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Old 08-13-2007, 12:20 PM
rpauls rpauls is offline
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Join Date: Aug 2007
Location: NW NJ
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Kohler MV20 crankshaft gear wear at camshaft replacement. Replace?

Hey guys,
I just spent last weekend replacing the camshaft in my Kohler MV20 and while the engine was apart I noticed some worn teeth on the crankshaft gear as well. Not sure if it is bad enough to be fatal. I showed this to two machinists, one said the motor will definitely fail again if the crankshaft is not replaced, and the other said the "right" way to fix it is to replace both crank and cam, but if it were his he'd just do the cam. What do you think?

Here is some background. The MV20 is from a 3-wheel Scag and has about 2000 hours on it. Top end was rebuilt last year. Last week while my son was mowing the lawn he said the engine sounded "funny" and he shut it off. It would not restart. I quickly determined it had no compression and the valves were not moving. Took it apart and discovered that half the teeth on the camshaft were completely gone and the shaft was sheared in half! Also the governor gear was destroyed. I saw only minor wear on the crankshaft gear though. It must be harder I assume. However, there was some damage here as well. I filed the sharp edges off the 2 or 3 teeth that appeared damaged and the whole assembly seems to turn freely. I replaced the camshaft and gov gear but not the crank because it is 400.00 and put it back together. Started it up and it appears to run fine. However, I think it is a little noisier in the lower end. (Though this has always been a very noisy engine imho but I could never find where the noise was coming from. All measurable clearances are good. Maybe the gears were always bad?)

Anyway, I will go home today and mow the lawn and see if the engine blows up. If it does I will get a short block as crank seems too expensive. So my question is, what do you guys think about reusing a crankshaft with gear wear like this? Is it really guaranteed to fail again? Also, are these engines supposed to be noisy mechanically?

Thanks for any input.

Rich
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  #2  
Old 08-13-2007, 02:14 PM
khouse khouse is offline
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With that many hours and you being able to perform your own work - you did the right thing. It should run for a long time. Good job!
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  #3  
Old 08-14-2007, 10:36 AM
rpauls rpauls is offline
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Join Date: Aug 2007
Location: NW NJ
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Mowed the lawn for about 1.5 hours last night with no problems so far. Keeping it below 2500 rpm for now. Will try 3200 tonight (which is the speed I normally use) Incidentally, is this a good rpm for this engine on a scag sm 61 deck? Seems ok.

Thanks,
RIch
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  #4  
Old 08-14-2007, 12:07 PM
thecrankshaft thecrankshaft is offline
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Location: Plymouth, WI
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Rich, full throttle is the only way to operate an air-cooled engine for various reasons.

The reason your engine failed is because the shaft that the governor gear rides on backed out, letting the gear fall. Before operating any more, make sure that the governor shaft is pressed to a depth of .285" from the outside of the crankcase. It is recommended to drill through and pin the shaft. This governor shaft is located near where the oil fill would be if this was a horizontal shaft.

For more information, refer to Service Bulletin 246.

For 2000 hours on metal gears, I would probably replace cam and crank due to wear patterns and now possibly stress risers if metal is missing from the gears.

BTW, a short block will cost around 2k.
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Old 08-14-2007, 12:21 PM
rpauls rpauls is offline
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Location: NW NJ
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Thanks thecrankshaft,

I did not notice anything funny about the gov gear shaft while disassembled. The broken gov gear was still sitting on the shaft, which appeared connected to the bottom of the engine. Since this is a vertical engine, the shaft is at the bottom of the engine. How could it back out? Does back out mean "up" here into the engine? And if it did why was the gear still in place? Maybe I misunderstood your point here. Even if the gear did get lose somehow, how could this nylon gear break the teeth off half the steel camshaft?

I would really like to read the service bulletin you mentioned. How can I get a copy?

THanks very much,
Rich
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  #6  
Old 08-15-2007, 09:56 AM
thecrankshaft thecrankshaft is offline
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Location: Plymouth, WI
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Looks like you now have a copy of the service bulletin. Backs out means the shaft comes down towards the frame, out of the engine, allowing the governor shaft to fall.
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  #7  
Old 08-15-2007, 10:37 AM
rpauls rpauls is offline
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Join Date: Aug 2007
Location: NW NJ
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Still running with no cam???

Yes. I got the bulletin. Apparently this was not the cause of failure in my case since my gov gear was sitting just where it should be on its shaft. I can not find any explanation for what happened. What sort of event would shear the camshaft and take half its teeth right off! Since only a couple of crankshaft teeth were damaged it seems to me that the crank may have stopped while the cam continued to spin for 180 degrees knocking out its teeth as it went. Perhaps this was when my son hit the rock and inertia kept the cam moving when the crank stopped momentarily. Funny this though, he said the engine didn't stall when he hit it, and in fact he continued to mow for a few minutes afterward with all appearing normal. Then it started "sounding funny". He then says he shut the engine off using the key. It never even stalled! How could this thing be running at all? I am puzzled.

Anyway. it works fine now.
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