What is the reccomended way of sizing a pen sac? If the nippple is a 12, do I use a 12 sac outside dia, or do you use a 10 sac o/d.

I know it is perceived wisdon to go down 2 sizes from the nipple dia. Is this though, from the internal or external dia of the sac, which thickness is going to be 1/64"-1/32" thick.

Thanks,
et

You're working on an incorrect assumption here.

Because some vendors sell sacs in even number sizes only, we think "oh, a 16 fits, I need to go down two sizes, so I should use a 12," with the result that you have a sac that's way too small for the pen. I can't tell you how many times I've found a ridiculously undersized sac in an oversized pen.

Sacs are made in even and odd number sizes - 14, 15, 16, 17 18 etc. So if you want to go down two sizes, use the next even (or odd if that's what you're going to use) size down.

Go with what fits easily into the pen. It should stretch a bit as you fit it on the nipple, but don't use one that's very much undersized.

No Ron, to go down 2 sizes I was going from a 12 to a 10. Point and question is do you take the internal or external dia. of the sac as the size for it. Nipple size 12, therefore sac external size 10. It's a rule of thumb I know, also it's just as bad to be putting a sac on that's too big as too small.

I have a knockout block that rises in increments of 1/32", from 8 to 32, which I have marked their sizes, making it easy to size sacs and nipples, lot of that one fits etc... went on before.

I appreciate that sacs also come in odd sizes, i.e. 1/64" increments, but they do tend to be a bit like hens teeth over here.

All the best,
Eric

Don't go down from the nipple size; go down from the barrel.
What I mean is, and what I do: measure the nipple. That's your sac size in 64ths of an inch. (So, 1/4" nipple = 16/64" = #16 sacs.) Some people go down 1 sac size, but two is too much unless it doesn't fit in the barrel well.
If you don't have an easy way to measure the nipple or it's not going well for some reason, see what slides into the barrel without forcing but without extra space, and then go down two sizes from that. (So, if an #18 just fits in the barrel, use a #16).
Sacs are sized by the nipple they're supposed to go on, not directly by their own size.

All perceived wisdom and literature is to go down two sizes from the nipple size.

All I am asking is this the internal or external size of the sac, which would be around 1/64" thick, therefore a differance of two sac sizes from internal to external sac size.

??????????
et

Again, it's neither. It's the diameter of the section nipple it's supposed to go on. And "all perceived wisdom and literature" is not two sizes down from nipple, it's two down from barrel. So says Richard Binder, and this thread, and this one, and I'm sure several others.

I'm with Jinnayah on this one and I use an even less scientific approach in that I don't even attempt to measure the nipple size - I believe it to be inconsequential. I find the sac that's a snug fit in the barrel, drop down two sizes, cut it to length, shellac it on the nipple and call it good. I keep a variety of sacs on hand, size 14 - 20 and that works for most any pen I run across. If I need a bigger sac than a 20 I'll count myself lucky and give the Pen Sac Co. a call.

As far as the OD/ID question, from the Pen Sac Company site: "The size of a pen sac is designated by two numbers: the outside diameter or O.D., expressed in 64ths of an inch, and the length, in inches. Thus, a 15 x 2 1/4 sac is 15/64ths of an inch outside diameter and 2 1/4 inches in length."