Hello Hermosil

There is some design 'flexibity, particularly with vert crank engines.

Manufacturers would typically offer a couple of different specs.
These would involve short or long crank lengths and a couple of
different crank widths.

The Viking was SB's high arch design (Model 520 or 720).
The bottom line is that any engine, boss or blade holder should place
the blade tips about equal height with the bottom of the base skirt.

I have seen Scott Bonnars with spacers to adjust the height - and these
came from the factory.

Below is a 520 and a 720 base.
Note how the first has spacers; the second seems to have a spacer plate.
That is promising in that either a short or long crank is possible.

I might add, in many cases, different cranks were not a hindrance.
Offering different engines for the same base was not restricted by
a specific crank. This enabled mower makers to best utilize
their inventory - they could use, say, a 3hp engine with a
No 1 crank; and a 4hp engine with a No 2 crank- on the same base!

[Linked Image]
[Linked Image]

So, the idea is to match and engine with a recognised boss
and blade holder. If the shaft is too long it will require spaces.

If the blades are way below the base, the machine will not catch
efficiently and will be unsafe. If way above the skirt, you will not
be able to cut short grass.

[Linked Image]

Hope this helps.
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Jack