G'day folks,
I'd say that the blade disc failure is a Masport issue for sure. Design for a start, in that the original design didn't handle the shear stress concentration, which naturally occurs in the blade disc, around the periphery of the boss.
The crankshaft failures are a bit more complex, and could be down to Masport, B&S, or just an unfortunate combination of both.
Time to go into some OPE industry history, I feel!
Older US-made Briggs crankshaft PTO's would bend a fair bit when an object strike occurred, without fracturing.
Masport recognised this issue, when they made a big push into the Oz market in about 1975. That was just after they had taken over Whirlwind P/L.
Bar blades were not used by any of the then Oz push mower manufacturers, so bent PTO's were not a common issue, until Masport and their bar blades. Masport consequently required their dealers to buy a 'crankshaft straightener' [a bench mounted 'special tool'], which I have described elsewhere on ODK.
It was a 'crank out of engine' setup, and the usual bend point was just below the oil seal. The amount of bend usually wasn't sufficient to prevent removal of the sump, and these would straighten to within spec easily.
What I'm getting at here, is that older Briggs crankshafts seem to have been heat treated more for 'toughness' than 'hardness', on the PTO ends at least. These two properties are essentially inversely related; i.e. as one increases, the other decreases.
These fractures indicate that heat treatment for a 'tough PTO end' may no longer be the case, with the Chinese-made Briggs cranks. Decreased toughness/increased hardness results in a greater tendency to brittle fracturing, which is what we may well be seeing here.
The inner top edge of the boss forms a bending stress concentration point, by its nature, so that's why some of the failures occur at that point on the shaft.
