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Honda GX690 into 1976 Sears ST12

honda gx690 sears st12 repower tecumseh hh120

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#31 John Arsenault ONLINE  

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Posted November 11, 2017 - 07:07 AM

 

 

It looks like the Subaru engine fit well in your tractor.

 

Yeah it did, I had to cut and nip a few things too. Well worth it though, I plan on doing it again to one of my other sears. 

 

I am on a few other sites and on one of them they guys are so picky about what I do to my tractor, they are so hell bent at keeping it "original". 

 

I feel the GX690 is going to be an awesome engine for that tractor. Now that I see how well it fits in yours maybe I'll push for a bigger engine next time around. 

 

One thing I have noticed is that I am barley using any gas compared to what I use to burn. I know they techy had a small tank but  it was not easy on fuel like the Subaru is. One mow with the tecky and stock tank and I needed a refill. I think its like a 1 gallon tank. I have mowed 4 times on one tank with the new engine. Its got a 7.4 qt. or 1.85 gallon tank. So its not even twice the size but goes 3 times as far. I know a lot of the savings is coming from the fact I put a larger pulley on the deck drive as well as the hydro drive and I can mow at about 1/2 throttle as opposed to 3/4 ish throttle before. 

 

I see you have the stock pulley on there so wanted to mention that because with your new found horse power  I am betting you will see the same results. Only difference is your tractor is a gear drive so you may not want to change your drive pulley , it may make your  tractor to fast. 

 

As far as the bigger deck pulley goes , no ill affects at all, I can hit real tall wet grass and it don't bog a bit. 

 

 

 

 

 

 


Edited by John Arsenault, November 11, 2017 - 07:10 AM.

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#32 rewilfert OFFLINE  

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Posted November 11, 2017 - 07:42 AM

Can you cut a round hole with a hole saw into the firewall to make an easy access to the rear plug? Then you just need to pull the battery out.

 

That was my original thought.  Though the hole would have to be quite large to be able to grab the spark plug lead, pull it off the plug and move it out of the way to access the plug.  It would also provide a heat path to the battery.  Once the engine was sitting in the tractor, I realized that removing the firewall when necessary would be the best plan.  It only has 4 bolts that are easy to access and it would take all of 2 minutes to remove.  Then I have unrestricted access to the plug and no hole to let heat get to the battery.  In all likelihood, I'd only be changing plugs every few years anyway.  This was initially one of my biggest concerns, but it has turned into a non issue.


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#33 rewilfert OFFLINE  

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Posted November 11, 2017 - 07:50 AM

 

 

 

It looks like the Subaru engine fit well in your tractor.

 

Yeah it did, I had to cut and nip a few things too. Well worth it though, I plan on doing it again to one of my other sears. 

 

I am on a few other sites and on one of them they guys are so picky about what I do to my tractor, they are so hell bent at keeping it "original". 

 

I feel the GX690 is going to be an awesome engine for that tractor. Now that I see how well it fits in yours maybe I'll push for a bigger engine next time around. 

 

One thing I have noticed is that I am barley using any gas compared to what I use to burn. I know they techy had a small tank but  it was not easy on fuel like the Subaru is. One mow with the tecky and stock tank and I needed a refill. I think its like a 1 gallon tank. I have mowed 4 times on one tank with the new engine. Its got a 7.4 qt. or 1.85 gallon tank. So its not even twice the size but goes 3 times as far. I know a lot of the savings is coming from the fact I put a larger pulley on the deck drive as well as the hydro drive and I can mow at about 1/2 throttle as opposed to 3/4 ish throttle before. 

 

I see you have the stock pulley on there so wanted to mention that because with your new found horse power  I am betting you will see the same results. Only difference is your tractor is a gear drive so you may not want to change your drive pulley , it may make your  tractor to fast. 

 

As far as the bigger deck pulley goes , no ill affects at all, I can hit real tall wet grass and it don't bog a bit. 

 

 

 

 

 

 

 

If you do look at the Honda V-twin engines, one thing to note is that they are all the same physical size. Only the internal dimensions change to get the different displacements.  The largest version (690) was the most readily available, so that's what I went with.

 

I'm curious how the gas consumption is as well.  The original gas tank is under the seat and quite large.  I've never measured it, but it seems to be at least 2 gallons.

 

I did consider using different pulley sizes for the transmission and mower deck.  Theoretically, I'd be able to run the engine at half throttle and have all the HP and torque of the original engine.  But then I'd have the ability to severely overspeed everything and I'm not sure that's a good idea.  I may play with it if necessary once I get it running again.


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#34 rewilfert OFFLINE  

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Posted December 05, 2017 - 08:07 PM

I was able to get back on this today.  I wanted to mull a few things over and had a fresh perspective on placing the engine.

 

The engine is now bolted to the frame.  I was able to use 4 holes that existed in the frame.  I just needed to open them up a bit to fit the larger mounting bolts on this engine (7/16").  As it now sits, the PTO pulley is in the same lateral and longitudinal position as it was with the HH120.  The only difference is that the GX690 PTO shaft is about 1/2" higher than the HH120 shaft.

 

I decided to move the battery box/firewall aft 1".  That allowed a little more clearance to the engine and also allowed me to get the PTO shaft in the original location.  I used 2 existing bolt holes and enlarged 2 others to make that change.

 

The wiring on the engine is just the right length to reach the voltage regulator when mounted in the original position next to the battery and also the key switch when mounted in the dash.  The key switch perfectly fit the hole where a cigarette lighter was before.

 

I was surprised how little modification was required to get it this far.  I was really expecting much more work.


38860831071_e14564fbe1_c.jpgUntitled by rewilfert, on Flickr


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#35 John Arsenault ONLINE  

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Posted December 06, 2017 - 05:40 PM

Looking good so far, did you check the grill and see it it will clear everything?



