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1976 Sears ST12 Repower options


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#1 rewilfert OFFLINE  

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Posted June 20, 2017 - 06:26 PM

I have a 1976 Sears ST12 that I am looking at repowering.  The original Tecumseh 12HP is getting long in the tooth.  It has intermittent ignition issues, smokes pretty bad and burns a noticeable amount of oil.  Given that the Tecumseh parts can be somewhat difficult to come by, I'm looking at repowering the tractor with a new engine.  The rest of the tractor works great.

 

I've looked at a lot of single cylinder engines around the 12HP size and they all look to be too long to keep the original grill in place.  Mostly due to the cylinder laying down at an angle vs straight up like the Tecumseh.  The twin cylinder V engines are a bit more compact in that respect.  After looking at a lot of V engines, it looks like the Subaru EH65 would be a good fit.  Everything appears to be in the correct orientation for clearance and access.  The only thing I'm not sure about is if the clutch pulley mechanism will clear the new engine.  I ran across one example of a EH64 installed in the same tractor and it looked like a great fit.

 

Does anyone have any constructive input on doing a swap like this?  I realize the hood will have to be replace or cut to allow the larger muffler to fit.

 

The EH65 is more engine that I need, but it has all the right specifications and readily available parts that would seemingly make the sway somewhat easy to accomplish.  I looked at Honda V engines and they appear to be a little more compact, but the documentation I found on them was a little lacking compared to the Subaru engines.

 

Thanks,

Robert



#2 mtoney OFFLINE  

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Posted June 21, 2017 - 07:28 AM

The Sears are a harder model to repower and retain the hood and grill as they were so compact around the engine area.  Have you looked into the single cylinder air cooled diesels from Yanmar/Clones?   The 6.5hp one I used in my WH has the power of a 10hp engine and should do all you need to do with your Sears, all while using way less fuel.  They also offer a 10hp model, most easily found in the clone of the Yanmar design and available from Carroll Stream Motors.  Thiers have one of the better reputations from the clone engines.  The slanted cylinder design makes repowering harder.  I think they went with that design to keep overall height lower since all engines are OHV designs now.   The diesels are upright like your old engine is.  Mike



#3 rewilfert OFFLINE  

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Posted June 21, 2017 - 09:33 AM

Thank Mike.  I had a quick look at Carroll Stream Motors and the 6HP version has dimensions that would work, but I would have to modify the exhaust and make a sleeve for the original pulley to work.  It also has about 1/3 the torque of the EH65 (but probably comparable to my current engine).  In general, I'm hesitant to go with clone engines.  I'd rather stick with the big manufacturers so that I don't run into future parts availability issues.

 

I couldn't find any pricing on the Yanmar, but suspect they are considerably more expensive.

 

This is what got me looking at the Subaru engines:

 

https://www.youtube....h?v=1JyjLVUs-aU

 

I can find the EH65 easier than the EH64 and it has the same external dimensions.  It looks like the engine fits well in the Sears, but there are no details of modifications (if any) required for the clutch pulley.


Edited by rewilfert, June 21, 2017 - 09:34 AM.


#4 mtoney OFFLINE  

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Posted June 21, 2017 - 04:44 PM

Sleeving the shaft is an easy fix, I had to do that as my genuine Yanmar diesel had a tapered shaft as it came from a generator.  Exhaust is easy was well, ditch the original muffler, get a flange that bolts in place with pipe nipple sticking out and pipe up what ever muffler fits your space.  I used one from a Honda.  You can get one of the genuine Yanmar engines for around $300 off ebay, then tack on $65 for a 12vt starter(one on it is 24vt), the taper shaft adapter is $61 and comes in 1" or 1 1/8th shaft size.  I have never been a fan of the Suburau/Robin and Generac engines personaly.  Go with one of the better dealers for a clone and you will be fine.  The yanmar parts are a direct fit.  But if you can find a good second hand Yanmar, the genuine version has better fit and finish.  The ones on Ebay are military take outs and have little to no run time on them.  Mine starts instantly, even when cold in the morning and without using the intake preheater.  Nothing like having a diesel powered tractor, just as they should be!   But good luck on whatever engine you go with.    Mike


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#5 rewilfert OFFLINE  

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Posted June 21, 2017 - 07:25 PM

Mike, thanks for the input on the diesels.  They do sound interesting, but I think I may just stick with the gas V twins that are a good fit.

 

I did a ton of research on the Subaru motors today.  I think they offer a really good package all things considered.  However, during my conversations with them today, I found out they are exiting the small engine business as of the end of September this year.  No engines are in production, only what is left in dealer/distributor stock.  Parts are guaranteed to be available for another 7 years and warranties will be honored, but I don't like the idea of starting a project like this with a product that's on its way out the door.

 

After that, I went back and looked at the Honda GX V twin engines again.  Their engines are slightly more compact dimensionally, but the information on them is a bit more difficult to find.  After talking to Honda and their local distributor, I was able to understand what engines were available with the features I need.  Right now, a version of the GX690 is looking like my best option for the $$ in their model lineup.

 

I also have a line on a hood from a twin cylinder Sears tractor that should clear the large muffler on the Honda engine without modification.  The only potential difficulty I see is the clutch pulley mechanism, but I won't know anything about that until the engine is installed.



