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520H Onan valve seat problem

6 posts in this topic

ID: 1   Posted

Hi, My 1993 Wheelhorse 520H (432 hrs owned since new) is back running on 1 cylinder. The cylinder close to the steering wheel has very low (20 - 30#) compression and pushes fuel up through the carb when the engine is started. This was a problem last summer and I pulled the engine and removed the head. I found the intake valve seat loose. I measured the valve seat od. and the valve seat bore and found .001" difference. I plated the valve seat .002 on dia. and re-installed with a new valve, seal and spring and peened around the diameter of valve seat bore. The plated valve seat pressed in very hard. The engine ran well for the fall and most of the winter (Snowblowing) It is now back to running on 1 cylinder and I suspect the valve seat is loose again. Is there a better way to fix this loose seat problem on Onan engines?

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ID: 2   Posted

I know nothing about Onan engines, but I've seen an oversize valve seat replacement on Ebay that claims to be a fix for this, as it appears to be a common issue with Onan Performer engines.

Kevin

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ID: 3   Posted

I know nothing about Onan's either, but I have worked in engine shops where valve seats were replaced. I didnt actually do it myself, but saw it done daily.

I do know that the amount of interference fit was critical (and this was on water cooled engines which are probably operated at more constant temperatures than air cooled engines). I dont know what the spec was for the fit, but it was really tight. The head was heated in an oven, and the seats were immersed in liquid nitrogen. If all went well, we could do about 4 seats before the head had lost enough heat and had to go back in the oven (probably about 45 seconds...). When the temp difference was right the seats would just drop in their bores (guided by a mandrel in the valve guide). It was a definate no-no to press or tap the seat into its hole.

I'd take it to a quality machine shop, and get them to do it. We would bore the holes first and then make the seats to fit the bore. I know the guy that made the seats was working to .0001 tolerances...

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ID: 4   Posted

Oops!

There was a dulicate reply, and I deleted it. Unfortunately, BOTH are now gone.

As posted by OldHorse earlier:

I have a 1992 520-H that had the same problem at about 500 hours, about 10 years ago. Same Onan, same cylinder, same intake valve. I had been told by my old school wheel horse dealer who sold me the tractor new, that was a common problem with Onan and their motors on "garden tractor" applications and the same reason Onan pulled that motor from those type of applications. Not typical in the commercial construction equipment applications but very normal in garden tractor, heat related.

If you consider the location of that cylinder to the rest of the engine compartment, it makes sense. Packed in the most enclosed area with the least amount of fresh air access for cooling. Combine that with the variation of heat change rates for the aluminum block compared to the steel valve seat (aluminum expanding faster than the steel), the rest is history.

Fortunately, an old drag racing foe of mine (really a good friend), ran a local machine shop.

I also tore mine down and removed the valve. Took the block and valve to his shop and discussed the surgical procedure.

We went to an oversized valve seat, which meant flycutting the valve seat bore in the block to a slightly deeper depth and to the press fit diameter called for by the press specs (about .002). Peened the seat after installation and we were on our way. Ten years later, I now have about 900 hours on the tractor and not a bit of problem with any loose valve seats.

As a note, if you have not yet done so, when you removed the sheet metal that wraps around that cylinder, I found gobbs of grass clippings that get sucked in through the shoud and deposited under that sheild and packs in between the fins on the cylinder jug. This crap can be devistating to these type of engines on a 95 degree day, and the fins are packed with grass clippings you can see because of shields. Trust me, the crap is there.

Since then, I have made up a little snake attachment for my air nozzle that gets me back into that area of the engine to blow he crap out. I've made it a ritual of doing this on a regular basis. It's a pain in the butt, but so is a loose valve seat.

With all those I've worked on in the past and the way the engine is shrouded and

guarded, I would have never imagined it could have looked that bad. I do believe without a doubt, that was the primary contributing factor to the seat failure on mine.

Live and learn and you only need to teach me once.

Good luck.

Old Horse

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ID: 5   Posted

Hi, Thanks for the quick replies. There was definitely a lot of grass built up under the engine shrouds when I first had this problem. THe engine is nice and clean now, but the damage is done. I will try to find a local machine shop that can handle this type of repair and go with a larger diameter seat that will yeild more interferance. The one I plated pushed in very hard so the machine shop will need the capability to heat the engine and cool the seat.

Thanks again,

John

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ID: 6   Posted

Hmmmmm--I thought the valve seats fell out because Onan took too much material away to build the 20 HP bore/stroke combo they needed. Boring the valve seats out bigger seems like it will only get worse--although I suppose you have no choice now. :omg:

Good Luck

Tony

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