Onan Head De-Carbing

[mattd860 162]

I want to pull the heads off my 20hp Onan with 630hrs on it to de-carbon them. What is the best method to clean off the carbon?

[TheReDStAnG 4]

Scotch brite, carb cleaner, compressed air, and clean rags

[AMC RULES 36,961]

I'm curious as to how one keeps the carbon dust from the top of the piston from getting down into the ring lands and cylinder?

Seems like to do it correctly, you would have to tear down the whole thing? :popcorn:

[1maidenfan 201]

I done it on my 16 hp Onan when I re-ground the valves. I just brought the piston out as far as it would go and then cleaned it. I did do an oil change after the cleaning, I used carb cleaner (aerosal can) to clean up after I used a wire wheel on my cordless drill. seam to work very well. :twocents-mytwocents:

[sorekiwi 761]

I'm sure someone will disagree with me, but unless the top of the piston is really bad, I dont touch it.

100 years ago when I was a young apprentice I was told by one of the old-timers that if I disturbed the carbon on the top of the piston, it would start to burn oil. While I am sure that this is an old wives tale, I learnt a lot from that grumpy old ba$tard, and I still do a lot of what he said.

You can smear a thin ring of grease around the bore, and then raise the piston over it. It will catch a lot of the crap that would otherwise get around the rings.

[KATO 115]

I agree . the grease trick works well . I' ve used it also.

[AMC RULES 36,961]

...unless the top of the piston is really bad, I dont touch it.

...if I disturbed the carbon on the top of the piston, it would start to burn oil.

Thanks Kiwi.

Any thoughts as to why that might happen?

Has anyone had this happen? :popcorn:

[mattd860 162]

Ok I took everyone's advice and cleaned up the heads. The carb cleaner and Scotchbrite pads worked great. I also put the grease around the cylinder and raise the pistons all the way and cleaned those too.

AND THEN...

I went to adjust the valves and saw the intake valve seat had come loose! :banghead:

[AMC RULES 36,961]

Then what did you do Matt?

[mattd860 162]

Then what did you do Matt?

Not me, I bought this tractor last year with 620hrs on it and haven't even put a full hour on it since then.

[RussC 1]

Do you just stake the seats back in like on a Briggs? That's what I probably would have done.

[boovuc 1,090]

Please don't leave us hanging. What is the fix for this from the Onan gurus! :bow-blue:

[mattd860 162]

The correct fix is to have a shop drill out a larger hole and press in an oversized seat. You can get oversized seats here - http://onanparts.com/index.php?main_page=page&id=3

[boovuc 1,090]

My 520 has had the valve seat repaired twice. (Something I didn't find out until I read the paperwork supplied by the PM after I bought it). The Toro dealer where the work was done said that "it cannot be repaired again".

This is why I have another P220 on my bench ready to go in the 520 when the current over sized seat fails.

It can also go into the 416 if need be.

i have a gasket set ordered and will be taking the heads off this motor for some decarbing later this summer.

I did read where "someone" wrote that, don't quote this, I like to buy up people's Onan's that have quit. Something about easily putting the intake seats back in them and they're as good as new. Unless this person is a machinist or has the bucks to take them to a machine shop, how can a "layman" with standard garage mechanics tools replace a "let go" valve seat in an Onan if at all?

[mattd860 162]

They can always be repaired but some required the bore to be welded up and then re-drilled entirely. A good shop will just make their own custom seat depending on the extent of damage. If someone had good welding skills and basic machine shop equipment, they can make a killing fixing these.

[boovuc 1,090]

Roger that!

[sorekiwi 761]

A lot of what has been stated in this thread is basically true, but I think there are a couple of things that should be clarified...

The basic problem with the valve seats coming loose is that the cylinder block is made from some sort of aluminum alloy, and the valve seat is made from some type of ferrous material. Steel and aluminum have different rates of expansion due to heat, so in order for the valve seat to remain tight in the block when it is warm, the designers call for the correct amount of interferance fit of the 2 parts at the time of manufacture.

The auto industry has been doing it for close to 100 years now on cars with aluminum cylinder heads, air colled engines are a little more tricky since the operating temps on an air-cooled vary quite a bit more than your typical water cooled engine. But motorcycles have been doing it forever as well.

Machining for an oversize seat isnt quite as easy as "drilling it out oversize and pressing in a new seat". The tolerances are critical and around a couple of tenths (of a thousanth of an inch). The guy that works out exactly what that fit should be will do a successful job, the guy that gets it wrong will have the valve seat come loose again

[mattd860 162]

You're exactly right. I was trying dumb down my answer to make it as simple to understand as possible. It should be noted that the seat that came loose on my motor was a .010 oversized seat that was repaired by the PO of my tractor. Apparently the PO and/or and shop that did the work didn't understand the tolerances between the two causing it to come loose again.

[RussC 1]

I realize that for some, stating repair techniques for a Briggs in a discussion about an Onan or Kohler may seem to be apples to oranges and not germane, but in reality air cooled aluminum block engines are air cooled aluminum block engines and the ratios of the coefficients of expansion between the aluminum block and valve seat material, and the acceptable interferance fit tolerances will be close to universal.

That being said, the material of the engine block which formerly held the valve seat fast in all likelyhood has not become eroded or consumed, but rather, displaced by repated percussion of the valve seating at high speed and a repair method which knurls or stakes the valve seat back in place can be a very effective repair. Of course it doesn't sound as professional if you are telling someone how you effected a repair in this manner versus saying "I had it machined", but what is more important for the typical homeowner, having your machine running, or sounding like Cliff Claven?

[varosd 1,185]

Great info for all us Onan owners!!

[AMC RULES 36,961]

...but what is more important for the typical homeowner, having your machine running, or sounding like Cliff Claven?

How 'bout the best of both worlds...running, over Cliff Claven. :)

[mattd860 162]

Just wanted to report that I successfully swapped the pistons from the motor with the bad seat to the motor with the good seat and now I'm up and running - better than ever. The motor runs very smooth and doesn't burn any oil (visibly anyways). I got to mow with the new 48" deck and powerflow bagger and really enjoyed it.

[zeedubbya 18]

I'm going to do a decarbon and valve adjustment on my 520h this evening. I've got everything ready to go. At the risk of sounding like a newbie ( which is really what I am!) what is the best method for getting at the rear head? Do I have to put the motor on a bench or can I just unbolt it and turn it in the tractor. I've done the decarbon on a case 448, but pulling the heads on it was easy because of the orientation of the motor? Thanks for the input everyone this is great info!

[AMC RULES 36,961]

:wwp: :wwp:

[Kelly 1,028]

The engine come out very easy, about a 20 min. job, I would leave it in the chassis and just turn it, to save my back they are not light.