Valve Grinding Questions

[1990Onan520-H 15]

Ok I might sound like a bit of an idiot here but... how do I know if I need my valves ground? My 88 520-H has 85 psi per cylinder and I have it all torn down at the moment to decarbon and do its first valve adjustment at 920 hours. I thought I should grind the valves while I was in there to be sure compression comes back up, but I think that means I have to yank the engine and bring it to a machine shop. I was hoping to not have to remove it. Any advice/input is appreciated.

[oliver2-44 10,370]

From a maintenance aspect you can just lap the valves to the seats using rough then fine lapping compound.  I'm not an Onan guy, but if you can remove the valves with the block in place, you can do this lapping. You would need these items.  There's probable some youtube videos on lapping valves on an Onan engine. 

 

[Stormin 9,983]

It would be an idea to lap the valves. Not a difficult job.  

[953 nut 57,932]

Too bad you have already torn it down. A leak-down test would give you a better idea if the compression loss is from rings or valves.

[lynnmor 7,566]

At 920 hours it would be a good idea to freshen up the valves.  There is a balance between valve to seat contact area that should be maintained.  If the seats are worn so that the contact area is too much, there is less pounds per square inch to seal the contact area and valve seat burning can result.  If the contact area is too narrow, the valve cannot transfer enough heat to the seat and valve burning can result.  Only cutting seats and getting the contact area to the correct width is the proper way.  Valves often warp a bit and can also have irregularities on the angle, only grinding will restore the valve.  I use Neway tools to correct the seats on small engines and a Hall Toledo valve seat grinder on large engines and on some small ones that are on the bench. I grind valves on a surface grinder.  Cheap tools are also available from the usual suspects.  In my opinion, lapping is OK only for a show tractor that will not be worked hard because valve contact area will only get worse.

[richmondred01 2,289]

Look at the seat? Any pitting or burnt areas?

if no, take the valves and have them resurfaced.

Lap and install.

Edited August 29, 2019 by richmondred01

[oliver2-44 10,370]

He got a lot of engine minor and major overhaul experience.   

[1990Onan520-H 15]

What is all this talk I hear about not lapping valves on the onan? I've heard only grind.

 

Also I didn't pull the heads yet. Loosened the bolts but haven't pulled heads. Should i sung the bolts back up and do a leak down? I worry that head gaskets could be leaking too. But I guess with even compression maybe not?

[bc.gold 3,403]

Onan engines of old were sold with stellite valve seats installed, a much harder material that that of the valve itself is made from, lapping the valve in place would damage the surface of the seat. Recommended reface the valve face only.

 

Stellite is a very hard material often used as tool steel. also welding rods used for hard surfacing to protect equipment from premature wear from abrasion..

 

Stellite, chromium, cobalt, tungsten and nickel alloy valve seats are commonly used for such high heat applications as are tool-steel valve seats. ... seat that has very consistent and uniform properties, and requires minimal finish machining.

[pfrederi 18,334]

8 hours ago, bcgold said:

Onan engines of old were sold with stellite valve seats installed, a much harder material that that of the valve itself is made from, lapping the valve in place would damage the surface of the seat. Recommended reface the valve face only.

 

Stellite is a very hard material often used as tool steel. also welding rods used for hard surfacing to protect equipment from premature wear from abrasion..

 

Stellite, chromium, cobalt, tungsten and nickel alloy valve seats are commonly used for such high heat applications as are tool-steel valve seats. ... seat that has very consistent and uniform properties, and requires minimal finish machining.

 

 

Interesting info on Stellite!!

 

Tecumseh used Stellite facing on the Exhaust valve of HH series engines (Not the H series)

[1990Onan520-H 15]

42 minutes ago, pfrederi said:

 

 

Interesting info on Stellite!!

 

Tecumseh used Stellite facing on the Exhaust valve of HH series engines (Not the H series)

Well that makes it easier. I'll just buy a valve compressor or rent one and take the valves to a shop.

 

Do the valves have a specific angle they have to be ground at?

Edited August 30, 2019 by 1990Onan520-H

[lynnmor 7,566]

Valves are 44 degrees and seats are 45 degrees.  Seat width is .031" to .047" wide.  You should get the Onan manual available on this site.

manual