Onan 20 Valve seat problem

[Geno 1,928]

I'm sure you guys have done this a few times.  If you want to send me to another thread I can do that or if you want to do it here that would be great too.      I know repeating the same testing or fix all the time can get to be a pain. 

 

I've worked on the outsides of these but never had to go inside one.  Pretty low hours on this one so I'd like to fix it.  Here's my problem.  If you need more pics or info just let me know and 

 

 

 

[Ed Kennell 35,814]

I never fixed one either Geno , but have read some of the fixes from peening to tighten the existing seat to overboring for oversize seats.   Just curious, was it the back cyl.?

And what was the condition of the cooling fins?

[Geno 1,928]

It was the rear cylinder.  It's clean but someone else already had the cover and head off. 

[woodchuckfarmer 333]

I had both intake seats come out of my onan .Took it to an engine shop and had them replaced the seats . It only cost about $130 bucks and he guaranteed they would never come out.

[decksetter 50]

I peened one back in to place like that, it held fine. Mowed an acre and a half for a season with it and no further issues.

I have had multiple Onans that the intake valves would run themselves so far out of adjustment the valve would no longer fully close, the lifter would still be holding it open.

Adjust valve so you have definite clearance and spring is holding the valve shut, peen the aluminum around the seat with a center punch and hammer at 12 o'clock then 6, then 3 and 9. Do this lightly, remember it's aluminum. V if you haul off and whack it you'll have a huge dimple in the block and push the seat out.

[km3h 543]

Gino

Isn't that what I told you to do last night? Boy you gotta listen to your elders. Ya here me!!!

[Geno 1,928]

I called the machine shop today.  They don't do a lot of Onans but do a lot of Kohler work.  They said they would look at it and they usually machine the block a little larger and put a larger seat in for around 50.00.  Gonna run it down to them in the next day or two. 

[KyBlue 652]

There's a shop by me that you might want to call Geno ... if you want the number, let me know

[Geno 1,928]

I might do that, I usually use Smith in Frankfort, he's on par with Coronet in Somerset, you aughta see his place.  If he can't tackle it I'll give you a call. 

[KyBlue 652]

Ive used Smith before too. Very nice shop!

[km3h 543]

Now you are on the right track. That really is the best way to fix it. Won't have to worry about that sucker in the future.

[MalMac 1,328]

Ok we all know about the Onan Valve Seat problem and a couple of different ways to fix them. What I would like to know is having the block cut out a tad larger and a new seat pressed in is gonna fix the problem. That's the way it was originally designed and do to the extreme heat and cooling it comes loose. What's going to prevent this from happening again? When the new one is pressed in, is some high temp adhesive going in there with it? I know things like trying to make it run cooler and cooling the engine down after running it hard helps but your still going to have the expansion and contraction of the aluminum VS. the steel valve seat. There are a lot of other engines out there that utilize a aluminum block and steel valve seats that have no problems. I am just trying to understand what's going to keep this from happening again. Maybe I am the one that only see's this but fixing it the same way it was designed in the first place (which is obviously a design flaw) is gonna be a long term fix. So you Onan experts out there try and make me understand. Because I have a Onan running great now but need to be prepared for what will come. Help me see the light.

[Geno 1,928]

From what I see the engine design is fine, it's the cooling design that isn't on this particular tractor design.  With the belt guard in the way and restricted air flow in that area it seems to be more prone to both clogging and general heat dissipation problems.  I would think the clogging may be more the problem as many people run these for well over 1K hours with no problems and this one is dead at 500.  From what I see if the air flow is restricted it can cause the the debris build up and if not kept clean then this sort of thing happens.

 

I think maybe making louvers the belt guard may help a lot, sort of like the hood. 

 

The bad news is I called 4stars machine shop and they can't do it.  I just got back from another who looked at it and the engine has to be completely disassembled to get it chucked up to fix it and I hate to do this with this low an hour motor. 

 

At this point I am looking for someone here to tell me if I can do a good fix myself.  I'll get some better pics of the damage on here in a few minutes.

[Geno 1,928]

Here are some pics. 

 

 

[km3h 543]

I would contact Boomer. If anyone can tell you how to fix it he can. 

[Hodge71 663]

I know a few small engine mechanics who say its not Just Wheel Horses...Its Onan.....They had Issues in those green tractors too with Onans

[Geno 1,928]

I do see one problem with the design, The oil filter blocks airflow to the entire bottom half of the cylinder bore, not good.  I wonder if the other part of the problem is also the belt guards on other tractors?

