Onan

[JAinVA 4,638]

Hello from VA,
     I know that the Onan P series has a bad reputation with some in the Wheel Horse family.The rear intake valve seat seems to be a problem for some.I am wondering if anyone
has had a problem with the rear exhaust valve seat?I know that there is a problem with cooling the rear cylinder in the way Wheel Horse installed the Onan and blocked air flow to the rear
cylinder.In other applications of this engine I don't see the problems that WH owners have.I own a 416 h and think I know why the intake seat for the intake pops out but need to know if anyone has had a problem with the exhaust seat to confirm my theory.This is question is meant for my own education only.So again has anyone had a problem with an Onan EXHAUST
valve seat popping out on the rear cylinder?I am looking forward to any and all replies.Luck,JAinVa

[boovuc 1,090]

JA,

I have read of both intake and exhaust valve seats going on the rear cyl of the P series Onans. personally, my only brush with the seat issue was the intake seat so I have no first hand knowledge of the exhaust seat going.
I'll check back to read your theory on why "most" of the failures are the intake seat.

[JAinVA 4,638]

BooVuc,
      I only have asked this question because everyone I have talked to about the p series Onan and all of the blocks I have seen have had a problem with the rear intake
seat.I just got a P220 block with close to 2000 hours on it and no hint of seat issues.This block was not used in a Wheel Horse but a Groundmaster with remote filter and better air flow to what we with Wheel Horses(stock) have.The airflow over the rear cylinder on the Wheel Horse is as we all know is blocked by the belt guard and its brace.
I have read the posts that give these seat issues as one of high heat and thermal expansion,but why only the intake seat.Thats the reason for the question.I think that the exhaust seat is kept close to the cylinder temp because of the flow of exhaust gases through it .The intake valve and seat are cooled by the incoming fuel and air mixture
coming through them.If the intake valve and seat stay cooled and the aluminum block they are located in is over heated by lack of air flow then thermal expansion causes
the cooled seat to become loose.Hope this makes sense.My thought is that if the cooling issues are addressed that a block with good seats will not have a problem with
loose intake seats.There are folks with much more experiance with the Onans than me and I welcome any critique of my posts.Luck,JAin Va

[bmsgaffer 2,053]

Seems like a solid theory to me! But I'm no expert.

I plan on venting my belt guard very well and I also have a remote oil filter kit planned for my 520 when I get her up and running.

[oldredrider 2,553]

As the Onans in stationary equipment (generators, welders etc.) seem to perform well and for a long time, says something about the quality. However, those applications are well vented for cooling: thus the long life.
IMO, Wheel Horse made a VERY poor decision regarding the use of an engine that REQUIRES adequate air flow in a tractor that doesn't provide it.
I'm with you on your theory.

[boovuc 1,090]

I'm with you on what would cause temp differentials from the exhaust valve to the intake valve. Not sure if those differentials would be that great between intake and exhaust once the engine is warmed up and running at full throttle. I can see it on a lot of start/stops. 
Other food for thoughts are the John Deere issues with the Onan's. They had a lot of valve seats go on their tractors also. Also note that generators, (the Onan Gensets), didn't run at 3600 RPM. They are governed at a much lower RPM. My place of work has a Miller Welder with an Onan. No#2 cyl had low compression to the point it was having trouble starting. Probably a valve seat issue! Never opened it up as it isn't part of my job and they needed a dependable welder so they opted for a new one with the old as a trade in.
The heat buildup issue with the Onan has other factors worth considering. The biggest is our ethanol loaded fuels. If you get a carb that starts to gum up and run crappy, a lean mixture can cause a lot of heat issues without the owner ever realizing it over a long period of time. This alone is bad but this and the WH air flow issue combined is real bad. The Onans need a lot of cooling. So much so that the air intake on the flywheel side grabs gobs of grass while mowing and deposits some of it in the cooling fins. If they aren't blown out and taken apart once and a while, you get hot spots. The oil filer is located in the rear cooling tins that:
1. Blocks about 25% of the total area under the tin thus blocking air flow under the tin on the very cylinder that is already susceptible to over heating in the WH configuration.
2. This filter is hot and putting out more heat in a trouble spot.
3. With the blocked air comes more grass and mower debris that settles in this area over time.

I think you have a contributing theory but I believe it is a lot of things that contribute to the issue. I think there are more intake than exhaust seats that fail. It may very well be the reason why the intakes fail..........first!