Onan Flywheel

J

jim curran

New member
Jun 12, 2009
54
0
0
Due to an accidental short circuit, my engine-side stator windings are cooked and I'd like to repair them. However, one of the flywheel removal
holes is cracked (see photo). I'd appreciate any advice on how to remove. Can I drill that broken hole further or all the way through and thread
it?

https://photos.app.goo.gl/nDQw7aRjX4AQPY4Q6

I know of the modification that uses a transformer and the house-side of the generator to operate the control board. But I'd rather not create a hack
if I don't have to. And I think if I get the flywheel off then I can fix an oil leak....yes? I am thinking that the fellow who rebuilt the
generator encountered the cracking and didn't bother with the flywheel.

Thanks,
Jim

--
Jim Curran
1976 Palm Beach
Alexandria, OH
 
If it were me, I would drill and tap the other two holes, and use them for
the puller. Probably not the best idea to try the broken side. There are
enough of those Onan engines out there not being used, that finding a
replacement flywheel should not be too difficult if you screw up yours.
Don't know how far you are from the Sirium's, but they might be able to fix
you up. Jim Miller also, on the West coast, jim K can help. In the
Northwest, Noble Warrant has some stuff as well.
Jim Hupy

> Due to an accidental short circuit, my engine-side stator windings are
> cooked and I'd like to repair them. However, one of the flywheel removal
> holes is cracked (see photo). I'd appreciate any advice on how to
> remove. Can I drill that broken hole further or all the way through and
> thread
> it?
>
> https://photos.app.goo.gl/nDQw7aRjX4AQPY4Q6
>
> I know of the modification that uses a transformer and the house-side of
> the generator to operate the control board. But I'd rather not create a
> hack
> if I don't have to. And I think if I get the flywheel off then I can fix
> an oil leak....yes? I am thinking that the fellow who rebuilt the
> generator encountered the cracking and didn't bother with the flywheel.
>
>
> Thanks,
> Jim
>
> --
> Jim Curran
> 1976 Palm Beach
> Alexandria, OH
>
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
>
 
I have had a few come off without a puller. You might try some light tapping with a hammer and some light heat with a propane torch.
--
Ken Burton - N9KB
76 Palm Beach
Hebron, Indiana
 
> However, one of the flywheel removal holes is cracked (see photo). I'd appreciate any advice on how to remove. Can I drill that broken hole further or all the way through and thread it?


Instead of drilling through, why not drill and tap the other two bosses that are 90 degrees opposed from the factory threaded ones?

Note that you don’t need tremendous amounts of pulling force to separate a tapered assembly - just put the puller in place, crank down on the jack screw as much as you feel comfortable and then smack the end of the jack screw with a metallic hammer. The instantaneous transfer of energy from the hammer blow will typically cause the taper to disassemble itself.

—Jim

Jim Miller
1977 Eleganza
1977 Royale
Hamilton, OH
 
Hi Folks,
Thank you all very much for the valuable advice! I was finally able to get back on this and the flywheel has been successfully removed.

I actually tried everyone's advice -

First I tried to work it off by hand. I heated the casting several times and tried to get penetrating oil into the gap. But all I got for my
trouble was some major-league blisters.

Next I had a look at the damaged hole. After looking very closely, I saw that there were still cracks around the hole and I wanted nothing to do
with that. Now that the wheel is off, I see that there is a lot of material available to extend the holes.

So I added threaded holes to the unused bosses. I used a high-speed drill to make a pilot hole and a few successively larger ones. The last bit was
17/64". The holes were about 1 1/8" deep. I tapped 5/16-18 threads and cleaned them out. The drill bits cut easily and the tap even easier. I used
a little bit of oil.

My puller wasn't in the best condition so I had to monkey with it quite a bit. Once I had the flywheel under tension I smacked the outer edge of the
wheel in an alternating pattern with a hammer via a short piece of 2x6 lumber. The puller released and the wheel was free! I pushed against the bare
axle (not the nut) and kept a foam cushion under the flywheel in case it got away. Keeping the wheel from turning was nerve-wracking because I was
trying very hard not to damage the vanes. I think I wedged some wood between the downpipe and the wheel to keep it still.

