Well now I've gone and done it...

   / Well now I've gone and done it... #21  
You got my attention.

Me too! I've had this fantasy about building the ultimate 425. Larger engine, larger displacement wheel motors (I'm going to do that for sure, just haven't had time), suspension seat, etc...

BTW, I thought we concluded the wheel motor circuit does not have any reliefs. The systems depends on wheel spin for relief, or engine stall. Correct.

In any case, VERY cool.
 
   / Well now I've gone and done it... #22  
BTW, I thought we concluded the wheel motor circuit does not have any reliefs. The systems depends on wheel spin for relief, or engine stall. Correct.any case, VERY cool.
My old PT-425 doesn't have relief valves nor case drains...
 
   / Well now I've gone and done it... #23  
I am not sure that is true about the relief valves.

Just about any hyd circuit would have a relief valve. You really can not afford to be building up pressure beyond the limits of the hyd components.

There are relief valves in my 1445 VSP pump, and when I go to the tow mode, the TOW bolt on top just opens the relief valves so the fluid can pass through with minimal pressure.

If you have your rpms up and full pedal, and you are stopped and are not spinning tires, then the reliefs are working.

It is just illogical to build a hyd circuit without relief.

You could almost bet that the hydrostatic pump circuit has a relief in the pump or the wheel motor circuit.

What is the brand name and model number on the 425 pump?
 
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   / Well now I've gone and done it... #24  
I did a little homework on this and both the Bondioli & Pavesi NT10 and M4PV21 have relief valves in the pump.
 
   / Well now I've gone and done it... #25  
I guess I was a bit hasty with my statement -- I meant no external bypass valves in the wheel motor circuits. I have no clue what's inside the pump...
 
   / Well now I've gone and done it...
  • Thread Starter
#26  
I still don't have time for pics & lots of details, but I've had a bit of time to think about it. I'm pretty sure the Robin big blocks will work in any PT 422/425 without major modifications to the tub. The basic big block Robin engine is really only about 1/2 inch longer than the small block, and is actually narrower. The Robin 22-25hp engines are dimensionally smaller than the Kohlers, so you Kohler guys should have plenty of room.

The Robin big blocks have a flywheel PTO shaft option, so there's hope for MossRoad and others with pumps on both ends of the engine, depending on how the pumps are supported.

The only real challenge in my conversion was modifying the pump adaptor to work with the different pilot diameter and bolt circle of the new engine. I have an old metal lathe, but most folks might need to pay a machinist to handle that. It's pretty simple, and not terribly precise by machinist's standards. +/-.005" is probably fine. It took me several hours, but I'm no machinist.

One thing that I initially missed is that the crankshaft centerline height is considerably different. I had to cut down the engine mounting spacers a good bit to keep the pumps at around the original height so they would clear other components. I also drilled the engine mount bolt holes a little farther back than I should have, so I had to grind a notch on the tub to clear a bump on the fan shroud. I will correct that error so the fan inlet screen will fit.

The rest was pretty straightforward. The electrical system was a straight plug-in.

That's all I have time for now. More eventually...
 
   / Well now I've gone and done it... #27  
Thanks for the update. Curious to hear about the muffler situation when you get time. Also, this engine has the potential in inject a LOT more heat into the engine compartment, which is already overheated in my view. You may want to consider adding a fan somewhere.

Also, I'd consider adding some screen to cover the air intact holes at the back of the tub. The screen will quickly get clogged with debris in some situations, but the main air intact shroud should still be free to pull air from the engine compartment. I need to do this to my tractor because the engine is getting clogged with leaves.
 
   / Well now I've gone and done it...
  • Thread Starter
#28  
Thanks for the update. Curious to hear about the muffler situation when you get time. Also, this engine has the potential in inject a LOT more heat into the engine compartment, which is already overheated in my view. You may want to consider adding a fan somewhere.

Also, I'd consider adding some screen to cover the air intact holes at the back of the tub. The screen will quickly get clogged with debris in some situations, but the main air intact shroud should still be free to pull air from the engine compartment. I need to do this to my tractor because the engine is getting clogged with leaves.

The muffler is whatever the local muffler shop had in stock. I asked for the smallest inlet/outlet size, oval shape, QUIET and 18-24" overall length. With an elbow on the outlet to point the exhaust noise away from my ears, I can now hear the actual mechanical noise from the engine. Turns out there's quite a bit of that, too.

The Robin big block comes with both a stationary inlet screen and a rotating screen that attaches to the flywheel/fan. Between the two of them, they add at least an inch to the length of the engine. Once I re-drill/slot the engine mount holes in the bottom of the tub, I should be able to move the engine forward enough to accommodate the rotating screen. I'm hoping that the rotating screen will throw debris off.

Regarding the heat issue: I have an idea that the bigger engine may actually run a bit cooler because the airflow and cooling fin surface area should be greater to be in proportion to the displacement. The 720cc engine was maxed out most of the time while mowing, and was running on the ragged edge of it's designed cooling capacity. The 999cc engine should be relatively loafing under the same load, and have plenty of cooling capacity in reserve. That assumes that the tub allows the hot air to escape adequately. If not, I'll take appropriate measures.

I did get to play just a little bit after work today. With the old engine at full throttle, climbing the steepest parts of my front yard wasn't any problem as long as the PTO was off and I feathered the treadle with a bit of care. With the new engine, it's still not any problem if I feather the treadle a bit...but today, I did it at about a quarter throttle.

It's still early in the game, but my impression at this point is that the big block engine hasn't turned my PT425 into a hotrod unstoppable beast. It will just let the PT do everything it should be able to do without panting and gasping desperately. I'm pleased so far. We'll see how it all works out.
 
   / Well now I've gone and done it... #29  
Please keep us updated. Would love to do this swap myself. Wish you were closer...I'd stop by to help.
 
   / Well now I've gone and done it... #30  
If the engine rpm is the same, you will have the same speed as before.

If you want more speed, get larger diameter tires.

What you have is extra HP above what is required to power all pumps at maximum.
 

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