520H Engine Tick

Help me make a decision.

I have a 1993 520H with a 20HP Onan engine. This is a high hour tractor. The PO said the engine had been rebuilt, but I don't know when or how intensive the rebuild was.

Anyway, the tractor is in my stable now, so I must address the problems. It runs like a top, but it develops a tick after it warms up. I've used a stethoscope and isolated the noise to the rear cylinder. It is a sharp tap, more like a valve out of adjustment. Not a dull knock like a rod or crank bearing.

So far, I have adjusted the tappets to spec. No improvement. My next thought was a loose intake valve seat. Looking from the intake port the rear valve seat looks just fine. It did not appear loose. Not knowing what else to try, I just pulled the rear head. The valve seats look just fine, not loose at all. The valves and seat surfaces look worn, but the compression is good at 125 psi.

I'm down to two thoughts.

1. The valve seat is only moving once the engine warms up?
2. What I am hearing is really a rod bearing knock.

I'm torn as to what to do. I could peen the seats and button it up hoping for the best. Or, go deeper into the engine and check the rod bearings.

I wish this thing had an oil pressure gauge, that would really help.

My thinking at this point is to peen the intake seats, button it up and run it for a while. If the tick gets worse, then its rebuild time.

Any other thoughts?

Thanks,

Kurt

You are out of my comfort zone....almost has to be broke for me to fix....good luck. If you feel comfortable with your idea go with it.....I would!

Thanks for the comments guys! I always hate decisions when you know something is not right, but you can't pinpoint the problem. Your encouragement really helps.

GT, I'm following your repower post. It looks inviting. If things go poof in this Onan, I may give your approach a try. Certainly is much cheaper than the other alternatives. Keep posting some updates.

I did make progress yesterday. I pulled the heads and checked things out. The valve seats looked good, as did the cylinders. I de-carboned everything and sat back and thought about it for a while. The only thing I found that concerned me was the base of the rear intake valve seat (as viewed from the manifold) I thought I saw a thin bright band of metal there - just like the seat had been moving. So, I made the decision to peen that seat. That went reasonably well. I just hope I did it right. Anyway, I buttoned up the engine and started it. I could not hear the tick. So I'm cautiously optimistic. I'm hoping to get everything back together this afternoon so I can work it hard and see what happens.

Fingers Crossed!

Well, I tried. I just mowed for about an hour and no improvement. The tick or tap is back once it heats up. Now I don't know what to think. If its a rod bearing, it won't last long. I already replaced one main bearing, but if its the other one it may last longer, but will get worse with time.

When I bought this tractor, I paid parts tractor price. I was intending to use it for parts and transfer the gear reduction steering to my 88 520H. I may still do that. But I really like this 93 520H. I've fixed the steering issues, the rear axle issues and oil leak issues. Those and a long list of little things has gotten this up to decent except for the engine tapping.

I think I'm going to run this a bit and see what happens. I hate to put too much more money or time into this one. Another 520H would be cheaper than an Onan rebuild. Maybe an HF re-power is in my future.

Thanks,

Kurt does the sound vary with the engine load? Loose rod bearings tend to make less noise under heavy load and the opposite is true of main bearings. When chopping the throttle from full to idle is the clatter worse? Could be something as simple as varnish on the valve stems causing them to stick when hot. Some MMO in the fuel would help with that. Keep us posted and good luck!

Yes Major it does vary with load. The tap only occurs when the engine is hot. The noise is more pronounced with the PTO engaged driving a 48" deck and going up hill. Going down hill it sometimes almost disappears. By your description, it sound like a main bearing. With a stethoscope the noise is more pronounced at the head, and almost quiet on the crankcase. Strange thing is that when I replaced the PTO side seal to cure an oil leak, the bearing plate main bearing was shot, but on the inside half only. The out side half was just fine. I replaced the bearing plate with a used one that had a bearing in like new condition. This did not help the noise a bit. I'm beginning to think the only way to find out is to do a full tear down.

At this point I am weighing options.

1. GT's approach with an HF engine. Price is good, but I worry about parts availability and longevity. Frankly, if I can get 5-7 years out of this tractor I would be happy.
2. Do what Geno suggests and make one tractor out of my 2 520H's
3. Find another 520H and use this one for parts.
4. Honda or Briggs repower
5. Used Onan
6. Rebuild or repair this one

Way too many options here. If money we not an option, I would be buying a Honda.

It costs nothing but time to tear this one down, Maybe that is my next project. Or I could just run this to failure, again a no cost option.

Thanks for the comments.

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