Everything needs to fit, it's true. But in reality there's no need to get too hung up on tiny measurements. I've been elasticating poles for decades have dozens of topkits but I've never measured a thing and you can be pragmatic about it and do it all by eye.
First, look at the tip section of the topkit you plan to elasticate and decide if and how much you need to cut it back relative to the size and strength of the elastic you plan to use. The thicker the elastic, the more you cut back.
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You can get the bush you judge likely to be right for your elastic and cut the tip to fit the bush. Or, cut the tip, go to your ts and ask for a bush to fit THIS, or take the whole thing to your ts and say " Can you cut and bush this to suit an elastic of around x?" The fit between elastic, tip and bush is not critical - you just need a fit that allows it to flow freely.
If you're doing it yourself, weigh the tip and bush up by eye and cut the tip down a bit at a time and test it each time until you get a nice fit. You'll find the bush will often "screw" rather than push on. Don't lop a big piece off in one go - you might find your tip is now too big for the bush ooer madam. One tip if you're cutting carbon tube: don't use a hacksaw. The teeth are very likely to snag and pull a split in the carbon. Slowly grind your way around with the edge of a file or something similar. One of those special little saws with tiny serrations is ideal if you have one, The carbon is surprisingly soft. You can tidy the cut end with a bit of fine wet and dry.
The bungs at the other end come in basically two sizes, according to how wide your topkit base is , are sectioned for easy cutting with a Stanley knife, and you fit them by cutting and testing until they fit where you want them - a couple of inches above where your female joint reaches, You'll need an extractor - a hooked bit of stiff wire - to reach in and pull them out.
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There are lots of step-by-step instructions online. Mandarin?
As a bit of angling diy, it's a small job. I've watched your mole trap video's Clive, and figuring those out is way trickier. It's one of those things that just looks baffling til you've done it once and see how it all works. And trickier if you have no ts around and you're trying to get bits online unable to inspect their physical sizes.
I may be misreading what you meant, Mark, but don't be tempted to tie a length of elastic to the end of the pole. When they first camemout in the 70's we used to stick a little aluminium crook in the pole end, hang a foot of braided elastic on that and attach the rig to the elastic. The outcome of having your rig dancing about on the exposed elastic was just what you'd imagine. Hopeless. The elastic need to run inside the tip and only come out with a fish on the end.