ONAN 20hp horizontal shaft opposed piston engine diagrams

You mods may want to add another section for Onan or "OTHER"

Here's some diagrams I came across on the onan engine I'm rebuilding for Red Fox. As soon as I find the link again, I'll add it. The page has everything you need to know to rebuild one of the engines, including part numbers











hope this helps someone other than me

I could have gotten a handful of Onan engines a while back... 2 16hp and 2 18hp if I remember.
Maybe I should have gotten them...    lol

The only thing I don't like about Onan is the parts pricing. IIRC, they are currently affiliated with Cummins. Bring your wallet. Lawren, thanks for posting. I have altered the categories to include Onan, and moved this in.

Doc Sprocket wrote:

The only thing I don't like about Onan is the parts pricing. IIRC, they are currently affiliated with Cummins. Bring your wallet. Lawren, thanks for posting. I have altered the categories to include Onan, and moved this in.

Thats partly why I didnt get them. Still, they would have been cool...

A good rule to follow is simply "NEVER turn down a free, or cheap motor"

Most old engines I've come across are victims of neglect... A good cleaning, maybe a gasket, and the thing runs like a champ.

I chose this motor for my build because:

1) since it came from an RV backup generator (60's Winnebago) it only had 78 hours on the hour meter.

2) since it was designed to run a 4000 watt generator head at full load, it has TONS of torque... While briggs has oppy's of the same hp that are much lighter, this one has automobile sized main journals and is all cast iron. If I want to mod it, it wont be difficult to swap in automotive parts (pistons, rods... I may even at some point consider machining the cyl block to accept WV jugs, pistons, and porche heads   

3) SINCE no one is snapping these motors up for mowers, I am finding them stacked everywhere, so junkyard parts availability is not an issue

These motors are great for high torque load applications where the extra weight of the motor is a help, not a hinderance. That's the upside... the downsides are

1) new stock or performance parts are STOOPID expensive, and the latter severely limited... standard hot rodding techniques will help a lot (port and polish, port match, etc) but you can't easily find race cams, etc.

2) the weight.... this thing weighs as much as a small 4 cyl vdub longblock. I wouldn't use this in a rally, or any other bounce around, pour the throttle on then hit the brakes application. For top end speed (NOT rpms, MPH) I'd run this motor with high gears, and let the weight add stability and torque power the high gears, Drag racing, I'd pick this motor. Pulling tractor, I'd pick this motor. Race mower? no way...Off road macine? I'd rather have a 12 hp briggs than this beast!