I was able to remove the stator by striking each of the slotted screws a few times and then backing them out. I'll need to replace them.

Next...need to rewire the stator or maybe bring it to an auto-electric shop.

Thanks again for all the advice. I understand it is very important not to over-torque the retaining bolt as it can lead to cracking the flywheel.

Jim

https://photos.google.com/photo/AF1QipPfrBdhU4XtSUkUO6MVUkLlGqACZ23sGvqIzMQb
https://photos.google.com/photo/AF1QipPWUMMMQw0awy02tiZkku7mc20rSsgvZbHMrVqT
https://photos.google.com/photo/AF1QipPivsuUf_vImgkViq9f0k6F-4nacOBPEooX4GH4
https://photos.google.com/photo/AF1QipP9-3d43kckLFdpA3T6NlB6wIayEAQ3S5kbymeN

--
Jim Curran
1976 Palm Beach
Alexandria, OH
 
Jim,
Google gives me a 404 error code that the requested URL was not found on this server. Might I suggest that you put your pictures up on the GMC Photo site.

JR Wright
Michigan

>
> Hi Folks,
> Thank you all very much for the valuable advice! I was finally able to get back on this and the flywheel has been successfully removed.
>
> I actually tried everyone's advice -
>
> First I tried to work it off by hand. I heated the casting several times and tried to get penetrating oil into the gap. But all I got for my
> trouble was some major-league blisters.
>
> Next I had a look at the damaged hole. After looking very closely, I saw that there were still cracks around the hole and I wanted nothing to do
> with that. Now that the wheel is off, I see that there is a lot of material available to extend the holes.
>
> So I added threaded holes to the unused bosses. I used a high-speed drill to make a pilot hole and a few successively larger ones. The last bit was
> 17/64". The holes were about 1 1/8" deep. I tapped 5/16-18 threads and cleaned them out. The drill bits cut easily and the tap even easier. I used
> a little bit of oil.
>
> My puller wasn't in the best condition so I had to monkey with it quite a bit. Once I had the flywheel under tension I smacked the outer edge of the
> wheel in an alternating pattern with a hammer via a short piece of 2x6 lumber. The puller released and the wheel was free! I pushed against the bare
> axle (not the nut) and kept a foam cushion under the flywheel in case it got away. Keeping the wheel from turning was nerve-wracking because I was
> trying very hard not to damage the vanes. I think I wedged some wood between the downpipe and the wheel to keep it still.
>
> I was able to remove the stator by striking each of the slotted screws a few times and then backing them out. I'll need to replace them.
>
> Next...need to rewire the stator or maybe bring it to an auto-electric shop.
>
> Thanks again for all the advice. I understand it is very important not to over-torque the retaining bolt as it can lead to cracking the flywheel.
>
>
> Jim
>
> https://photos.google.com/photo/AF1QipPfrBdhU4XtSUkUO6MVUkLlGqACZ23sGvqIzMQb
> https://photos.google.com/photo/AF1QipPWUMMMQw0awy02tiZkku7mc20rSsgvZbHMrVqT
> https://photos.google.com/photo/AF1QipPivsuUf_vImgkViq9f0k6F-4nacOBPEooX4GH4
> https://photos.google.com/photo/AF1QipP9-3d43kckLFdpA3T6NlB6wIayEAQ3S5kbymeN
>
>
> --
> Jim Curran
> 1976 Palm Beach
> Alexandria, OH
>
 
Not quite. There was advice to apply a hammer to the shaft, which I don’t
see in your list.

I learned it from a boat mechanic. We needed to pull the flywheel of an old
Johnson 25hp outboard so we could replace the magnetos. He took a brass
drift, held it in the tapered shaft with the nut removed, and whacked the
other end of the drift—hard—with a large ball pein hammer. On the third
whack, the flywheel popped off the taper.

I never removed the flywheel of an Onan, but that trick worked to remove
the flywheel of my Generac when I replaced the shaft speed sensor.

Rick “glad it worked out in the end” Denney

> Hi Folks,
> Thank you all very much for the valuable advice! I was finally able to
> get back on this and the flywheel has been successfully removed.
>
> I actually tried everyone's advice -
>
> --
Rick Denney
73 x-Glacier 230 "Jaws"
Off-list email to rick at rickdenney dot com
 
> Not quite. There was advice to apply a hammer to the shaft, which I don’t see in your list.

In a previous life I worked in a machine shop where we regularly assembled and disassembled very large gears (up to 10 feet diameter) and shafts (up to 15” diameter) used in the mineral and mining industries using 800 and 1000 ton hydraulic cylinders with improvised pulling and pushing fixtures. I can say from personal experience that applying a bit of impulse force to the shaft with a hammer (or an air chisel with a blunt tip tool) while the puller is in place is almost universally the best way to disassemble tapered items. This method instantaneously deposits force into the assembly and it is surprising how little effort is required in the hammer swing to achieve success.

Glad to hear that the flywheel is now off and progress is being made on the rest of the unit.

—Jim


Jim Miller
1977 Eleganza
1977 Royale
Hamilton, OH
 
> Due to an accidental short circuit, my engine-side stator windings are cooked and I'd like to repair them. However, one of the flywheel removal
> holes is cracked (see photo). I'd appreciate any advice on how to remove. Can I drill that broken hole further or all the way through and thread
> it?
> https://photos.app.goo.gl/nDQw7aRjX4AQPY4Q6
> Jim

Interesting. Today iexovered a broken starter bracket on my Onan. So ended up pulling the flywheel on that one and on the parts Onan. Each flywhee
had four threaded holes for the puller.
--
Steve Southworth
1974 Glacier TZE064V100150 (for workin on)
1975 Transmode TZE365V100394 (parts & spares)
Palmyra WI
 
> Interesting. Today iexovered a broken starter bracket on my Onan. So ended up pulling the flywheel on that one and on the parts Onan. Each flywhee had four threaded holes for the puller.

I’ve seen several different versions of the Onan flywheel. Some have 20 degree timing marks and others have 25, some have two puller holes and others four. Some have “Rotation” and an arrow cast into the flywheel face, others do not.

I am testing a new ignition system for these gensets and my development unit has TDC and 20 degree marks on the flywheel so I had to do some minor math to find out where my desired 26 degree timing point was. It would be interesting to know how many different flywheel versions there are.

—Jim
Jim Miller
1977 Eleganza
1977 Royale
Hamilton, OH
 
I have worked on Onan engines most of my adult career life. There are, as
you say many versions of flywheels out there. The cast iron genset engines
that run at 1800 rpm have a 20° advance and a 10 ball governor. The 3600
rpm engines use the 26° advance mark and NORMALLY use a 5 ball governor.
The BF series of engines, for the most part are used in garden
tractors, and run at a variety of engine speeds. Their flywheels resemble
the NH iron block flywheels, but are balanced differently and sometimes are
made of materials other than cast iron, like aluminum alloy. Some have rare
earth magnets on the inside to power a charging system, some don't. The
crankshaft tapers are nearly the same between series, and flywheels have
been known to be swapped between them.
Out here in the west, they race garden tractors, and build mini
pullers out of them as well. Lots of hot rodding tricks, multiple carbs,
different cams, no governors, nitrous oxide, etc. I have seen some strange
stuff.
Jim Hupy
Salem, Or
78 GMC ROYALE 403

> > On Sep 16, 2018, at 6:04 PM, Steve Southworth

>
> > Interesting. Today iexovered a broken starter bracket on my Onan. So
> ended up pulling the flywheel on that one and on the parts Onan. Each
> flywhee had four threaded holes for the puller.
>
> I’ve seen several different versions of the Onan flywheel. Some have 20
> degree timing marks and others have 25, some have two puller holes and
> others four. Some have “Rotation” and an arrow cast into the flywheel face,
> others do not.
>
> I am testing a new ignition system for these gensets and my development
> unit has TDC and 20 degree marks on the flywheel so I had to do some minor
> math to find out where my desired 26 degree timing point was. It would be
> interesting to know how many different flywheel versions there are.
>
> —Jim
> Jim Miller
> 1977 Eleganza
> 1977 Royale
> Hamilton, OH
>
>
>
>
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
>
 
You must log in or register to reply here.
Applied GMC
GMCMI Fall 2026 Rally

Share this page

Share this page
Top Bottom
Back