# Sears ST10 engine replacement



## turk (Oct 6, 2010)

Hello all.I've got this vintage Sears ST 10 tractor and trying to replace
the tired tecumseh 10 HP in it with a twin cylinder Onan 10 HP motor. 
I need some help from the engine guru's here on exactly how to go
about rewiring the stock wiring harness into the Onan engine.The onan
has a 12V starter as does the tecumseh,but what would I need to do
to hook up the starting and charging systems between the 2 engines.
Any advice or comments greatly appreciated.Ron


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## Bill Kapaun (May 8, 2007)

Don't know if I can help much-
What's the Sears 917.xxxxxx number of the tractor?
One MIGHT be able to find a schematic for it.

Was the ONAN off another tractor?

Are both engines battery/points ignition or????


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## turk (Oct 6, 2010)

Hello Bill. Tractor model# is 917.25722.Yes,the Onan is a battery/points set up.
Don't know what is was used for,but don't think it was in a lawn tractor.Motor is
in excellent shape and a rare one(twin cylinder,10HP),not many made.Looks
like it would fit nicely in the ST10 frame.I do have the original ST10 owner's manual
with a wiring schematic.Of course,the Tecumseh uses the solid state ignition
and a charging stator mounted to the flywheel.Any ideas on rewire would help.Ron


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## Bill Kapaun (May 8, 2007)

This manual will have a schematic for an Onan with battery ignition.
http://www.managemyhome.com/mmh/lis_pdf/OWNM/L0707017.pdf


The ignition switch will have to be different or at least some "surgery" performed on the wiring.
Maybe the switch Part# for this tractor will fit right in???

I don't have a schematic for YOUR tractor.

The main thing that will be different-
The wire going to your current ign. coil GROUNDS with the key OFF to kill spark. (M terminal)
The ONAN requires 12V to the coil in order to run. (*I*gnition terminal)

The other wiring should be very similar regarding-
B From battery
S to starter solenoid
R input from the alternator

Look at the "truth table" at the lower left of the schematic for both tractor.
It shows what connections are made in the different key positions.
You may need to verify with an ohm meter to the new switch (if used) to make sure they are in the same positions you "expect".


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## turk (Oct 6, 2010)

*ST10 repower*

 Sorry to be so late replying to your excellent info on repowing my
Sears ST10 with a 10HP twin Onan.I had to put the project on the back
burner for awhile,got into refurbishing my Sears ST16.Just wanted to let
you know I greatly appreciate all of the info and advice you gave.I expect to get
into the little ST10 in a few weeks.Just one last question though,would it be
a good idea to consider using new wiring when possible in making up wiring
connections for the ignition ,battery connections,etc.Thanks for all your advice.Ron


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## Gibby (Oct 17, 2011)

Don't forget to add about 4 or 5 inches to your ST. If I remember right, that is about how much shorter the ST is than an SS, and it's all in the front half of the frame, you may have trouble squeezing an Onan into the ST's engine bay. Not saying it can't be done, but without some fabrication work, you're going to have "issues".


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## turk (Oct 6, 2010)

*ST10 engine swap*

:usa::Thanks Gibby.The little 10HP twin onan is slightly wider than the
tecumseh in there now.Looks like I might need to extend the grill
and hood about 2-3 inches.Thought about using the grill extender from
a SS16 and maybe use the hood also.I plan on doing away with the gas
tank under the hood and put a slightly bigger tank in back of the seat,similar
to some of the SS16's.I'll need to fabricate a bracket mount for it.The grill
extender also raises the hood about an inch upward,so I'm hoping the little
onan will clear.One of my concerns is keeping the onan's engine pto as close
as possible to the original tecumseh.I'll probably need to fabricate an engine
plate to raise or lower the onan to keep the drive and deck belts in line with
the onan's pto pulleys.If I use the Sears onan hoods,I could route the onan's
twin mufflers through the cut out of the hood because I plan on using a duel muffler
set up.My goal is to repower and restore the little ST10,but keep it's compact
size and give it a little more torque and smoothness with the onan twin.If 
you have any advice or comments,please let me know.I have a plasma cutter
and mig welders,so changing things around is a possibility.


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