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Honda GX690 into 1976 Sears ST12

honda gx690 sears st12 repower tecumseh hh120

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#1 rewilfert OFFLINE  

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Posted July 04, 2017 - 01:32 PM

I'll use this new thread to document the installation of a Honda GX690 into my 1976 Sears ST12 tractor.  After looking at all the major manufacturers for a new V-Twin engine, I settled on a Honda GX690.  The specific engine model is the GX690 TXA2.  All I really needed was the GX690 TDW, but the TXA2 model includes an oil level indicator and a factory mounted control box that includes a key switch and hour meter.  The oil level indicator circuit will shut the engine down in the event of a low oil quantity condition.  Since the ignition wiring is different between the HH120 and the GX690, I plan to remove the control box key switch and wiring and install it in the tractor dash.  The hour meter will be an added bonus for tracking when maintenance is due.

 

I will be using a Honda GX690 muffler, P/N VHRM6 that has the exhaust outlet facing towards the front of the tractor.  I looked at several other mufflers and this one seemed to be the most well constructed (and hopefully the quietest).

 

My initial measurements suggest that I will need to cut the hood and both upper and lower grill sections to allow clearance for the new engine and muffler.  I was initially against cutting these items, but I changed my mind since this is likely the last engine that this tractor will ever get and I have no plans to go backwards to the original Tecumseh engine.

 

Once the parts arrive, I will remove the front bodywork, existing engine and wiring.  Once the new engine is installed, wired up and running, I will clearance and reinstall the bodywork.

 

I have concerns that the existing clutch pulley installation will hit the new engine, but have a few ideas to gain clearance if necessary.  I also expect to modify the firewall and battery box to allow access to the rear sparkplug.
 
Here are a few pictures of the tractor as it currently sits with the Tecumseh HH120 engine.

35549551842_df479988f3_c.jpg2017-07-04 11.17.04 HDR by rewilfert, on Flickr

35331166550_95bd660b8e_c.jpg2017-07-04 11.16.47 HDR by rewilfert, on Flickr

35587388031_36005f128f_c.jpg2017-07-04 11.15.33 by rewilfert, on Flickr

35718297745_7c298a173b_c.jpg2017-07-04 11.15.48 by rewilfert, on Flickr

35331170580_1d4a9dc64d_c.jpg2017-07-04 11.16.03 by rewilfert, on Flickr


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#2 stiemmy OFFLINE  

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Posted July 04, 2017 - 02:15 PM

Why the repower??

It looks to be in very nice condition.
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#3 chieffan OFFLINE  

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Posted July 04, 2017 - 02:45 PM

Unless there is something drastically wrong with the Tech engine, why go to all that expense and trouble?  Nice looking tractor the way it is and cutting thing up to fit a bigger power plant don't make any sense to me.  Now if your building a pulling tractor, that is another story.


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#4 KennyP ONLINE  

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Posted July 04, 2017 - 02:58 PM

That is way to nice of a tractor to be cut on!


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#5 rewilfert OFFLINE  

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Posted July 04, 2017 - 03:00 PM

The Tecumseh engine is shot and unreliable at this point. My cost analysis says this repower is more cost effective than a rebuild for what I want. This wasn't a decision taken lightly, I did a lot of research to get to this point.

The previous owner did a rattle can respray on the tractor. It looks good from 5 ft away, but it's not pristine by any means.
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#6 EricFromPa ONLINE  

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Posted July 04, 2017 - 03:12 PM

Here's a much better solution to cutting the Grill and Hood.No matter how you cut it will not look very good.

 

The Opposed Twin cylinder Sears suburbans (must people use the SS16 to repower as they are everywhere) Hood and bottom grill spacer will bolt up to your tractor and give you the room you need without having to cut anything.SS16 Hood is about 2 1/2 inches longer and the SS16 lower Grill "spacer" Lifts the front grill Up about 1 1/2" and out front 2 1/2.

 

11212.jpg

 

This hood is a bit to expensive IMO.

http://www.ebay.com/...tcAAOSwPWRZTHpQ

 

Or Briggs twin Powered GT16 Hood.Same length.Much Cheaper but needs painted.

http://www.ebay.com/...AcAAOSwtZJY-7J4

 

Here is a SS16 with a Honda engine swap.As you can see no cutting of the hood or grill was needed.

http://gardentractor...an-repower-r124

 

I suggest you find an Onan or Briggs powered Parts tractor or sell your tractor and use a SS16 tractor for the Engine Swap.

