Tasks & Costs Associated with Rebuild?

[chesbaycruiser 83]

I am inquiring about an Onan 20 HP that is losing power and showing low compression in the rear cylinder. In reading other posts, I see that a common cause of this is the valve seat, although I realize that might not be the only thing to cause this problem.

Acknowledging that I don't know a lot about small engine repair, can someone who does give me a brief rundown on the major tasks involved in this repair? I don't want directions how to do it, just a "big picture" of what has to happen, major parts needed and likely (ballpark) costs for any work or tasks that would likely require a machine shop.

Same with the pistons/rings, as I'm guessing that could be a source for the problem too? I'm sure I could tear down to them, but don't have a clue about what's involved with having the cylinders machined, etc.

Finally, any thoughts on other potential causes for this problem would be appreciated.

Again...NOT looking for detailed directions or price "quotes", just looking for general info. As always, thanks for putting up with my intellectual short-comings!

Chris

(Boy, this guy doesn't ask for much, does he!? )

[Shuboxlover 479]

Interesting topic, I'll be watching this one closely....

[rickv1957 73]

A simple call to a machine shop can anwser your questions, Rick

[chesbaycruiser 83]

A simple call to a machine shop can anwser your questions, Rick

I do plan on going to my local small engine repair shop tomorrow to ask some questions, but I guess my point is I'm not sure exactly what questions to ask.

Trying to get a little better "educated" first....

[DoctorHfuhruhurr 137]

A simple call to a machine shop can anwser your questions, Rick

I do plan on going to my local small engine repair shop tomorrow to ask some questions, but I guess my point is I'm not sure exactly what questions to ask.

Trying to get a little better "educated" first....

I found this which may be of some use .........

http://www.mytractorforum.com/showthread.php?t=91204

[refracman 167]

Onan rebuild parts and machineing costs will run between 800-1200 generally speaking.

[Save Old Iron 1,568]

Chris,

you might want to Google "leak down test". You will get some visual clues as to what bad valves and valve seats look like. Its not rocket science - just a exacting fit of an angled valve face mating perfectly with an angled metal ring call a valve seat. If the ring loosens in the block or the face of the valve or seat becomes pitted or burned away, you lose the seal and lose compression.

In a leak down test, you use an air compressor to pressurize the suspected cylinder with compressed air.

The source of your low compression will be identified by where you hear the air leaking from out of your engine block.

Pull the oil dipstick - hear air hissing ? that's air leaking past your piston ring to cylinder wall interface - could be worn rings, could be scored cylinder walls.

Listen to the muffler outlet - hear air hissing - that's a sign the valves (exhaust or intake) are leaking and are your source of lost compression.

Hear air around the cylinder head to cylinder head junction ? maybe a blown head gasket or loose head bolts.

Onan engines can suffer overheating at the rear cylinder if the cylinder fins are never cleaned of debris picked up during normal mowing. Remove the rear cylinder head tin and check for clogged fins. If its packed full of grass or a mouse nest, the cylinder probably overheated and suffered a loose valve seat.

Also check to make sure the large rubber gasket surrounding the oil filter is present. If not, the air intended to go around the cylinder cooling fins can "escape" around the oil filter before proceeding up to the cylinder head.

Google is your best friend on this one.

[Confused99 2]

Take a look at the link

http://onanparts.com/index.php?main_page=p...products_id=124

Jason

[sorekiwi 761]

I had an interesting problem diagnosing the engine problem with my 520 when I got it.

I bought it running on on one cylinder, and I (and the previous owner) both just assumed it had a loose valve seat.

With the inlet manifold removed you can see a lot of the the valve seat, and it actually looked OK on mine. So I did a leakdown test, and found less than 5% leakage on the bad cylinder and 10% on the good cylinder. I could hear next to no air leaking into the crankcase, or in either of the ports. Huh?

Finally I decided to do an old fashioed compression test and found no compression on the bad cylinder. But the leakdown test said it was OK. Huh?

The problem turned out to be a broken rod. The piston was sitting in the bottom of the bore, still sealing it up pretty good. But you dont get compression unless it goes up and down!

[chesbaycruiser 83]

Thanks guys! Excellent info...exactly what I was looking for!

:omg:

[wallfish 17,610]

How to make a leak down tester

http://www.wheelhorseforum.com/index.php?showtopic=9161

[chesbaycruiser 83]

Great article, Wallfish!

:omg: