I saw on a question on PSI that the ID on pencils are different than the pens. You have to order a specific 8mm tube
I'd be curious to hear more details on this (before it sneaks up and bites me in the ass).
I don't have any 8mm pencil kits handy, but I just measured a couple slimline "7mm" tubes (non-PSI, from Exotic Blanks) and the pen & pencil tube IDs were too close to measure a difference (at least on my cheap vernier calipers - I don't have a small enough telescoping gauge to fit inside). The tubes in both kits were in the range of 6.275-6.3mm.
The OD (measured with a mic) varied even within a kit, though - the pencil had one at ~6.74mm and the other at ~6.78mm (while the pen's were both ~6.75mm), so I suppose there's a chance it's a quality control/tolerance issue, rather than a different spec. Or maybe it's just the 8mm kits. Or maybe it's just PSI kits. Or maybe I'm just guessing and have no real clue.
Hrm... although it does occur to me that when I ordered from EB they were selling the generic 10" 8mm tube pack as spares for the 8mm DuraClick pencil (which
is a PSI kit), which suggests they're at least
meant to be the same... but I didn't think to compare measurements before making up the kit.
-Bats
(probably just guessing. definitely no clue)