Why won't my cassette tool not fit?

^
A different tool to would be ideal - ask your lbs because you might want to try it first.
Alternatively, get a steel shimano lockring - that will work better.

With the tool in the vice, you can use the rim to tighten the lockring.
I give it a firm turn, but no need to yank. You'll feel it tighten down.
It's there really to snug the cassette onto the fhb and hold it there - riding doesn't produce forces to unscrew it.

I've found a cassette tool with a locating pin really helps with this.
Like this one.

Thanks guys. I have reservations about another tool fitting. But I'm going to order some (I can always send back). Failing that I'll try the methods advised.

Thanks!

You could get a length of m5 Threaded rod from a hardware shop and bolt that through with a couple of nuts&washers to hold your tool in place... hey presto!

Yep.

So what did they do back when this stuff was new? Cant imagine all these 'bodge jobs'

Unless the lock rings and tools differed slightly.

That's it.

If you had the period shimano cassette and tool (honestly a modern one would probably be right too) it would fit right.
But that hub is 30 years old,
and the cassette (lockring at least)
is from another company, and neither of them made the tool -
And it's a generation later.

The very fact it still fits at all is amazing.

There's almost no other industry where this is the case.

Bought a parktools tool (short stubby one) the splines slipped in further and the quick release also went through enabling me to hold it in place while I tightened it down

I clamped the socket bar in my workmate (a vice works too) then put the wheel on it and using the wheel as both leverage and a weight holding itself down onto the tool and just turned it off with not a great deal of effort

Contrary to what's been posted and speculated about patents, the Sram and Shimano lockring patterns are the same. There is no patented design or design registration of this feature.
The Shimano freehub patent was poorly written, and all they were able to protect was the method of fitting the freehub body to the hub shell with the hollow bolt. Hence in the early nineties a plethora of the lightweight CNC outfits (Paul, Phil, Ringlé, TNT, WI , Nuke Proof etc etc etc) all started producing hubs that both a Shimano cassette would fit on to, and that it would also be attached with the conventional lockring. What they all avoided was retaining their freehub body with a hollow bolt.
So, what a lot of them also suffered with was difficulty in tighteneing and loosening the lockrings, unless you either remove spacers or nuts and washers.

Your existing cassette lockring tool will fit your early nineties cassette, you just need to remove the axle first by removing the non-driveside locknut and cone.

As other have speculated, some tools have very thin walls, some don't. Thin wall is useful for the spatially challenged, but also more likely to crumble with an over-tightened or seized lockring.