520-8 Engine Repower Decision

[wh5208speed 646]

Just thought I would throw a topic out for the group for feedback. Over the years I have owned a few 520-8's (12-13 of them...lost track somewhere). Anyways, this past summer I sold off the last 520-8 speed that I had kept for about 10 years to a collector since I really wasn't using it very much as I my favorite tractor is the 78-79 C-161's (of which I have 3). Well, this really nice low hour (414 hrs) 520-8 speed came on the market near a place where I had to travel for work this past week. The owner said everything was tip-top and that the engine ran great. So, after I completed the project for work for the week, I went to purchase this 520-8.

 

When I got there, you could tell it was kept indoors and looked like a nice low hour machine. I started it up and something seemed off, as anyone knows, a good running Onan has a unique smooth sound. This seemed like it was misfiring. Decided to pull the front spark plug wire off and the engine died. Put the front wire back on and started it up and pulled the rear spark plug wire (nothing). I didn't bring a compression tester with me, but my best guess is the infamous rear exhaust valve seat. I could tell the owner did not know a lot about them and he was sorry for not sharing this as I had come a long way. He asked if there was a price that I would be comfortable with, and we agreed on a number I feel good about. 

 

So now the dilemma.....I now have a very nice, low hour 520-8 speed with a bad motor. I know they are rare (225) and there will be some purist that would think it to be crazy to not find a 20hp Onan to put back on it. Here is what I am considering:

• 20hp Onan-from another 520-biggest challenge is to find an Onan that is low hour that wouldn't cost a fortune
• 16hp Kohler Magnum-I have a brand new Kohler just sitting on the shelf. I have a parts 312-8 that I could use for the parts to get the electrical system compliant
• 23hp Briggs Vanguard Repower- Since this is such a nice 520-8, maybe go all out, an put one of the Vanguard repowers in it. Never been a big fan of Briggs, but I know a lot of small engine mechanics are fans of the commercial Vanguard line.

 

Edited December 18, 2021 by wh5208speed

[lynnmor 7,813]

First thing I would do is properly diagnose the problem.  If you do find a loose valve seat it may not be the end of the world, often they can be repaired for a reasonable amount of money if you do the grunt work preparing it.  The valve seat issue is most often the intake on the rear cylinder.

[squonk 43,780]

Do a compression test. It may be an intake manifold leak or something similar. A leaned out cylinder will act the same way.

[wh5208speed 646]

4 hours ago, lynnmor said:

First thing I would do is properly diagnose the problem.  If you do find a loose valve seat it may not be the end of the world, often they can be repaired for a reasonable amount of money if you do the grunt work preparing it.  The valve seat issue is most often the intake on the rear cylinder.

I appreicate the advise. I am on vacation next week, so I plan to check it over before deciding on the path to take.

[RED-Z06 2,567]

I had one come in and the guy was told the back cyl was dead.  It was a dead spark plug...ran great at 2100hrs (416-8)

[kpinnc 14,628]

Agree with what has already been said. Might be a simpler fix than you think.

 

I'm no purist- not one of my tractors is all original. That being said, that is a nice looking machine. If there ever was a candidate for keeping original, you already have it.

 

...and if you have no other options, the Vanguards are good engines. I have 3 of them... 

 

 

[Bill D 2,247]

I vote for a Vanguard swap if the Onan is beyond repair.  Small Engine Warehouse sells a kit.  If you go the Magnum 16 route, a new 9 pin connector and two 5 pin relays should allow you to make the electrical systems compatible.  No need to chop up the wiring harness from your 312-8.  Use one relay to kill the motor and the other to mimic the function of the oil pressure switch.  Good luck with it.

Edited December 19, 2021 by Bill D

[RED-Z06 2,567]

I had a deere in that was swapped from an 18hp onan to a 20hp Vanguard, very smooth and responsive but you could feel the lighter rotating mass under load, that massive onan flywheel stores alot of energy

[kpinnc 14,628]

1 hour ago, RED-Z06 said:

I had a deere in that was swapped from an 18hp onan to a 20hp Vanguard, very smooth and responsive but you could feel the lighter rotating mass under load, that massive onan flywheel stores alot of energy

 

I swapped in a 14hp Vanguard on a 520-H for a while, and it struggled far less than my Onan 518-H when mowing with the same 48 deck in heavy grass. The Vanguard never even pulled the governor, while the Onan most definitely did. Both did the job well, but the Onan seemed more sensitive to loading.

 

I would say that the nose of the tractor is definitely lighter- maybe as much as 75lbs so, but power should be no different from comparable HP rating between the two. And the Vanguards drink far less fuel.

Edited December 19, 2021 by kpinnc

[wh5208speed 646]

14 hours ago, kpinnc said:

Agree with what has already been said. Might be a simpler fix than you think.

 

I'm no purist- not one of my tractors is all original. That being said, that is a nice looking machine. If there ever was a candidate for keeping original, you already have it.

 

...and if you have no other options, the Vanguards are good engines. I have 3 of them... 

 

 

That is a sweet setup. How did you mount the exhaust shield without the tabs that comes on the Onan muffler?

[kpinnc 14,628]

4 hours ago, wh5208speed said:

That is a sweet setup. How did you mount the exhaust shield without the tabs that comes on the Onan muffler?

 

The Vanguard muffler had a partial shield already welded on. I just used sheetmetal screws to mount the WH one to it.

[Tuneup 1,452]

You are in a really fun place! What to do? All options result in countless enjoyable hours. For me, second the view of seeing exactly WHAT is the problem with the current power plant. If not repairable, look to see what you plan to do with the machine? Is the torque of a twin really needed? If not, a single works fine, if you're handy electrically.

[kpinnc 14,628]

3 hours ago, Tuneup said:

You are in a really fun place! What to do? All options result in countless enjoyable hours. For me, second the view of seeing exactly WHAT is the problem with the current power plant. If not repairable, look to see what you plan to do with the machine? Is the torque of a twin really needed? If not, a single works fine, if you're handy electrically.

 

Agreed 100%. Part of the hobby I enjoy the most is using some old fashioned ingenuity to keep my tractors working. Of course I love all original gems, but finding them here is extremely rare. I make do with what I have. Very few products in history were built on such a flexible design, and I take advantage of that to the best of my ability! 

[WheelhorseBob 1,550]

Full disclosure, I’m a purist. I would do some basic testing first. If you find low or no compression, pull the engine and remove the cylinder heads for a proper decarb and inspection. You can have it out and on the bench in an hour. If memory serves the parts for this (gaskets) are reasonable. If the exhaust valve seat is loose but the damage isn’t too significant, I’d bring it to a reputable machine shop to repair the seat. 
 

being this is a rare model I would do everything to keep it original. If it were an H I might have a different opinion but even then I wouldn’t give up on the Onan unless it was severely damaged. My 2 cents and worth every penny!😉 Btw, 161’s are becoming my favorites too.