#36 rewilfert OFFLINE  

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Posted December 06, 2017 - 08:21 PM

I did hold the lower grill in position to see how it would fit.  I will need to trim it slightly on the upper left side to clear the engine shroud.  Otherwise, I should have just enough room to get in there to replace the oil filter without disassembling the front end of the tractor. 



#37 rewilfert OFFLINE  

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Posted December 14, 2017 - 08:06 PM

I've made it to the point that I was able to drive the tractor around today.  The engine self primed the new fuel line and fired right up.  Initial impressions are that the GX690 is MUCH quieter and smoother than the HH120.  It was a good shake down.  There are a few items I need to address.

 

I was originally going to use an aftermarket hour meter and oil level low light mounted in the dash.  Today, I was able to determine that the oil level low light has logic within the Honda hour meter and separating the two may be difficult.  For that reason, I've decided to just mount the Honda hour meter to the frame of the tractor and use it.  When a low oil level condition is detected, it kills the engine and sets a red warning light on the hour meter.

 

The original throttle lever does not have enough friction to hold high throttle settings.  The springs on the engine are too stiff.  I need to find a way to give the lever a bit more friction.  I'd really like to replace the whole throttle lever/cable assembly, but haven't found anything that will fit in the original position.

 

I did have to trim the top of the dash tower above the battery in order to get the battery to go in.  I ended up removing 3/8" across the whole top.

 

The tractor never had a proper battery hold down, so I ordered a Deka 06161 battery hold down.  It fit perfect and holds the battery nice and tight in the battery box.

 

The picture of the drive belt looking aft shows a slight misalignment between the engine and clutch pulleys.  This is due to the clutch pulley pivot point bushing being extremely worn.  Replacing the bushing doesn't seem too difficult and I'll get to it after everything else is finished.

 

The next step is to get the grill and hood trimmed to fit around the engine.

 

After I get the bodywork reinstalled, I will be adding new voltmeter and oil pressure gauges along with a low oil pressure light to the dash.  Then I will install all new wiring for the lights.  I'll also need to find a way to patch the hole where the original ammeter was installed.  That position is no longer usable due to moving the battery tray aft.


25186910848_6abe9bb67e_c.jpgUntitled by rewilfert, on Flickr

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#38 rewilfert OFFLINE  

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Posted December 15, 2017 - 08:49 PM

I mounted the Honda hour meter on the left side tractor frame. I just enlarged a hole that was already there. It's out of the way, but easy to see when you need to get to it.  The low oil level light is on the upper left side of the hour meter.

39080007491_bd04dcc2d4_c.jpgUntitled by rewilfert, on Flickr


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#39 John Arsenault ONLINE  

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Posted December 18, 2017 - 07:36 AM

I expect to see this complete soon , very soon? 

 

Video?



#40 rewilfert OFFLINE  

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Posted December 18, 2017 - 10:49 AM

It's going to be a while yet.  In order to finish the job, I need to get the steering wheel off to remove the dash tower.  I'd really like to find a replacement steering shaft.  The existing one is damaged on the top side where I had to drill out the broken bolt that holds the wheel on.  I can cut the shaft off, but I'm reluctant to do so until I have a replacement shaft in my hands.

 

I also need to source the lights and switches I'm going to use in the dash for the headlights and low oil pressure indicator.

 

It's progressing, but down to a lot of finish details that I need time to research and source parts for. 


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#41 KennyP ONLINE  

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Posted December 18, 2017 - 12:50 PM

I'm watching as I have an ST/16 with no engine amongst other things!


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#42 John Arsenault ONLINE  

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Posted December 19, 2017 - 04:50 PM

I'm watching as I have an ST/16 with no engine amongst other things!

Repowers are easy on these, if you ever need help you got Rewilfert and I to help ya.


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#43 rewilfert OFFLINE  

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Posted December 22, 2017 - 09:50 PM

Now that the steering wheel is off the tractor, I am able to remove the dash and start reconfiguring it.  I will be installing a new voltmeter, mechanical oil pressure gauge, low oil pressure warning light and headlight switch.

Since I moved the battery tray back, I could no longer use the location of the original ammeter. There was just enough rear clearance on the portion of the dash below that to install the voltmeter and oil pressure gauges.  The added bonus is that portion of the dash is angled slightly towards the driver's eyes, so it's easier to read the instruments.

 

I'm looking at options for the low oil pressure light right now.  I should be able to install it on the same section of dash between the oil pressure gauge and choke.  I decided to use a Honda oil pressure switch that comes installed on other versions of the GX690 engine.  That way the set point is correct and if it ever fails, it's a readily available Honda part.  It will get plumbed in under the dash in the line that feeds the oil pressure gauge.

 

I sourced a new headlight switch from the local golf cart shop.  I'm considering installing it either above the key switch or the choke.  I haven't decided yet.


27453449729_8955b7f65d_c.jpgUntitled by rewilfert, on Flickr


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#44 rewilfert OFFLINE  

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Posted January 09, 2018 - 07:45 PM

I sourced the indicator lights for "low oil pressure" (red LED) and "headlights on" (blue LED) over the holidays.  Today, I completed mounting everything in the dash.  The next step will be to wire up everything and plumb the oil pressure gauge to the engine.

 

38707630375_54f99e6ae6_c.jpgUntitled by rewilfert, on Flickr



#45 Dayton_King ONLINE  

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Posted January 09, 2018 - 07:58 PM

You will love this motor. When I repowered a tractor with the gx670 I was impressed. Weighs less then the old engine, twice the power, and better fuel economy.
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Also tagged with one or more of these keywords: honda gx690, sears st12, repower, tecumseh hh120

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