This fella may have a hood and lower grill mount for a SS16 in his stash.

https://atlanta.crai...6158072326.html

Or this fella has a SS16 with no engine.Might be able to work out shipping for the hood and lower grill mount.Or grab the whole tractor for $175 and use it to repower if your willing to travel.

https://akroncanton....6204169464.html

 

As you can see here the SS16 has plenty of room under the hood for aany big V-Twin engine and you don't have to cut anything.


Edited by EricFromPa, July 04, 2017 - 03:35 PM.

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#7 EricFromPa ONLINE  

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Posted July 04, 2017 - 03:17 PM

Pull the deck and mule drive and sell the ST12 as is for a couple hundred smackers.Then go get yourself a Twin cylinder suburban that needs an engine. I linked you a SS16 for $175 obo on Craigslist.If your willing to travel I would go with that option.

 

Even after modifying the hood and grill you Might run into a serious problem with the Clutch mount.Not 100% sure about your tractor but the older Tecumseh powered Suburbans clutch mechanism were Bolted to the engine. SS16 Clutch mechanism is attached to the Battery Box and sits back torwards the dash further to clear the rear head.

 

You will have to cut the heck out of the inside of the grill to clear the front cylinder and valve cover. This will make your hood hinges snap off the grill 1st time you open it.

 

The top transmission bracket on a SS16 is similar to the bottom grill mount.I can't find a pic of the grill mount but did find pics of the transmission mount.

 

Similar but not the same.

 

s-l16001.jpg s-l1600.jpg


Edited by EricFromPa, July 04, 2017 - 04:13 PM.

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#8 chieffan OFFLINE  

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Posted July 04, 2017 - 09:07 PM

The Tecumseh engine is shot and unreliable at this point. My cost analysis says this repower is more cost effective than a rebuild for what I want. This wasn't a decision taken lightly, I did a lot of research to get to this point.

The previous owner did a rattle can respray on the tractor. It looks good from 5 ft away, but it's not pristine by any means.

Sounds like you think you want more power for some reason so you convinced yourself the Tech is "shot".  By the time you get through with that re[power you will have a lot more invested than a rebuild, and still may not have anything.  I know the larger side shaft engines are getting hard to come by but they are still out there.  Just have to hunt for them and find them.  Be much better that chopping up a good tractor that will never be worth much.  Just my opinion - it your $$$.



#9 Sawdust ONLINE  

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Posted July 04, 2017 - 10:10 PM

Regardless of the bad paint job I agree this tractor is in way too good of shape to repower. Sorry your not getting much support but we all hate to see these gems cut up.
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#10 EricFromPa ONLINE  

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Posted July 05, 2017 - 01:23 PM

Looked through the ST-12 parts manual and the clutch mechanism MIGHT be welded fast to the bottom of the battery box depend on the year.

Have a close look up under the battery box from the Flywheel side of the engine.Should be clearly visible from that side.

 

This battery box is off of a ST-12.

 

s-l1600.jpg

 

 

You can do the swap Without cutting anything if you get these 2 parts.

 

Lower Grill bracket off of a Onan powered suburban push the grill out and lift it up. $12.99 shipped.

 

http://www.ebay.com/...BAAAOSwopRYiiw0

 

And 1 of these 2 hoods. 1st one is expensive but won't require cutting for the muffler. Both of these hoods are Longer than your stock hood and will bolt rite up to your Upper grill section.

 

http://www.ebay.com/...tcAAOSwPWRZTHpQ

 

http://www.ebay.com/...AcAAOSwtZJY-7J4


Edited by EricFromPa, July 05, 2017 - 01:25 PM.

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#11 rewilfert OFFLINE  

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Posted July 05, 2017 - 08:47 PM

Looked through the ST-12 parts manual and the clutch mechanism MIGHT be welded fast to the bottom of the battery box depend on the year.
Have a close look up under the battery box from the Flywheel side of the engine.Should be clearly visible from that side.
 
This battery box is off of a ST-12.
 
attachicon.gifs-l1600.jpg
 
 
You can do the swap Without cutting anything if you get these 2 parts.
 
Lower Grill bracket off of a Onan powered suburban push the grill out and lift it up. $12.99 shipped.
 
http://www.ebay.com/...BAAAOSwopRYiiw0
 
And 1 of these 2 hoods. 1st one is expensive but won't require cutting for the muffler. Both of these hoods are Longer than your stock hood and will bolt rite up to your Upper grill section.
 
http://www.ebay.com/...tcAAOSwPWRZTHpQ
 
http://www.ebay.com/...AcAAOSwtZJY-7J4


When doing my research for this swap, I've looked at everything you've mentioned as options.

