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How Bent Is Too Bent - Mower Crankshafts


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  #21  
Old 05-04-2019, 08:32:53 AM
gdstew gdstew is offline
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Default Re: How bent is too bent - mower crankshafts

"Many air-cooled shops have a fixture to successfully repair"


I had a "Lil Brute" straightener. Mounted the engine, and a bolt would push the crankshaft sideways until straight. Quit using it many years ago when B & S went to thin sumps that would crack. Had a 50-50 chance of breakage. Also, a 1"long crank doesn't give a guy much leverage to work with.
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  #22  
Old 05-04-2019, 09:08:45 AM
Tracy T Tracy T is offline
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Default Re: How bent is too bent - mower crankshafts

short crank, find a junked pressure washer they have the short crankshaft. and pressure washers are everywhere here, always the same failure the pump is shot. think people forget water turns solid and expands below 32 deg....
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  #23  
Old 05-05-2019, 01:12:15 AM
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Default Re: How bent is too bent - mower crankshafts

When the blade bent it set considerable stress at the bend point. Trying to bend it back will introduce more. If it decides to fail at that point you would have a piece of blade flying out at considerable speed and force. Plus, the resulting out of balance condition from that would certainly cause more damage.
Buy a new blade.
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Old 05-06-2019, 10:10:29 PM
K-Tron K-Tron is online now
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Default Re: How bent is too bent - mower crankshafts

I never considered bending the blade straight again. I think some of you guys need to put your reading glasses on! I pulled out four mowers of this same type that I have used commercially over the last few years and none of them had a bent crankshaft. They were all within 0.003" of runout. I determined that Briggs is full of it with a 0.0005" maximum allowable runout. There is three times that figure between the crank and the oil sump as a running clearance. I find it incredibly hard to believe that this small incident caused the crankshaft on this engine to bend as much as it did. I pulled down my trusty 1985 Snapper 21401P with the 4hp Briggs 110902 engine and the crankshaft had just 0.009" runout. I pretty much intentionally mowed anything and everything in its path with that thing. It ran a Pac-N-Sac, a Snapperizer and winged blades for 18 years. It would run full throttle and damn near lug down to stall tens of thousands of times mulching branches, leaves and the occasional rocks and roots. I wore out three sets of valves and guides in that engine. That one mower almost single handedly paid off all of my college debt, which was in the six figures. I am disappointed in this Briggs 126702. One small mishap and the crank is bent 0.022". This has to be partially due to recycled content being used in these modern crankshafts. I will tell you first hand that my Snapper has hit and sheared hundreds of more roots than this Briggs with just 73 hours on it. The engine will come apart tomorrow for a full diagnosis.



Chris
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  #25  
Old 05-06-2019, 10:38:44 PM
Tracy T Tracy T is offline
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Default Re: How bent is too bent - mower crankshafts

trust me you will be disapointed when you get inside! some things will be obivous! do a full observation, some things take some close looking. the last time i was in a new briggs was 20 years ago! and it was bad back then. take some pictures, i gave up on briggs 20 years ago! so i am very interested to see what practice they are following now!
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  #26  
Old 05-07-2019, 09:12:04 AM
kwfiggatt kwfiggatt is offline
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Default Re: How bent is too bent - mower crankshafts

Quote:
Originally Posted by Tracy T View Post
trust me you will be disapointed when you get inside! some things will be obivous! do a full observation, some things take some close looking. the last time i was in a new briggs was 20 years ago! and it was bad back then. take some pictures, i gave up on briggs 20 years ago! so i am very interested to see what practice they are following now!
Yeah, I think it's amazing that the newer briggs engines don't even have an oil drain plug anymore. WTF? There's no way to drain the oil except to either vacuum it out or turn it over and drain it out of the fill tube, while spilling the contents of the gas tank, too.

I just bought a $100 212cc, ~6.5 hp predator engine (harbor freight Honda clone) for my son's go-kart and it appears to be MUCH better than what the briggs engines are nowdays... Then again, I'm sure the push mower factory doesn't pay $100 for a briggs engine, either...

Kevin
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Old 05-07-2019, 09:16:47 AM
Bruce Dorsi Bruce Dorsi is offline
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Default Re: How bent is too bent - mower crankshafts

There is the "theoretical" and then there is the "practical."

I think that guy "Mike" may have been blowing smoke up your butt. ....I have never heard of B&S replacing a bent crank under warranty on a rotary mower. ...The .0005" run-out may be the spec as the c/s is being machined, but certainly not as installed in the engine.

Before going too far, I would install a new blade, and with the plug out, rotate the crankshaft noting where the tip of the blade is, in relation to a point on the mower housing. ....Now, rotate the crankshaft 180* and see where the other tip of the blade is in relation to that same spot on the mower deck. ...If the deviation is 1/8" or less vertically, keep using the mower. ....Lateral deviation should be limited to the ~.020" bend in the c/s.

We had a crank straightener in our shop. ....Long shafts straightened easily. ...The more they were bent, the easier they were to straighten. ....Short shafts, not much luck straightening.
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  #28  
Old 05-07-2019, 09:40:38 AM
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Default Re: How bent is too bent - mower crankshafts

Quote:
Originally Posted by Bruce Dorsi View Post
There is the "theoretical" and then there is the "practical."

