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Author Topic: Onan Mill Motor Maintenance  (Read 2364 times)

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Offline Andries

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Re: Onan Mill Motor Maintenance
« Reply #20 on: July 26, 2014, 05:55:04 PM »
Beenthere - last winter was the coldest winter on record for over 100 years. A perfect time for me to be in my son's nice warm shop, figuring this stuff out.
WM sent me a gasket kit for the Onan and I set to replacing both cylinder head gaskets.
 
Gotta admit that I'm flummoxed by the fact that I can order up this material from WM and Cummins and get supply on short order.
Try that with a twenty year old Detroit product.  :-X
LT40G25
Ford 545D loader
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Offline Andries

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Re: Onan Mill Motor Maintenance
« Reply #21 on: July 26, 2014, 06:01:14 PM »
The valves looked like the photos of 'normal' valves on the web,
 
and the old head gaskets came off easy using a pocket knife.
 
Replaced both of 'em and then it was time to put the heads back on.
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Offline Andries

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Re: Onan Mill Motor Maintenance
« Reply #22 on: July 26, 2014, 06:10:46 PM »
After fitting the heads back on the cylinders with new gaskets, it was time to tighten down the head bolts. That's when I had to head into town to get a torque wrench that could be adjusted for the light settings needed.
 
Turns out the block and heads are aluminum and its really easy to make a knuckle-headed mistake by putting too much muscle on the wrench.
 
Also paid off big time to keep the decals on the inside of the cowlings.
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Offline Andries

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Re: Onan Mill Motor Maintenance
« Reply #23 on: July 26, 2014, 06:16:49 PM »
Well, it all turned out pretty good. The compression readings were really close to what the manual called for. After replacing the cowlings, changing oil and filters (air and oil), doing an alignment, it was spring and time to get back to milling.
 
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Offline thecfarm

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Re: Onan Mill Motor Maintenance
« Reply #24 on: July 26, 2014, 09:40:27 PM »
A happy ending.
Model 6020-20hp Manual Thomas bandsaw,TC40A 4wd 40 hp New Holland tractor, 450 Norse Winch, Heatmor 400 OWB,YCC 1978-79

Offline Jim_Rogers

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Re: Onan Mill Motor Maintenance
« Reply #25 on: July 26, 2014, 10:12:34 PM »
I hope while you were doing all of that, you adjusted the valves?

Jim Rogers
Whatever you do, have fun doing it!
Woodmizer 1994 LT30HDG24 with 6' Bed Extension

Offline Andries

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Re: Onan Mill Motor Maintenance
« Reply #26 on: July 26, 2014, 10:21:16 PM »
Y'know, I didn't, but I should've.
The gasket kit I got from WM included gaskets for the valve cover plates, located just under the air intake manifold.
I'll do the valves and gaskets together was my thinking, IF the compression issue was fixed.
So giddy to have dodged the "engine replace" bullet, and milling with a fully powered machine made me put it aside for a while.
Thanks for the reminder, it'll be good to do both valves and gaskets in one shot.
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Offline biggkidd

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Re: Onan Mill Motor Maintenance
« Reply #27 on: July 27, 2014, 09:03:20 PM »
  I've never seen an L head engine with adjustable valves and I work on small engines for a living. The newer overhead valve engines do mostly have adjustable valve clearances.

 Larry
Echo 330 T, Echo 510, Dolmar 7900, Jinma 354 W/ FEL, & TPH Backhoe
 Building a Band Mill  :)

Offline Delawhere Jack

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Re: Onan Mill Motor Maintenance
« Reply #28 on: July 27, 2014, 09:38:26 PM »
The Onan has valve lash adjustment in the tappets (lifters). Fairly easy job, and fairly critical to keep adjusted.

Andries, thanks for the photos. I've got the same engine on my LT40 and it needs new valve guide seals. I was thinking that it may require removing the engine from the mill, but looking at your pics I think probably not.

Should be able to pull the heads, remove the valves and mic the valve stems with the engine installed. Got good compression on mine, but it smokes for a few seconds after starting due to oil leak down from the valves.

Looks like I've got the next item on my to do soon list.

Offline Delawhere Jack

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Re: Onan Mill Motor Maintenance
« Reply #29 on: July 27, 2014, 09:45:41 PM »
Getting gas in the oil can have an effect like using a "Super Detergent" oil, and loosen up all the gunk that has built up in the engine. Might not be a bad idea to do an oil and filter change ahead of schedule now that you've corrected the misfire.


