520H Broken Rod?

[Doco2279 0]

I'm having an issue with my '96 520H with just over 1000 hrs.  It started as a considerable loss of power. So, I started with electrical.  Changed out the fuse block, new spark plug wires, new plugs, bench tested coil.  Moved on to Carb, disassembled, cleaned, put it back on.  Still bogged down and and noticed front cylinder was getting hot.  So, next step, check the valves, since I was showing zero compression.  While I had the head off, while turning the flywheel, the piston was not moving.  Of course the valves were doing their thing.  So, problem discovered, broken rod.....I haven't torn it down any further so I don't know how bad the damage is.  I'll be doing the tear-down today when I can pull the engine. 

 

Some background on me.  I dabble in the world of mechanics...everything has been self taught.  Basic maintenance is where I'm comfortable...but I'm never one to turn down a challenge.  This is the furthest I've ever dug into an engine, so I'm a little apprehensive going further.  I'm equipped with a service manual, forums and youtube.  I've got a nice little shop in the barn to work on this project, with most of the tools I'll need. 

 

So, is it worth my time to dig into this?  Is this something that should be relatively simple? I'm sure someone will chime in and tell me to repower...and I've considered the diesel route....but I'm viewing this as an opportunity to up my mechanical knowledge. 

 

Game plan follows: 

                        Remove engine

                        Degrease

                        Ascertain damage to rod/piston/cylinder/crankshaft/bearings/rings                      

                        Remove heads/decarbon everything

                        Replace rod and any damaged components

                        Replace crankshaft oil seals (since they are leaking already)

                        New Head Gaskets

                        Put it all back together

 

SO...I guess I'm asking for some advice.  Am I biting off more than I can chew.  Am I missing something.  Should I be looking at doing more when I've got everything torn down? Any advice would be much appreciated.

                        

                       

                      

 

[MarkPalmer 80]

Bottom line is you have nothing to lose by dis-assembling and assessing the engine internals right now.  There is a reason why the rod broke, and you need to find out why.  As the engine is in it's "autumn years," it won't be as simple as throwing a new rod in and going back to town, but the work can be done by anyone with decent mechanical skills and knowledge of engines in general.  With the engine torn down, you should measure all the internal parts as described in the service manual, and replace and/or machine whatever is out of tolerance.  It can be an expensive process with an Onan, but my thoughts if undertaking this project would be go in and do it once and do it right. 

 

-Mark-

[rmaynard 14,864]

  Good advice.

 

and to Doco2279,

 

[aremai4_27 0]

I work as a mechanic for Exton Mower Service in Washington Township, PA, and one of the certifications we have to get is a Kohler engine certification, the study guide for this test is available free online, not only does it explain how the engines work, electrical systems and everything else, but it also gives helps on assess problems and causes, one specifically, connecting rod breakage.  You can tell by the type of break, and conditions of other internal engine components of what caused what.

[Doco2279 0]

Thanks for all of the advice.  I got the engine out yesterday, opened up the oil pan.  Total chaos....rod broke  about 3/4 inch below piston.  The rest of the rod is in pieces in the oil pan.  Lots of gas in the oil, which I'm sure led to the failure.  Looks like some chatter marks towards the bottom of the stroke, and a small gouge on the crank.  So...at the bare minimum....new crank (or turned, if possible), new piston, new rod, new head gasket.  Locally, I found a '91 520H going for around 500.  I haven't talked to the guy, but that might be a better investment if the hours are low enough....use that for yard work....and I can take my time rebuilding the original engine and doing a total restore on my original machine....which I've been putting off....since it does such a good job mowing.  Much better than my MTD with a 21HP Briggs.  Comments? 

[MarkPalmer 80]

If the 520H for sale is operating good, it would be the way to go as I'm sure you'll be in to over $500.00 to rebuild your Onan.  

 

-Mark-

[sorekiwi 761]

I bought my 520 running on one cylinder.  The previous owner, and myself just assumed it was a valve seat issue.  It turned out to be a broken rod on mine.

 

 

I pulled the engine apart and found very little wear in the bores, the crank was within spec (after a polish) and most importantly my valve seats looked good.  I weould rather have an Onan with a broken rod than a loose valve seat.

 

I bought another dead engine (one that had lost a valve seat) as a donor and took both rods from that.  I used a couple of the bearings from that engine as well.  There have been a couple of spec changes on Onan rods, so I wanted a matched pair to avoid any balance issues.

 

I combined the two engines, added a new set of rings and put it back together.  Its been running well for 3 years now.  I think I have about $300 in the engine.

 

Here is a good thread on MTF if you want to see what is involved:  http://www.mytractorforum.com/showthread.php?t=91204&highlight=onan

[MarkPalmer 80]

That is one thing I recall about Onans, that if the crankpin bearings on the rod develop too much side clearance on the crank, the rod bolt can come in contact with the bottom lip on the cylinder and shatter the rod. I don't think it happens too often, but it's another, "can happen."  The big Kohler opposed twins are known to throw rods every now and then also.   

 

-Mark-

[Doco2279 0]

Well, that other Wheelhorse got taken out from underneath me.  So....I'm planning on replacing the crank, rods, pistons, and some new gaskets.  Just to get it back up and running for the season.  This winter I'll be stripping it all back down and getting it all re-done.  I just don't want to go to long without this mower up and running. The three acres I mow is alot more enjoyable on the wheelhorse than the MTD.  I'll let you guys know if I run into any problems....or heaven forbid, if my engine blows up.  Thanks for all the helpful advice.  I greatly appreciate it.