John Deere Mower Crankshaft Repair


Old 09-15-10, 09:42 PM
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John Deere Mower Crankshaft Repair

I hit a hard object in the yard with my mower. It went into convulsions instantly and I shut it down immediately. Thinking it might be a blade problem I started it once without the blade engaged. It started but still shook. I again shut it down immediately. It hasnít been run since then. On inspection, after disconnecting the spark plug I had someone pull the cord. The blade brake clutch (BBC) moved up, down and sideways.

To date, I have disassembled the top of the engine down to the starter cup. I have the BBC and pulley of the PTO end of the crankshaft. With the just the shaft showing on the PTO side it is clear the crankshaft is bent when I turn it. I like the mower and want to put it back in service.

I am considering alternatives. I am interested in opinions and options.

1) I found a shop that will straighten the crank, inspect/replace flywheel key and check and re-hone the blade. I am a bit leery of this based on cautions I have read about the risk of straightening a crankshaft.

2) I am considering replacing the crankshaft. I am interested in the advisability of this and other options. I am looking into Outdoor Power Equipment as mentioned in another thread. But I donít know if they will have an engine that matches mine and how cost stacks up compared to a repair.

If I were to replace the crank, I would rather not proceed if other internals to the engine are damaged. Can I gauge this if I remove the plug (to relieve compression) and turn the crankshaft to see if it turns smoothly? Iím thinking if it does, then other things in the engine might be alright. If I were to replace the crank is my shopping list a crank, two oil seals and a crankcase gasket? Thanks for any help people can provide on this.

Mower: John Deere JA 65
Engine: Briggs and Stratton 6.5HP Intek OHV
Engine Model #: 121602 0281 E1
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Old 09-16-10, 01:31 AM
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Well, I know that once a crankshaft bends, it somewhat weakens it and all that, but I have never seen or heard of a crankshaft breaking after it bent that I recall. That doesn't mean it can't happen. What would happen if it broke while mowing? Probably nothing serious since the blade should be fairly well balanced. Then again, strange things happen. I have straightened some cranks, and left some to be replaced, but if it were mine I think I would straighten it. A lot of people disagree and have good reasoning for their point of view, so listen to both views and come to your own conclusion. I myself don't see much risk, if any. If anyone knows of an instance when a crankshaft broke after straightening it, I would like to hear about it.
Old 09-16-10, 02:46 PM
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Thanks for that reply. The repair shop says they can get the shaft back to 98% of original trueness. After a crank is straigtened to this degree is the risk of an engine that is not balanced shaking itself apart, as I've read elsewhere, pretty much eliminated?
Old 09-16-10, 08:00 PM
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I'm sure there is factory specs for allowable runout on a crank.
The balance on your crank has not changed unless it was damaged and material removed.
The vibration from a bent crank is actually caused more by the extension of the blade.
I know shops in my area that do it, and it is not much if you take them the crank.

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