Onan Microlite 4000 shaft locks up on occasion

No, not a "Microlite".



That is a picture of my 120 yr old engine cylinder.

I was the first in the family to have needed to crack it open to replace the 120 yr old head gasket which had developed some seapage of the antifreeze into the cylinder.

The black stuff is carbon build up which is normal for all engines, where the carbon stops is where the rings stop at top dead center. The top of the piston stops just slightly below the cylinder deck.

Once I removed the carbon, I was able to verify with a straight edge that the cylinder have almost no ridge.

I stopped short of removing the piston since The cylinder most likely has some egg shaped wear and getting the rings back into the correct wear position is very tricky. Engine does need rings and I debated on the idea of replacing them but that would open a new can-o-worms up which requires boring to remove any egg shape and new custom rings.. No off the shelf rings exist for a 5.5" bore..

ALL engines can develop a "ridge" and any ridge developed will not make the piston stick.

You "issue" most likely came from the RINGS getting stuck in the piston grooves. That can happen from excessive carbon buildup and the engine sitting for extended periods of time with no use..

The carbon over long periods time sitting without running hardens like a rock which can lock an engine up.. That is one of the many issues I faced in freeing up my engine without tearing it down and removing the piston..

At 1800 hrs on a air cooled engine as you worked on most likely needed new rings as they most likely were allowing excess oil and blowby to build up carbon in the piston ring grooves..

I have also worked on various 1970's-1980's v8s and inline 6s with over 100K miles and along with some 1960's small aircooled engines, never once had an issue removing pistons from the cylinders without deridging the cylinders or having them lockup due to ridges.

There is zero "need" to randomly pull the cylinder head and deridge.

In the OPss situation for this engine, it is common to have the STARTER GEAR to hang up just enough to interfere with the ring gear and preventing you from turning the engine over.

Yes sir that’s a iron engine, the Onan microlite is aluminum and not made to last like that engine.

Kpackpackkelley wrote:
If it has a ridge at the top you’ll have to get it reamed if it still runs good and makes electricity.
You can look on YouTube and see ridge on cylinder wall and how they ream them and it may be in your service manual.

I've heard of a ridge forming. I should have said I've never heard of a cylinder sticking on it. If you've seen it, I'll take your word for it. I never say never.

I can fix most things, but once you start talking engine tear down, I'm out, and would likely just get another generator.

Kpackpackkelley wrote:
Granted he didn’t take care of it as it had 1800 hrs on it.

Not sure running 1800 hrs means he didn't take care of it. As I said, I know a couple guys that have had 3000+ hrs and 1 guy that had 5000+ hrs on their Onans (different models).

I take care of mine (mostly) and maintain regularly, although I've had carb problems where the cylinders walls were washed out by gas (gas in the oil). And even though I know I shouldn't run it for only a few minutes a day to run a microwave, that's what I do. So who knows, you may be right.

I appreciate and listen to any suggestion or theory.

Kpackpackkelley wrote:
Yes sir that’s a iron engine, the Onan microlite is aluminum and not made to last like that engine.

You DO realize that ALL "aluminum" engine cylinders ARE iron sleeved, right?

Onan is no exception to sleaving.

HERE is just one sample of an Onan gen engine which does state..

Engine
Engine Brand Cummins Onan
Starting System 12-Volt Button Start
HP 7 HP
Consumer Engine Warranty 2 Years
Engine RPM 3600 RPM
Cast Iron Sleeve Yes"

Basically in a nutshell, there IS a IRON cylinder sleeve that was either installed by pushing a sleeve in or using the iron sleeve as part of the aluminum molding process.

The outside of the cylinder is aluminum and finned for best heat dissipation for air cooling and less weight. Sleeved engines are used exclusively by even the automotive world in any aluminum or aluminum alloy block.

The sleeving is done so well that it is impossible to see any difference between the aluminum and the iron sleeve.

Some engines the sleeve is called a liner and is removable and replaceable, find that in big diesel engines.

The next air cooled engine you tear apart, take a magnet to the cylinder sleeve.. Bet the magnet will stick..

Gdetrailer wrote:

Kpackpackkelley wrote:
Yes sir that’s a iron engine, the Onan microlite is aluminum and not made to last like that engine.

You DO realize that ALL "aluminum" engine cylinders ARE iron sleeved, right?

Onan is no exception to sleaving.

HERE is just one sample of an Onan gen engine which does state..

Engine
Engine Brand Cummins Onan
Starting System 12-Volt Button Start
HP 7 HP
Consumer Engine Warranty 2 Years
Engine RPM 3600 RPM
Cast Iron Sleeve Yes"

Basically in a nutshell, there IS a IRON cylinder sleeve that was either installed by pushing a sleeve in or using the iron sleeve as part of the aluminum molding process.

The outside of the cylinder is aluminum and finned for best heat dissipation for air cooling and less weight. Sleeved engines are used exclusively by even the automotive world in any aluminum or aluminum alloy block.

The sleeving is done so well that it is impossible to see any difference between the aluminum and the iron sleeve.

Some engines the sleeve is called a liner and is removable and replaceable, find that in big diesel engines.

The next air cooled engine you tear apart, take a magnet to the cylinder sleeve.. Bet the magnet will stick..

aka...Biral cylinder/s

In the service manual page 10-9 shows how to ream the ridge That may not be the problem but I’ve seen it happen. http://www.skilledcrafting.com/onanfiles/981-0503%20Onan%20KY%20MicroLite%204000%20Series%20RV%20Genset%20Service%20Manual%20(02-1997).pdf.

There are multiple possibilities from dirt dauber nest to a loose screw being caught in the flywheel or a fastener vibrating loose.
It could be the swear ridge at the top of the cylinder but only with a worn con-rod or crankshaft.
Not all aluminum cylinders are sleeved just the better ones.

Rigdon wrote:
There are multiple possibilities from dirt dauber nest to a loose screw being caught in the flywheel or a fastener vibrating loose.
It could be the swear ridge at the top of the cylinder but only with a worn con-rod or crankshaft.
Not all aluminum cylinders are sleeved just the better ones.

Kback claims the onans are cheaply made but obviously as you pointed out, the better ones are sleeved.. That makes the Onans not so much cheaply made..

Heck, even the bottom rung B&S lawnmower engines are sleeved..

Per HERE

"BRIGGS & STRATTON

Vertical Shaft -Twin - OHV

22 HP - 724CC Intek

MODEL: 44N677-0045

Cast Iron Sleeve, 12 Volt Electric Start, 9 Amp Alternator,

1” Diameter x 3.16” Long Crankshaft,

Dynamically Balanced

Keyed, Drilled and Tapped 7/16-20,

Choke a Matic, Oil Filter, Fuel Pump,"

As far as some aluminum engines being not sleeved goes, perhaps a few of the cheep 2 cycles used in say weed whackers and low grade consumer chainsaws..

My '87 BMW K75 motorcycle has an all aluminum sleeveless block. As far as I know there have been no issues with it. But then there was the Chevy Vega with a sleeveless aluminum block which had major problems mainly because it had a cast iron head. The different metals did not play well together.

I’m not arguing with anybody. Onan had to move to 3600 rpm from 1800 rpm to stay competitive in the market for generators that’s the name of the game. They are not made like the Ws3 or the Cck that were made to last forever.
I’ve seen a spec K microlite piston rings stuck on the ridge, if the Op can’t find any reason it’s getting stuck he needs to pull the head and look at the piston. As is posted here It could be anything.