Onan questions broken rod

[welderman85 53]

i just picked up a free 520h from a freind of the family. the bad news is it has a broken rod.ts i the grandson pulled the motor and pulled some of it apart i have all parts i think .So i pulled the rear head and pulled the piston or whats left of it and cleaned the chunks of rod from the pan. the cylinder walls didnt look bad ans it came with a set of used pistions and rods he got from ebay. I dont rember the hours it had on it i can check wensday. i was wondering what would be my next action on the motor can it be rebuilt or will it cost more than a good v twin swap. thanks for the help

[sorekiwi 761]

I bought a 520 with a broken rod. In my opinion I'd rather fix one with a broken rod than a loose valve seat.

On mine I switched in a set of used pistons and rods (new rings). There are several different rods (and maybe pistons, I dont remember) so I wanted to keep a matched set in the engine.

Mine turned out well, it blows a puff of smoke on start-up but once warmed up its clear.

I'm thinking I had about $150 in gaskets and rings to get it running again.

[HorseFixer 2,012]

What you need to do is carefully take it apart noting carefully the disassembly. Then take the crank, cam, block, valves, pistons to a trusted motorshop and have them Mic the cylinders, crank, cam, and see if they are within or how far out of specifications. I have been where you are at a lil more than a year ago and thats what i did. The engine that I am speaking of is the one in my Snow Chucker. Having the engines critical components analized Bore, Crank, and so forth will tell you if its worth a rebuild the cost depending how many parts need replacement. You might get out easy with just replacing the bad Rod and set of gaskets. But if you have much wrong and decide to go a full rebuild then you can plan on a cost depending on the condition between $1200.00 and 1800.00 also depending on machining costs. Mine was nearly $1800.00 The Rod in yours does not surprise me the old style Rods are junk Onan has solved this problem with stronger Rods. If you are thinking of rebuilding go to Onan parts.com

Cheers ~Duke

Edited January 16, 2013 by HorseFixer

[JackC 617]

There is the correct and expensive way to approach it or the quick and dirty way.

You could just try replacing the broken parts, put it back together, and if it runs it runs.

For the cost of doing it correctly you might be a able to find a complete low hour 520H.

What year is the free 520H and how many hours on the clock?

[sorekiwi 761]

There is the correct and expensive way to approach it or the quick and dirty way.

I'll agree with Jack on that one. I figured it would cost me around $1000 to rebuild the Onan properly. I didnt have $1000 to spend so it was either patch up the Onan with used bits and try to coax a few more years out of it, drop in a 16 HP Kohler (which I already had), or (if I was going to have to buy an engine for it) the other temptation was a 10 hp Chinese Diesel.

Kinda depends what your goals are and how fat your wallet is!

[artfull dodger 394]

I disagree with Duke, respectively on the older Onan rods, Having that opening above the big end helps with the recprocating load and forces, Jaguar did this for years on thier high performance engines. Onan's loosing rods are the result of either being run low on oil to many times, allowing one or both jugs cooling fins to get plugged up with oily debris, the result being normaly being an overheating condition which causes the rings to loose thier temper with a resulting loss of compression on that cylinder. This in turn causes an out of balance situation in the engine, and the resulting harmonic vibration does bad things to rods. If the overheating goes on long enough and a rod doesnt let go, the hardened valve seat can pop out. But, the valve seat tends to be more prone to engines that dont get thier valves adjusted properly. Out of adjustment valves can also cause overheating, hard starting and leading to a loose valve seat. I do fully agree you need to get the engine checked out by a engine shop if you are unable to take the proper measurements yourself. DO NOT use those used piston heads till you have checked the ring grove clearance with a feeler gauge, there is a spec for it and if they are to loose, they will dig into the cylinder walls and in some cases, get ripped right out the top of the piston. Seen that one myself on a B48g in a 420 JD. Done properly, then maintained properly, Onans can live in excess of 2000 hours of operation. Duke's snow chucker will out live him by many years because he took the time and spent the money to have it done right. Piston and rod styles MUST match on both cylinders, used is ok if they are good parts and match. Onan made several styles of piston heads for the same engines, and you have the 2 different style rods, solid I beam and the older style with the opening just above the big end. Onans were designed for gen-set operation, so they are high performance engines by design and require the maintance to go with that performance. Sadly many didnt see proper maintance and were abused, overheated ect and we are now reaping the effects. Thankfully, word is that Cummins/Onan has released the rights to produce parts to the aftermarket, so rebuilding costs should come inline with overhauling a Kohler KT or Mag twin as Stens and others catch up with parts production. Stens already has parts like voltage regulators out, at about half the price of the OEM Onan part. Build it right, or set it on a shelf and put that Kohler on till you can afford to do it correctly. There is nothing like a Onan P220 in full throaty roar. The power and torque is hard to equil, and nothing I have seen has that throaty exhaust roar of the Onan twins. Good luck and great score on a free 520!

[welderman85 53]

the tag on the seat pan says 41200e03 serial 2001810 hour meter reads 557 i dont know how ture it is but the tractor is in pretty good shape and was owned by an older couple so it could be close

[JackC 617]

41-20OE03 is a 1992 520H. O is for ONAN and E is for Eaton. 557 is a relatively low hour number. There should not be a lot of wear on that engine.

It would be good to know the conditions when the rod went. Was it run low on oil? Was the machine mowing and maybe the flywheel screen got plugged causing the engine to over heat?

[artfull dodger 394]

With that low hours, unless the rod breaking did damage to the block or bore, it should be toward the lower end of the price spectrum to fix the Onan. I am better it got run low on oil. Most Onans I have dealt with do use oil, even when new. Not checking it every single morning you fire it up is like Russian Roulette and the Onan is the loaded gun. All the broken rod engines I have dealth with were either overheated due to mouse nest, oil leak causing build up of chaff on front side of the jugs or being run low on oil, all it takes it one time low enough on a hot day, oil overheats and breaks down, then boom. Onan is now recommending 15w40 oil from 0' to 100'F for all P and B series engines. I would find a matching good used piston and rod, clean up or replace the crank, hone + new rings and put it back together again. . Mike

[welderman85 53]

I have a new used rod and piston for it I was thinking just put her back together and see what happens money is kinda tight for a full on rebuild

[artfull dodger 394]

I hear ya, money real tight here as well, in school and not employed anywhere at the moment. Mine probably needs a hone and rering as well. Just keeping a close eye on the oil level for now. Mike