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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. 







Also tagged with one or more of these keywords: honda gx690, sears st12, repower, tecumseh hh120

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