520-H loose valve seat?

[1990Onan520-H 15]

Hey all, I popped open my 88 520 to do the decarbon, reseal intake, clean carb etc. and found this. There was a light tapping noise when running, but I'm not sure it's a seat issue or not. Cylinder compression was 85psi on each cylinder. Just under 1000 hours on the machine. Intake valves seem to be just ever so slightly held open. They hold brake clean and veryyyy slowly seep. Exhausts are perfectly liquid tight no leaks and were loose lash wise. While cleaning, a sliver of aluminum came off in my hand. Top left of the pic, you can see a piece of what I'm assuming retains the seat missing. Is this the start of a valve seat issue? Can I peen the seat in and be ok?

[lynnmor 7,302]

When the engine was built, the seat was staked in by upsetting that bit of metal you are seeing that is flaking off.  I suspect that the seat is coming loose when the engine is hot and will eventually wear to cause a serious problem.  Keep in mind that I am only giving it a good guess from a distance.  The aluminum block will expand when heated to a greater degree than the steel seat, so try heating the block with a heat gun, or other method, for quite some time and then check for looseness.  I am not a fan of just peening around the seat and prefer tight press fit of about .005" to .006".  The Onan manual is incorrect, showing an impossible .0305" for the P220, but is correct for the P224 showing .0045".

[Greentored 3,201]

Just went through this on rear intake on my recently purchased 520. I am brand new to this and never even saw the inside of an Onan til now, however been building engines for 30 years and even had a speed/machine shop for a bunch of them, so installing seats is far from new. I am very fortunate to have the ability and equipment to do it right, and also totally understand the 'stake it and put it together' method to keep one alive. I am shocked that this works at all haha. Lynnmor explains it perfectly- the aluminum expands much more than the steel seat, so a press fit is needed, or itll be loose once warmed up.

From an engine builder standpoint, Onan's .0045 spec is not nearly enough, especially on an air cooled engine, and wonder if this is the actual issue. Then again, if it were, one would think we'd be seeing the much hotter running exhaust seats falling out instead of the intakes, right?  Any seat I ever slapped into V8 aluminum heads got .008-.010 and heated a couple hundred degrees, Iron heads got .006-.007, Harley heads got .010-.011 and heated to 300+ degrees. Seats always got the freezer treatment, and a thin wipe of loctite sleeve retainer, more as a lubricant.

***If you fellas with Onan seat troubles forget everything you read about these seat issues, remember this one thing:  a Harley head at 300 degrees with a frozen seat, it will literally DROP INTO PLACE! Thats with a .010 press measurement at room temp! Whats that tell us about .0045 press? (The intake seat I just replaced in the Onan got .010.)

  Ok, so you just read two paragraphs of useless, rambling BS haha, sorry. 1990Onan- how loose are the seats in your block? If you lucked out and caught it early and the seat is still snug but can be removed with your fingers, you will PROBABLY be ok to get a pair of Onan +.010 seats, heat the block up, freeze the seats and drive em home. If you have the ability (or a friend) to measure the seat bore, you can figure out how much press youd have.  If the bore is wallowed out or the seats have pounded themselves down into the block, you dont have much choice but to have a machine shop cut a new bore and install a much larger oversized seat.

Edited January 3, 2020 by Greentored

[1990Onan520-H 15]

How can I tell if it's loose? I can't move any of them with my fingers. That was why I was hoping a peen and send it. This 520 has seen better days and I can't keep throwing money at it. But I also don't want to purposely let it blow apart. This is the front cylinder by the way.

[lynnmor 7,302]

Here is a good article that should answer most of your questions.  Your engine returned to good condition is worth as much as the tractor, so tossing money at it shouldn't be much of a waste.  You may not have a loose seat, that aluminum flaking off may just be a poor job of staking when built, read thru the article and there is one idea there to determine if the seat is loose.  Like I said earlier, try heating the block and then test for looseness, try tapping with a punch and hammer from the inside and also try to twist the seat to see if you can get rotation.  A tiny notch ground on the bottom edge of the seat will allow for testing with a punch.

 

The low compression readings may return to normal if you freshen up the seats and valves.  I have seen worse and only minor valve and seat grinding was needed.