for a good seat contact pattern. I'll think about it some more and maybe I will remember exactly. It will probably come to me in the middle of the night, hopefully I will remember in the morning!!! Share this post Link to post Share on other sites
decksetter 50 #14 Posted October 4, 2013 From the ONAN P Series service manual. IMPORTANT: Do not lap valves. The sharp seating surface between the valve and seat will be removed resulting in shorter valve life. Wish I would have read this a couple months ago... Hope I didn't hurt it too bad, the p216 is running great so far after lapping valves (and replacing one exhaust valve). Share this post Link to post Share on other sites
JackC 616 #15 Posted October 4, 2013 Here is a video on valve adjustment. Worth learning how to take care of your valves. http://www.youtube.com/watch?v=slMP05G8LnQ 1 Share this post Link to post Share on other sites
