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I’ve been trying to find somewhere that states the minimum blade dimensions or minimum reel diameter of a usable reel, ie. I need to know how much, in any life I have left in the reel on the mower I am restoring.
Hello madnkn and Glo Mod BB
I'm guessing this is a general question about reel life.
As BB said, an image would be helpful for informed comment....

The two issues here are:
[1] how far can a reel be ground?
[2] what is the life of a reel?

Reels can be ground right down to just above the supporting collars.
[collars may be called 'webs', 'spiders' or whatever!]

The life of a reel is proportionate to use, grinds, and severity of the grinding process,
and the expertise on the set of reel to bottom blade.

In other words, a definitive answer is simply not possible.

I have owned a mower that held an original reel, still with life in the reel,
and it was a century old!

A professional reel may last just a few seasons.
Domestic reel life will vary wildly, depending on a few factors.
Certainly, the quality of 'set' of the user (reel to bottom blade) and the training
of the re-grinder in grinding the blades is crucial to component life.

I have heard of horror stories about this.
I worked as a tradesman for Ransomes machinery, so I feel I have
a competent understanding of the theory and practice.

As a general stab in the dark, if you have 10mm of blade above the collars
you have a reel that will probably see you out, fingers crossed grin

I hope this helps
-------------------------------
Jack