Jessem Paralign doweling Jig Drill Bit

[Bill Huber]

9.7mm is too loose for 3/8.
9.6mm works fine.

I think a lot of it depends on where you get your dowels and where you live, the humidity levels in your area.
The dowels I have if I use a 3/8" bit I have to hammer them in and that is just not very good to get the glue all around the dowel.

[mreza Salav]

It could be true Bill. But 9.7mm is about 0.006" larger than 3/8". If you make a M/T joint with that much gap you'll see how loose they are.
My dowels are mostly very close to 3/8" and I find 9.7mm way too loose (like they'll fall out of the hole if you keep the piece up side down). That is not right IMO.
If your dowels are larger than 3/8" then the fit might be good.

[Chris Padilla]

Let's see here:

3/8" = 0.375" = 9.525 mm
9.6 mm = 0.378"
9.7 mm = 0.382"
V bit = 0.377" = 9.5758 mm

If we knew what KIND of wood the dowels were made of, we could look up info on how much they could shrink or grow depending upon moisture content. At these tiny changes between sizes, I'm almost CERTAIN the local humidity and wood species plays a significant role in the dowel's effective size.

[Bill Huber]

It could be true Bill. But 9.7mm is about 0.006" larger than 3/8". If you make a M/T joint with that much gap you'll see how loose they are.
My dowels are mostly very close to 3/8" and I find 9.7mm way too loose (like they'll fall out of the hole if you keep the piece up side down). That is not right IMO.
If your dowels are larger than 3/8" then the fit might be good.

I understand what you are saying and if you notice I also said in post number 13 that I have to pull them out with pliers so I would thing that is tight enough, wouldn't you?

[Mel Fulks]

That's what we liked about the plates I described, the dowels go in easily and glue gets between dowel and work. You can
make a test piece ,then rip it along length of dowel and find it's glued so well that you can break dowel in half and find you are removing big chunks of the structure. Even when we bought long dowel lengths in quantity their sizes varied.

[Jack Clay]

Thanks to all for the help.

[mreza Salav]

I understand what you are saying and if you notice I also said in post number 13 that I have to pull them out with pliers so I would thing that is tight enough, wouldn't you?

That would be tight. Maybe our dowels have different sizes or maybe your climate is a lot more humid than ours.
Anyway, I think if the OP has too loose of hole the proper solution is to get a smaller drill bit that is closer to the dowels he uses.