Vent liner install

[JamesT]

Currently installing a 5\16"-32 Chambers vent liner in a .54 cal 1 1\8" breach barrel. I had a total brain glitch. I referenced my tap and die set for the drill size which called for a 9\32"(.2812). The liner calls out a letter "I"(.2720). I have drilled the hole but havnt cut threads yet. Is this a safety issue and if so what are my options?

[rich pierce]

Yer good.

[DGB]

I would use that same drill you used and drill and tap a piece of scrap to see how lose it is.
You should have enough barrel size to go to the 3/8 White Lightning so you sleep better at night.

DGB

[P.Bigham]

Good idea Dave.  If I did the math correctly your only talking of a difference of 5 hair widths to give a visual. 

[Scota4570]

https://www.efunda.com/designstandards/screws/unified.cfm?start=64&finish=147

The major and minor diameters of 5/16 x 32 TPI NS  are 0.312" and 0.275".  To get a full thread you need to start with a 0.275" hole.  The threads are 0.018" deep per side.  IF you drilled oversized with a 0.281 drill you get a hole 0.281 or a bit larger.  281 -275 = .006".  divide 2 and you get 0.003".  3 / 18 = 16% less than a full thread.   

Keep in mind drills love to cut oversized.  Unless you made a pilot hole your  final hole will be larger than the drill bit.  If it did you have sacrificed more engagement than what I calculated. 

I do not know how much effect 16% has on the ability of the liner to resist blowing out.  I make my liners and breech plugs with an interference thread fit.  I am paranoid and obsessive about safety matters.   It was also declared good enough by another here with more experience than I have.  I have no opinion other than if it were mine I'd install a 3/8" liner. That is based on my practice on doing all safety related operations to the best of my ability every time. 

Perhaps measure the hole and see what you have?     

[JamesT]

Thank you all for your advice!

[577SXS]

Why take a chance, go with a 3/8" liner.

[smallpatch]

My tap chart shows 9/32 just like yours for a 5/16-32.  It shows letter I for 5/16-24.
I’ve always used letter I but the chart shows 9/32 for 75% thread.
I’d use it.

[Bob Roller]

My tap chart shows 9/32 just like yours for a 5/16-32.  It shows letter I for 5/16-24.
I’ve always used letter I but the chart shows 9/32 for 75% thread.
I’d use it.

I would also use the "I" size drill and then carefully countersink the hole the depth of one thread.
A few drops of oil or tapmatic if you have any.I also start this kind of drilling with a center drill so
wandering by the point of the drill is elemenated.
Bob Roller

[J. Talbert]

Not trying to pile on, but this is one of the reasons I routinely use the 1/4 “ vent liners.  They work quite satisfactorily  and they preserve the option to go bigger in the future if needed.
Jeff

[577SXS]

If tap drill chart shows 9/32 tap drill size then don't worry about it. I thought about this and we forget that this is a 5/16-32 thread, 32 TPI would require a slightly larger hole than 28 TPI. When you screw the liner in if it tightens up and doesn't strip you're good. Anyway I think you are fine and you should use some locktight on it anyway.