In article om,
"Dave Liquorice" says...
On Mon, 4 Apr 2005 16:43:28 +0100, Rob Morley wrote:

It doesn't need to be a cone - just a cylinder long enough not to
fall into the pipe that the plunger is stuck in.

True but how long is that?

If he pulls the plunger back as far as it will go then stops it
moving forward again, then not very long at all - but it would
probably need to be a bit longer to discourage it from twisting
sideways and jamming, as the rod pushing it would be substantially
off-centre. The longest it would need to be is the amount that the
stuck rods can be moved backwards and forwards, plus a bit.

And it's only a guess that the plunger has
entered another pipe and not just gone past a badly aligned join.

Same difference really - the plunger needs to be aligned centrally in
the pipe, the cylinder will do that.