I have just had the mast foot replaced on my Canterbury tales. The new foot is smaller than the old one, which I no longer have as a reference point. Is there a baseline measurement that people use for the position of the mast foot? (do I pack the front of the mast step or the back?). Furthermore, how much slack should be allowed at the mast foot to allow for raking the mast?

There are quite a few measurements for you in this forum thread

I think that the new mast foot is about 10mm shorter. I was a bit worried about it too, but the way it sits in my step it makes almost no differance as there is a tufnol wedge that stops the mast jumping out of the step with extreme rake. Since this limits the position of the front face of the mast the length of the tenon heel doesn't matter as the mast will automatically sit where it was.

The best way around this is to put another stop in your mast step in the middle. Then cut a piece out of the middle of the tenon heel on the mast so that it sits roughly where it did before, IE with about a 5mm gap front and back. You may find that you can jiggle the front or back stop so that it rests against one of these too, but you won't it to fit against both. Remember while this measurement is critical, 1 or 2 mm here or there will not matter too much.

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Tenon heel will look like this:  |____|  |____| for want of a better way of showing a diagram!