520-8 rebuild ?

[857lover 291]

so i bought a 520-8 with 600 hours and i wanted to take the heads off and decarbon the valves and put new head gaskets  but when i took off the front most head i saw a nice size puddle of oil in the cylinder and im wondering if i should really take it down and try and rebuild the whole thing new piston valves and such or just try and re ring it since it only has 600 hours

[lynnmor 7,323]

Run the piston all the way down and measure the bore.  The measurements can be found in the Onan Service Manual on page 3-1, they are a bit confusing, just go by the amount of taper specification.  600 hours isn't much but abuse is the major cause of failure. Abuse is not changing oil, oil and air filters, running with fins plugged and a missing oil filter gasket that should seal the opening around it. New rings and a hone might take care of it but check for play in the valve guides.

[857lover 291]

4 hours ago, lynnmor said:

Run the piston all the way down and measure the bore.  The measurements can be found in the Onan Service Manual on page 3-1, they are a bit confusing, just go by the amount of taper specification.  600 hours isn't much but abuse is the major cause of failure. Abuse is not changing oil, oil and air filters, running with fins plugged and a missing oil filter gasket that should seal the opening around it. New rings and a hone might take care of it but check for play in the valve guides.

seems like it’s in spec maybe i have stuck oil control ring ? 

[lynnmor 7,323]

6 minutes ago, 857lover said:

seems like it’s in spec maybe i have stuck oil control ring ? 

It is possible but I never saw one stuck.  It isn’t too hard to pop the pistons out.  If you put an old ring in the bore the gap can be measured with a feeler gauge, and that will give you an idea about abuse.

[857lover 291]

10 minutes ago, lynnmor said:

Ithat will give you an idea about abuse

guy i bought it from was the second owner he bought it from the original owner who passed away and the tractor sat for a while until the second owner purchased it so i don’t really think it’s been ran hard and the cylinder wall dosent have any crazy scores or anything i’ve also never had a onan apart i’ve done small k181s and stuff so i really don’t know how difficult it is to work on theese  also just doing quick looking but is the rings and piston and valves really the same on a p216,218,p220?

Edited June 29 by 857lover

[lynnmor 7,323]

1 minute ago, 857lover said:

guy i bought it from was the second owner he bought it from the original owner who passed away and the tractor sat for a while until the second owner purchased it so i don’t really think it’s been ran hard and the cylinder wall dosent have any crazy scores or anything i’ve also never had a onan apart i’ve done small k181s and stuff so i really don’t know how difficult it is to work on theese 

Pretty standard stuff, that service manual will be a big help.  Some aftermarket parts and gaskets are poor quality, so buy genuine Onan for the more important things.  I keep mentioning abuse because an engine that has been dusted will need much work and one that was kept clean may need very few parts.  You will need a torque wrench capable of reading the values in the service manual.  There are a few quirks and when you run into a problem just post here and we will help.

[kpinnc 12,199]

6 hours ago, 857lover said:

so i bought a 520-8 with 600 hours and i wanted to take the heads off and decarbon the valves and put new head gaskets  but when i took off the front most head i saw a nice size puddle of oil in the cylinder and im wondering if i should really take it down and try and rebuild the whole thing new piston valves and such or just try and re ring it since it only has 600 hours

 

Just my unqualified opinion:

 

I would button it back up and run it a while to see what it's doing first. Do a compression test on both cylinders. 

 

I'm no mechanic, but with 600 hrs combined with that kind of blowby, your piston should be a mess, and it isn't.

 

An Onan that sits a while can do some weird stuff. It might be perfectly fine as is. 

[857lover 291]

4 minutes ago, kpinnc said:

 

I'm no mechanic, but with 600 hrs combined with that kind of blowby, your piston should be a mess, and it isn't

yeah i have been using seafoam in the gas for like a tank or two so that might have helped in cleaning the piston but i did plan on waiting for it to cool down to get a compression test going on it

 

[Handy Don 12,310]

My first thought, given no indication of damage, was that the oil was overfilled and just seeped around--possibly aided by coincidental ring gap alignment?

[857lover 291]

1 hour ago, Handy Don said:

My first thought, given no indication of damage, was that the oil was overfilled and just seeped around--possibly aided by coincidental ring gap alignment?

definitely not overfilled it was doing it when i bought it then i changed the oil so i knew what was in it was good quality oil and it’s actually low now 

[c-series don 8,743]

I’m with @kpinnc , I’d put it back together and run it. You could be tearing it down unnecessarily. If it smokes excessively or you hear strange noises then you can address it. 

[857lover 291]

1 minute ago, c-series don said:

I’m with @kpinnc , I’d put it back together and run it. You could be tearing it down unnecessarily. If it smokes excessively or you hear strange noises then you can address it. 

will do i will probably run some seafoam with the oil and change it again but i will run it hard and see if anything changed