Edited September 24, 2014 by Geno

[Hodge71 663]

I do see one problem with the design, The oil filter blocks airflow to the entire bottom half of the cylinder bore, not good.  I wonder if the other part of the problem is also the belt guards on other tractors?

 

From my understanding the Onan engines sat opposed in JD tractors but they were heavily guarded with screening so the front of the tractor looked more complete...I think therein lies the problem....low or no airflow again. Onan generators seemed to do better. We had them on all the Verizon bucket trucks and many huts used them for back up power. Although the constant use wasn't like that of a tractor that gets used very week for extended periods...

 

And thats all Im' saying about that as I'm getting bumps and sweating from all this green tractor talk......and also the last thing I need is the Onan in Dads new 416 to poop the bed....

Edited September 24, 2014 by Hodges Dodges

[km3h 543]

I had a Motor Home with an 7 KW Onan generator and it had 1500 hours on it when I sold it. It sat in an enclosed compartment. Of course that generator was designed to run at 1800 RPM. Gino, Harbor Freight has a 23 HP engine for $699.00. You could be the first to try one out. That would make you a pioneer.  

[Ed Kennell 35,814]

Nick, are the HF engines all capable of taking the axial thrust from the PTOs ?  Thinking about the $99  6.5HP.

[km3h 543]

All I can tell you is that on another forum, the talk about using these engines as replacements for snow blower, tillers and tractors. The brand is Predator and is said to be a clone of Honda. I don't know this for a fact, but it is said that the parts are interchangeable in many cases. At least one guy on that forum who is in the repair business installs them for customers when they need a new engine.

[Ed Kennell 35,814]

Ok we all know about the Onan Valve Seat problem and a couple of different ways to fix them. What I would like to know is having the block cut out a tad larger and a new seat pressed in is gonna fix the problem. That's the way it was originally designed and do to the extreme heat and cooling it comes loose. What's going to prevent this from happening again? When the new one is pressed in, is some high temp adhesive going in there with it? I know things like trying to make it run cooler and cooling the engine down after running it hard helps but your still going to have the expansion and contraction of the aluminum VS. the steel valve seat. There are a lot of other engines out there that utilize a aluminum block and steel valve seats that have no problems. I am just trying to understand what's going to keep this from happening again. Maybe I am the one that only see's this but fixing it the same way it was designed in the first place (which is obviously a design flaw) is gonna be a long term fix. So you Onan experts out there try and make me understand. Because I have a Onan running great now but need to be prepared for what will come. Help me see the light.

The thermal expansion rates of aluminum and steel are fixed.  0.0000123  and 0.0000073 in/in/deg F resp.

  So it is easy to design the correct interference fit to keep the seat tight at a given max operating temp. 

    So the problem is clear,  the rear cyl temp gets above the design temp. and the seat gets loose.  

So the solution must be to keep the  engine (front and rear cyl) within the design operating temp.

Vent the belt guard, make sure the oil filter/tin seal is installed, and clean the cooling fins.    

 

                                                               

BTW, I doubt if a steel seat could be cold pressed into a aluminum bore successfully.  The bore would need to be machined with the correct interference with the seat.   Then a temp differential created so the seat could  be dropped in the bore.

Edited September 25, 2014 by ekennell

[MalMac 1,328]

The Onan is a good motor but it never should have made it out of it's intended environment and put into a Garden Tractor, other brands of tractors have had the same failure. Any air cooled engine is susceptible to proper cooling. The Onan is just extremely sensitive. I don't know how many times I have seen people mowing with a 520 or the likes with a Onan and see the grass screen totally plugged up. Most people who bought Wheel Horse's with Onan's in them are not tractor Guru's and all they have on their minds is getting the yard cut. If they do clean the machine off I am sure it was not on their mind that grass might be clogging up the fins or other areas. Like I said the Onan is a good motor and strong, it just should have never made it into a garden tractor application. It should have stayed where it belongs in a nice and clean RV environment. It's really ashame that Toro chose to ignore this problem. It does not take a rocket scientist to see a Onan will suck up enough grass in the first five minutes of mowing to cover the grass screen compared to other motors. They all get some grass sucked up on there but the Onan is one of the worst of the lot. By putting in even more air flow restrictions along with grass build up you have the makings of what we are all finding out, premature failure. It's a shame that something was not done engineering wise to help overcome this. It's a good motor just out of its environment as far as I can see. Now if Geno can fix this Onan, I am sure he will be very alert to what causes this and will take appropriate measures to keep it from happening again and get a long life out of it.

[Ed Kennell 35,814]

  Gotta keep em clean and cool.    That's one reason I dedicated mine to snow blowing duty.

[Geno 1,928]

That would do it.