The Dirty Little Secret about Cub Cadet Walk-Behinds

[HarmonySeeker]

Having successfully dealt with the Honda Harmony 216 blade clutch repair, my next candidate is a Cub Cadet walk-behind, SRC 621 with Honda motor and caster wheels.
I am researching how to remove the bolt securing the blade assembly to the motor shaft, which has broken about 1/4" inside the shaft.
So putting the search terms into YouTube, lo and behold, the mower with this problem, by and large, IS the SRC 621.
So I kick it out to the ol' IH gang here on the forum: What's causing this and has anyone stumbled upon the real cure?
Many Thanks!

 

[Romore]

Either the bolt is too soft or it was overtightened.

 

[HarmonySeeker]

I have considered the possibility. But a hardened bolt would be more 'brittle', you do realize?
A 'softer' material (or more 'ductile' as we say in the business) would be less likely to fail at impact.
I removed the blade and personally tightened the bolt with a 3/8" socket wrench a week before it broke, admittedly without a torque wrench, but I applied no mechanical levers during tightening.
I'm 62. I have two bad shoulders and a bad elbow, so I sincerely doubt I over-tightened, but cannot masculinely deny the possibility.?
And it appears to have happened before, as the self-propeller pulley and adapter have '9.99' written on it in paintstick and is remarkably shinier than an under-deck component should be.
An interesting side point that just occurred to me is the bolt used on the Honda Harmony I just fixed has a 'special' code bolt affixing the blade to the shaft.
Thanks for the hint. I'll see if I can find that code on the bolt I need.

 

[bertsmobile1]

Not quite right.
Hardness & brittleness do not go hand in hand
While we all use the word hard for a stronger bolt what we really mean is a higher tensile strength bolt .
And a grade 8 bolt is both harder & tougher than a grade 5 bolt which is the standard strength bolt .
When it comes to nuts & bolts what makes them harder is a stronger grade of steel is used to make them from in the first place .
So Ronmore is fairly well on the mark.
What he missed is a bolt that is too long .
Or in the case of mowers a lazy person with a big impact driver who does not bother to clean the hole before shoving the bolt back in.
Mower blade bolts are designed to tighten in use but Bozo the clown does them up to 220 ft lbs because that is the highest setting on his impact then wonders why it breaks off when he tries to undo it.
Now it is not beyond the relems of reality that Cub got a bad batch of bolts or even the engine supplier sent a batch of engines where the bolt hole was just a fraction too short .

However you are the very first & only person who has come here with a broken blade mount bolt.
Your post has been up for a while and the dealers on the forum have not come back with a Cub recall notice or dealer service bulletin .
Thus I would speculate that the "problem en mass " is an owner / operator problem & not a factory problem and close to the IQ of people who post on U -Tube which for the bulk of what I have seen is slightly lower than your average Resis monkey .
The blade retaining bolt should be a grade 8 bolt and would usually be a hot rolled bolt to boot.
Normal ( grade 5 ) bolts are actually made from a far lower grade of steel than they used to be when they were cut because the thread rolling is a cold forging process and that increases the strength & hardness of threaded length while leaving the strait section soft which is why they generally break either just under the head or on the first turn of thread.

In your case I would imagine you just got a bad bolt .
Down here it is SOP to replace blade bolts whenever you replace the blade.

Honda, along with most japanese automotive bases companies order their bolts with the torque stamped into the head that 8.8 is 8.8Nm and not the grade of the bolt.

 

[HarmonySeeker]

'However you are the very first & only person who has come here with a broken blade mount bolt.'
Perhaps everyone else found their answer on YouTube, perchance.