I think that guy "Mike" may have been blowing smoke up your butt. ....I have never heard of B&S replacing a bent crank under warranty on a rotary mower. ...The .0005" run-out may be the spec as the c/s is being machined, but certainly not as installed in the engine.

Before going too far, I would install a new blade, and with the plug out, rotate the crankshaft noting where the tip of the blade is, in relation to a point on the mower housing. ....Now, rotate the crankshaft 180* and see where the other tip of the blade is in relation to that same spot on the mower deck. ...If the deviation is 1/8" or less vertically, keep using the mower. ....Lateral deviation should be limited to the ~.020" bend in the c/s.

We had a crank straightener in our shop. ....Long shafts straightened easily. ...The more they were bent, the easier they were to straighten. ....Short shafts, not much luck straightening.
I would like to add one thing. Rotate blade 180 deg and check again! I rarely see new blades that are perfectly straight, and used blades always have a slight bend. The slightly bent shaft and imperfect blade can be installed so that they subtract from each other to give minimum run out.

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  #29  
Old 05-07-2019, 09:49:05 AM
Tracy T Tracy T is offline
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Default Re: How bent is too bent - mower crankshafts

K-Tron, wouldnt you divide the number you came up with regarding the runout by 2? that would put the actual bend at .011, still too much IMHO.
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  #30  
Old 05-07-2019, 09:08:03 PM
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Default Re: How bent is too bent - mower crankshafts

I decided against pulling this engine apart for now since I have no clear workspace to tackle another project. Instead I grabbed a good flywheel off of another mower engine, replaced the flywheel key and gave it a whirl. The engine started right up and ran surprisingly smooth considering how bent the crankshaft is. There is about 3/16" of vertical travel in a known straight blade on this mower. I had to run my near identical kool-bore version of this mower side by side to test for vibration and noise. The I/C engine always rattled and vibrated more than the kool-bore from day one, but now it is significantly worse. I have some good video of when this I/C engine was new, it definitely ran smoother than it does now. Based on my extensive testing I would say that any crank with under 0.005" of runout is perfect. Running a crankshaft with 0.022" of runout is just asking for trouble. I would never suggest or consider straightening a crankshaft on a mower. Unfortunately all of my offers on the crankshaft I wanted to replace this one with were rejected, so the hunt continues. I may just buy a NOS kool-bore shortblock and swap the crankshaft into my I/C block. You can practically buy the entire kool-bore shortblock for what you can buy a crankshaft for.





Chris
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  #31  
Old 05-08-2019, 12:57:04 AM
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Default Re: How bent is too bent - mower crankshafts

In your picture, looks like Starboard side of blade has a tad more pin engagement than the Port side. Turn blade 180 deg and try again- maybe a little better. The other thing I have done is to clean back of high side of blade where it contacts pad to bright metal, along with high side of pad. That reduces runnout by a few thousandths.
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  #32  
Old 05-08-2019, 12:57:41 AM
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Default Re: How bent is too bent - mower crankshafts

Here’s a valuable little tool that I’ve had for years.
I’ve had brand new blades noticeably out of balance.
Don’t want to have any more reason for vibration than necessary.
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  #33  
Old 05-08-2019, 10:54:40 AM
Andy Williams Andy Williams is offline
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Default Re: How bent is too bent - mower crankshafts

the easiest fix for the problem is just to grind a bit off the end of the blade on the side that is to the bend . because of the bend the blade will not be centered in the deck. i had an old 50s Reo power trim that had a bent shaft. by doing this the whole assembly is kind of balanced and it stopped the vibration . i know it is not the correct way to do it but it works. i still use it and no problem.
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  #34  
Old 05-08-2019, 10:44:54 PM
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Default Re: How bent is too bent - mower crankshafts

Power, you have a good eye. I had to go out and double check that the blade was torqued down straight and flush. It sure was, but it does not look that way in the video. I ended up stripping the entire engine down for inspection. It looks brand new inside. There is no wear on any of the rotating surfaces. Knowing that the oil sump, connecting rod, camshaft, piston and block bushing look perfect I ordered a new crankshaft and gasket kit.

Chris
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  #35  
Old 05-17-2019, 07:40:09 PM
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Default Re: How bent is too bent - mower crankshafts

My 1993 Briggs & Stratton 126702 is all completely rebuilt with a new crankshaft, gaskets and seal set. It has been completely stripped down, bead blasted and ultrasonically cleaned before re-honing the cylinder, cutting the valve seats, lapping the valves and grinding the valve stems for proper valve lash. I did not find any wear inside this engine, every rotating surface was immaculate and measured up right at specification. Every bolt has been torqued to factory specification after chasing the threads. $73 and two days of work and it runs better than it did when the engine was new.



Chris
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  #36  
Old 05-19-2019, 03:02:19 AM
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Default Re: How Bent Is Too Bent - Mower Crankshafts

Here is a crankshaft straightener I made to fix a bent Kawasaki shaft (John Deere push mower).

It doesn't put any strain on the main bearing as there are two bolts. One (with allen key) is used to bend the shaft, the other (long, hex head) at 180 degrees and near the main bearing, taking the strain.

I copied the basic idea from those seen on the internet, but added a couple of holes so the "clock" (dial indicator) can be used to check as you go, without removing the straightening device.

It worked perfectly, though admittedly it was a long shaft.
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