The clutch pulley on my ST12 mounts to a pivot point on the right side frame. If you look at my pictures, you can see it just aft of the muffler. From what I've been able to tell, there are 2 versions that mount to the bottom of the battery box. They don't seem to be consistently installed on certain model tractors from what I've found. The picture you posted is from a tractor that has the battery and fuel tank mounted longitudinally side by side behind the engine. That version is too long and will create a clearance problem to the back of the engine. There is another version that only has the battery mounted laterally under the hood (like mine). That's the one to get and I'm actively searching for one right now. The other option would be to cut out the existing pivot point and flip it 180 degrees. That would give me a little over 1" more clearance to the engine. I won't be able to tell if that is enough until I get the engine set in place though.

 

I've seen the videos of the tractor with the Subaru engine.  The hood wasn't cut, but the upper and lower grill sections were.  That conversion did not use a standard Subaru muffler either, it was something custom built.  The stock Subaru muffler is tall and would require the hood to be cut.  The Honda muffler is roughly the same height as the Subaru and will require the hood to be cut.  I won't know how much until I set the engine into place and see how much it protrudes from the side of the tractor.

 

Based on the measurements I have taken so far, it looks like I will have no problem with the hood clearing the top of the engine.  There is plenty of room to spare.  As far as the length of the engine compartment goes, I shouldn't have any clearance problems there either.  Having room to access the rear spark plug will be an issue, but everything else should clear the firewall and inside of the upper and lower grills without a problem.  I might have to trim away some of the sides of the upper and lower grills, it just depends on how much the engine protrudes on the left side of the tractor when the PTO pulley is put into the proper position to align the drive belt.

 

I looked at the longer hoods with the longer twin cylinder engine muffler cutout.  The cutout is still not high enough to clear the Honda muffler.  If I estimated correctly, that cutout also extends a lot further aft than where the Honda muffler will sit.  If I cut one of the longer hoods to accommodate the Honda muffler, it will look a blt odd as I would have to cut through the side vents and leave part of them aft of the muffler.  If I use my existing hood, it would be a clean cut that completely removes the existing side vents.  This is all based off of rough measurements and guesses.  Everything is still an option at this point until I set the engine in place and measure everything.  
 


Edited by rewilfert, July 05, 2017 - 08:51 PM.

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#12 MNGB ONLINE  

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Posted July 10, 2017 - 12:35 PM

Hi have you made any progress on the repower? I was looking at my SS16 this morning got measurements of the B&S V twin mounting base so if all goes better I'll start removing the Onan one of these days



#13 rewilfert OFFLINE  

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Posted July 10, 2017 - 01:43 PM

I now have the Honda GX690 engine in my hands, but the muffler appears to be backordered.  I'm going to continue getting parts together in the near term.  The original engine has an intermittently failing stator.  Sometimes it runs, sometimes it dies and won't work for days.  I figure as long as it keeps running I'll keep getting the parts together.  If I'm lucky, it will continue to run until October and I'll install the new engine then.  If not, I'll do it as soon as I have all the appropriate pieces together.  When I do make any progress, I'll post it here.

 

I'm looking at adding an oil pressure gauge to the dash.  The new engine has a pressurized oil system and a tap to measure the pressure, so why not?  I'm considering installing an oil pressure gauge, voltmeter and hour meter in the dash.  I was just looking around for options where they would all somewhat match.  I've also considered a tachometer, but haven't found anything that looks like it will work yet.


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#14 MNGB ONLINE  

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Posted July 10, 2017 - 01:51 PM

Hi thanks for the update, when I get the B&S installed I'm going with a warning light on the oil pressure I've installed gauges and the light and on the GT  I prefer the light, a tiny tach should work good simple to install and it keeps track of hours.


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#15 rewilfert OFFLINE  

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Posted July 10, 2017 - 10:03 PM

I like having an oil gauge just to monitor the pressure, but a low pressure light is definitely more useful.  You'd never catch a low oil pressure condition in time to save the engine with just a gauge.  I was thinking about installing both. 

 

I'll look at the Tiny Tach and see what it offers.  The engine comes with an hour meter, but no tachometer.  I'd really like to have a dial gauge tachometer, but I just haven't found anything yet that looks like it would work.

 

Looks like my muffler is now on the way, should have it later this week.







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