Is there a standard for pen turning bushes?

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Pen mandrels are often referred to as 7mm as that is the size of the hole you drill to fit a 7mm tube (actually a touch smaller or it wouldn't go in the hole) for kits like the slimline. The actual size of the mandrel is 0.247"+/- 0.001"(6.274mm) depending on the seller. The bushings that slide over the mandrel are a touch bigger or they wouldn't fit.
Did that help or muddle you up more?

Pete
 
To be honest I avoid the entire mandrel/bushing size stuff by turning between centres (TBC). I use stock bushings to get close to size and then remove them and finish off between the centres only, checking the sizes with a caliper. Might take a few minutes longer but I don’t have any out of round problems either.
Pete.
 
Phil is correct mandrels are not 7mm (not in the UK anyway), you drill a 7mm hole in the blank to suit the outside dia of say a slimline tube. However even though 1/4" is standard or more often 6.25mm which is slightly less, they are often not especially accurately machined, bushes can vary a bit as well.
 
It all depends on the pen kit and where it is from as the bushes are drilled out (As already stated) with either a 1/4" hole of a 6mm. Some that I have brought in the past from Axminster tools have had the 6mm which I drilled out to 1/4" as that is my mandrel size. I have however gone to turning between centres using some tapered bushes and measure the kit and turn to that it save having to buy bushes every time I turn a new pen design.
And now if I do decide to make a run of pens I can turn up my own bushes
 

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