Offline biggkidd

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Re: Onan Mill Motor Maintenance
« Reply #30 on: July 27, 2014, 10:02:43 PM »
Goes to show that working on small engines day in and day out doesn't teach you everything. In all the years I've done it I've only worked on two Onan's that I can remember and both of those were carb problems. There just aren't many Onan's around here. Thanks for setting me strait! 

  Larry
Echo 330 T, Echo 510, Dolmar 7900, Jinma 354 W/ FEL, & TPH Backhoe
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Offline Chuck White

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Re: Onan Mill Motor Maintenance
« Reply #31 on: July 28, 2014, 07:06:58 AM »
The Onan has valve lash adjustment in the tappets (lifters). Fairly easy job, and fairly critical to keep adjusted.

Andries, thanks for the photos. I've got the same engine on my LT40 and it needs new valve guide seals. I was thinking that it may require removing the engine from the mill, but looking at your pics I think probably not.

Should be able to pull the heads, remove the valves and mic the valve stems with the engine installed. Got good compression on mine, but it smokes for a few seconds after starting due to oil leak down from the valves.

Looks like I've got the next item on my to do soon list.


Nope, no need to pull the engine.
~Chuck~
Retired USAF 1989, Retired School Bus Driver 2012, now semi-retired Mobile Sawyer, 2018 Silverado 4X4
1995 Wood-Mizer LT40HDG25 Kohler - Cooks Cat Claw Sharpener and single-tooth setter, 4-foot Logrite cant hook.
Basic mechanical skills are all that's required to maintain a Wood-Mizer

Offline bandmiller2

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Re: Onan Mill Motor Maintenance
« Reply #32 on: July 28, 2014, 07:33:34 AM »
Valve lash adjustment is critical especially on air cooled engines due to their wide temp. swings. Not enough clearance will tend to not let the valve seat properly and can/will lead to burning the valve. A dead give away is lack of compression when you pull over the engine or if the starter spins it faster than usual. A friend has a flywheel type splitter with an overhead valve Honda, wouldn't start, pulled it over very low comp. Valve lash was almost non existant adjusted and first pull. Likewise diesel injector timing is critical. Frank C.
A man armed with common sense is packing a big piece

Offline StimW

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Re: Onan Mill Motor Maintenance
« Reply #33 on: July 28, 2014, 10:17:51 AM »
Valve lash adjustment is critical especially on air cooled engines due to their wide temp. swings. Not enough clearance will tend to not let the valve seat properly and can/will lead to burning the valve. A dead give away is lack of compression when you pull over the engine or if the starter spins it faster than usual. A friend has a flywheel type splitter with an overhead valve Honda, wouldn't start, pulled it over very low comp. Valve lash was almost non existant adjusted and first pull. Likewise diesel injector timing is critical. Frank C.

You have to be careful, Honda uses compression release!
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Offline Andries

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Re: Onan Mill Motor Maintenance
« Reply #34 on: July 28, 2014, 11:51:24 AM »
Thanks for the reminders re: the importance of valve adjustment. Jim Rogers and Delaware Jack both seem to have handle on this, but I don't have their experience, yet.  :D
My files on the Onan included the attached from Otto Farfan at WM in Indy, which was also probably a big fat hint to get'er done buddy!
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Offline Jim_Rogers

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Re: Onan Mill Motor Maintenance
« Reply #35 on: July 28, 2014, 08:13:25 PM »
This is what happened to my valves from not adjusting them as they needed to be:



Jim Rogers
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Woodmizer 1994 LT30HDG24 with 6' Bed Extension

Offline Delawhere Jack

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Re: Onan Mill Motor Maintenance
« Reply #36 on: July 28, 2014, 09:19:10 PM »
Goes to show that working on small engines day in and day out doesn't teach you everything. In all the years I've done it I've only worked on two Onan's that I can remember and both of those were carb problems. There just aren't many Onan's around here. Thanks for setting me strait! 

  Larry

We all learn something new everyday.  ;)  As small engines go, the 24HP Onan is on the larger end of that scale. I learned about the valve adjustment issue with this engine right here on the FF. A lot of the mid 90's WM mills used that engine. Probably as much info here on the FF about it as on the rest of the Internet combined.

It's a pretty reliable design so long as you keep up with maintenance, very similar to the old VW Beetle engine.

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Re: Onan Mill Motor Maintenance
« Reply #37 on: July 28, 2014, 09:25:30 PM »
While we're on the subject, has anyone had to replace the valve guides?


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