#6 mtoney OFFLINE  

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Posted June 21, 2017 - 07:52 PM

Usually the tapped bolt holes in the crank case cover are fairly standardized so that electric PTO clutches will fit many different brands of engines.  As long as the crankshaft size is the right diameter for your clutch, I think you will be able to adapt it fairly easily.  Keep in mind your fuel tank size and length of run time between refuelings.  While the Vtwins are better than the old flat twins of years ago. She will still burn more fuel than your single cylinder did.  Also make sure both cylinders are able to vent thier hot air easily out the PTO side of the tractor.  Honda's use a plastic cooling fan on the flywheel. On some of the JD 318 repowers, when running the side covers it was found the engines would run hot and the cooling fins would "flatten" out and deprive the engine of enough cooling air.  Running over temp isn't good for any air cooled engine.  Honda's are awsome power plants, but when you need parts they are difficult to figure out which are the correct parts and then salty price wise.  Just a heads up on that one.  All of the Honda engines I have dealt with were great till they needed parts.  At that point it was lots of head banging trying to get the correct parts and the prices for said parts made Onan parts look cheap.  The Vanguard series by B&S is one of the best out there, right up there with Kawasaki as the preferred engine in commercial ZTR mowers.  They were designed in Japan by Diahastu and not by Briggs.   Kawasaki's are awsome as well but not cheap.  You can get the Kawasaki's with fuel injection now as well in some models.    Mike



#7 rewilfert OFFLINE  

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Posted June 22, 2017 - 06:56 AM

Mike, I looked at the Kawasaki V twin engines. It appears that they no longer produce an air cooled horizontal shaft version due to EPA regulations.

I looked at the B&S Vanguard engines as well. It looks like they are the right size dimensionally, but they have long starters that may interfere with my clutch pulley mechanism. I originally passed on them for this reason. The Honda and Subaru engines have more compact starters.

My tractor uses a foot pedal controlled idler pulley to place/relieve tension on the drive belt to the transmission. This mechanism is close to the driver's side of the engine. The only thing that attaches to the PTO shaft is a simple pulley for the drive belts to the transmission and mower deck.

I have had nothing but good luck with the Honda engines. I have a 1988 Honda commercial push mower with the original engine. One pull after sitting all winter and it fires right up. Getting the correct parts for an engine *shouldn't* be an issue if you have the correct service manual and serial number of the engine.

#8 det12187 OFFLINE  

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Posted June 22, 2017 - 07:24 AM

A Honda clone (13hp) from Harbor Freight would work too. Some trimming
around grill would have to be done and exhaust would have to be from the
front as well. I have done two so far on older Super 12's one was a 1967
and the other was a 1968. A sleeve for the shaft would have to made as well.
Its worth a look.

#9 rewilfert OFFLINE  

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Posted June 22, 2017 - 04:11 PM

I'm purposely trying to steer clear of the clone engines.  I want something new that has an established parts/service network behind it.  I'm also trying to avoid any modifications to the grill and hood if at all possible.  I'd like to use as many off the shelf, unmodified parts as I can to make future repairs easier.  It's a lot to ask, but the more I look into it, the more I think I can make it all work.   



#10 mtoney OFFLINE  

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Posted June 22, 2017 - 04:35 PM

I agree, while many have used the clones to great success, others have had less than steller results.  I think a V twin is going massive overboard but its not my tractor.  I think with some very carefull trimming, some custom exhaust work(if you can weld), one of the smaller single cylinder Hondas might fit.  Sears notched out hoods and made panels that fit around the motor.  So if the trimming is done carefully and very clean and smooth.  Once repainted to match, it will once again look stock.  I am anxious to see you find your engine and start the project.   Mike



#11 rewilfert OFFLINE  

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Posted June 22, 2017 - 06:42 PM

Mike, a V twin is definitely overboard, but they seem to have the best fit with the smallest amount of modification.  They have fuel pumps, higher amperage charge coils, the correct size PTO shaft, etc.  It just makes the installation easier to do with fewer modifications and without requiring so many additional accessories.  The larger engine will also be less stressed than a smaller one.

 

I found a posting on here that is almost exactly what I am trying to accomplish.  The tractor model is similar and the engine is an older model, but they are both close enough to what I am working with to show that I shouldn't have any major issues.

 

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

 

I have a Honda motor in mind, I'm just trying to get a complete list of parts together so I can order everything at once.  This swap will also allow me to replace all the electrical systems on the tractor and install a few electrical features that it was not originally equipped with.


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#12 dodge trucker OFFLINE  

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Posted June 22, 2017 - 06:42 PM

how about a new-ly rebuilt Kohler?



#13 rewilfert OFFLINE  

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Posted June 23, 2017 - 05:54 AM

I did look at Kohler engines, but they were too tall to fit without hitting the hood. Also, if I'm going through this much effort, I'd like to get something brand new. Thank you for the offer though.

I did look into rebuilding the original Tecuseh 12, but the cost of all the parts I thought should be replaced on a 40+ year old engine (along with machine shop work) was around 2/3 that of installing a new V twin. Given the difficulty of finding the older parts, I decided that wasn't going to be my most cost effective path forward.

#14 rewilfert OFFLINE  

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Posted June 24, 2017 - 02:28 PM

I was able to track down some information on the Vanguard engines.  Briggs and Stratton keeps the information on them well hidden, it wasn't readily available like Subaru and Honda.  Their engines have a lot of "stuff" mounted on the starter side and the starters come out almost flush with the PTO side of the engine.  For those reasons, I don't think that they will be the most ideal for my application.  After looking at all these manufacturers, I think Honda is the way I am going to go.

 

After doing more research, it appears there may be difference in the frame length of the single and twin cylinder Sears tractors.  That explains why I am seeing more apparent engine room on some of the swap pictures than I am seeing on my tractor.  I need to understand the frame differences a little more.  I can fit a Honda V twin in my engine compartment, but won't have any room to move it fore/aft.  I need to see how that will affect the PTO position compared to the original HH120 engine.



#15 rewilfert OFFLINE  

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Posted July 04, 2017 - 01:35 PM

I am moving forward with the engine repower on my tractor. I started a new thread here to cover it:

http://gardentractor...976-sears-st12/




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