G'day Norm, Gadge and Mowerfreak.

No stockpile scenario here.
I believe Victa worked to a deliberate marketing plan.

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How long until all full cranks were 160?
Recent research I conducted seems to indicate this:-

The demise of the 125cc occurred in 1977 prior to the 1977-78 range year.

The evidence and the logic support this.
Victa's 2-stroke range would have included 5 variants without the demise
of the 125cc.

In late 1977 (for the 1978 year) Victa introduced a new small engine -
the 85cc 'half-twin' used on the Lightweight and Compact lawnmowers.
The line-up would be: 160cc Power Torque, 160cc full-crank, 170cc Twin cylinder,
and 85cc single. The 125cc had to go; given that Victa would concentrate upon
just one engine - the 160cc half-crank - within just a couple of years.

Also note that the imperial reel mower were dropped in 1977,
a user of the 125cc.

The 125cc - Victa's original engine - would be no more.

The rest is history.
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Jack