pt 425 engine alternatives

[scotts562]

I have a 2005 pt 425 with the Robin Subaru engine that will be getting a hydraulic pump rebuild soon. I would really like to change the Robin engine out for a more dependable/easier to start engine, such as a Honda or Kohler, at the same time. I know Power Trac used to use Kohler engines in 425s, but I am under the impression that there were extensive modifications made on the newer models to accommodate the Robin engine. After several hours of searching this forum for this, I have not found a clear answer.

I did find a couple of engine conversions on 422s but the gas tank/battery/heat shield configuration is different than on my machine. I also found a post where someone converted a 425 to a Kubota diesel, but the original poster did not give any details.

Can someone recommend an engine that will fit into a 425 without a bunch of modification to the machine?

Thank you for the help

 

[Marks481]

What is your configuration?

-two pumps bolted together on front of engine
-pump on front and rear of your engine
-muffler inside the engine compartment on front of engine
-muffler inside the engine compartment on back of engine
-muffler mounted outside engine compartment with exhaust header on front of engine

PT changed the 425 arrangement as many times as they changed the 422 arrangement. The engine in the 425 and in the 422 are identical dimensions, so what they built into one, they built into the other.

If you have the stacked double pump, then engine replacement with a like size Kohler should be simple. One challenge is that there is no telling what size the stub shaft on your engine is until you take the pump off, since it seemed to change a few times over the life of the 425. Assuming you measure before buying, then the only modification required is (usually) to cut the shaft on the engine shorter than stock. Buying an engine with the wrong diameter shaft (because it is cheaper or available) just means you have to purchase a spider (love-joy) connector of the right size, which isn't that big a deal. You probably want to change out the spider anyhow.

If you have the pump on the front and back of the engine, then you will need to buy the "Front Drive" AND "Front Drive Adaptor" kit to mount a stub shaft on the fan side of the motor.

The threads you read about extensive modifications were probably ones where people were trying to shove a water cooled Kawasaki EFI 27 hp (short story: doesn't fit) into the PT.

When I replace the engine in my beast, it will probably be with a 27HP EFI model. Both Kohler and Robin make one, and it is the same size engine as the stock 25 hp engines that is in the PT425.

 

[scotts562]

Thank you for the information. I suspected a Kawasaki, or any other water cooled engine, would not fit. I will officially scratch it from the list. I will take a look at the 27HP EFI Kohler.

I have two pumps bolted together in front of the engine with exhaust outside the engine compartment from the front of the engine. I have tried to attach a picture (it is the same configuration shown on the Power Trac website.

It will not be a problem determining shaft size. I have already changed this engine out twice and have a re-built spare sitting on the shelf. I was going to re-use the love-joy coupling, but now that you mention it, I will replace it.

 

[Marks481]

Looks almost like my 2005 PT-422, except my battery is on the floor of the tub instead of in front of the exhaust manifold. From the looks of it, your engine tub must be about 4" longer then mine.

I think the Kohler ECH749 (26 hp EFI) would be a nice engine, and Small Engine Warehouse has a couple for $1600-2100. Specs on this state it is 2.3" longer than the Robin EH72, so it might be a bit crowded.

 

[woodlandfarms]

You have read the post where one person in this group has moved to Harbor Freight engine. Basically a honda rip.

 

[scotts562]

Tmarks: It is kind of neat that our machines were both manufactured in the same year. I took a look at the Kohler ECH749. Assuming I can get a shorter air filter for it, it might work. It would be very tight. I have 2.75" from the end of the pump to the hydraulic tank (only 2" from the hydraulic line elbow to the PTO valve). The engine sits tight to the engine cover hinge bar and I have 1.25" exhaust to heat shield and 2" engine to heat shield. Overall tub measures 32.75" (engine cover hinge bar to hydraulic tank) and 33 3/8" (frame to hydraulic tank). If it is not too much to ask, could you measure the distance from the hinge bar to heat shield on your machine and the end of the pump to the hydraulic tank? Maybe I can do similar to what others have done with PT 422s. I have been doing some research and am now considering a 23 HP Briggs Vanguard. The dimensions look like it would fit nicely. I am not sure if I am reading the power curve data correctly, but it appears as though this engine provides more power at the lower RPMs than the Robins.

Woodlandfarms: Thank you for the reply. A Predator engine was my original thought. Gotta love Harbor Freight. Predator Tracs did a great post on his conversion of a 422 to a Predator engine. From the pictures he posted, I assumed I would have clearance issues with the heat shield. The battery on my 425 sits behind the gas tank, not under. Tmark's input makes me wonder if my assumption is wrong. From information I have gathered from this site, I know the 22 HP Robins is the same size as the 25 HP Robins. It never occurred to me that the tub on a 422 may be shorter than the tub on a 425. I assumed the frames on a 425 and 422 were identical and only the engine/ROPS/lift height were different.

 

[Gravy]

Cliff's Notes version: 35hp Robin EH90. Requires drilling new holes in the tub and shortening the spacers under the engine. Chuck the pump adapter in a lathe to accommodate the different centering spigot and drill a couple more holes for the asymmetric bolt circle. Hack and weld an exhaust system. Enjoy 33% more displacement and around 40% more torque. Cackle maniacally. Don't think too much about fuel usage. It will still vapor lock on occasion.

Oh, yeah,

Feel slightly guilty because you still haven't posted the pics & build thread...

 

[Frank Sorbello]

Today, I ordered a 25 HP Kohler from Equipatron to replace my 22HP Robin/Subaru. $1529 free shipping. We shall see how it goes.

 

[Marks481]

...35hp Robin EH9...feel slightly guilty because you still haven't posted the pics & build thread...

so what do we have to do to make you feel more guilty?



Because we WILL do it...

Back to the OP... more power is better. I know my 22HP just doesn't have what it takes for field mowing (I have a choice: drive fast or spin the blade fast.... don't get to do both). I know that if I re-powered I would look for more hp at the rpm of interest, not just the same or slightly better.

 

[agjones]

The Robin Subaru engine combined with the lack of heat management added up to a failed engine and no warranty support from Subaru. Lame. 349 hours.

Sticking with gas...I would highly recommend the Honda GX670 or 690 for the PT, same form factor as the Predator (which is a gx670 clone)

However, I replaced my terrible Robin engine and hazardous muffler...with a Kubota D662, complete with radiator. I did have to modify the back of the tub and cage a bit to accommodate the extra 2" of engine/radiator. This engine (as configured by Jacobsen for a golf couse mower) produces 18hp @ 3000rpm. Any 3-cylinder kubota should fit as easily as this one did for me. At $500 bucks the mower carcass offer me a lot of hydraulics and the diesel engine and was worth it...but I'd go for the D772 or 905 if doing it again. The oil cooler stayed in the same place, same battery and gas tank (purged and cleaned) I even re-used the wiring harness...I'm adding a hyd circuit to eliminate the attachment change shuffle and I'll try to get some pictures up.

Now that trail building season is back for me, I'll be re-hardening the back of the PT425 with the grill/guard of a little dozer....that will give me filtration to keep grass etc out of the radiator as well as rear facing lights! and steel between me and whatever I back into....

I have slightly less power in ground engagement, like trenching, but far better low end response and almost 40% better fuel economy. I just broke raw ground with the tiller and power rake to make a start-up farm patch for a landscape client. No problem. Basically you just have to re-learn where the power curve and stall points are....

I don't feel the hit on power since I have resurrected a toro dingo from the scrap yard and switched it to the Power-Trac QA setup. With it's higher pressure, it handles the ground attachments when I need it to.