# Best drill bit for housing threads?



## brownsfn2 (Feb 1, 2015)

I have several Bock housings and I have the m7.9 x .6 tap that goes with them from Classic Nib.  

Can someone recommend a drill bit size for this?  I have tried "L" and one other size but having trouble. 

Thanks for any help.


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## lorbay (Feb 1, 2015)

7.3
Lin


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## randyrls (Feb 1, 2015)

Check out MSC


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## Carl Fisher (Feb 2, 2015)

I put together a quick reference sheet a while back that has most of this, but the easy method is to subtract the pitch from the diameter to get your drill bit size.

So officially 7.9 - 0.6 = 7.3 as mentioned above.  I use a 7.25 because it's a bit more common.

http://content.penturners.org/library/general_reference/drillbit_alternatives.pdf


* Note there is one error it appears in the spreadsheet and that's for the first size in the tap/die section.  That should be 6mm not 5.5.


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## mredburn (Feb 2, 2015)

7.3mm is .287. I always go a hair larger on the hole because the threads are not always fully formed or sometimes the feed is out of round from the manufacturer. Small amounts but it can affect the way it screws in. i would go with an "L" drill bit. 7.4mm is .2915 and if you measure the drill bit it is probably undersized to start with. Mine are all a couple thousand under the stated sizes.  There is a great chart in the library that I printed and keep on my drill storage box. I usually have  a copy of it on my desktop for printing or reference as well.


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## brownsfn2 (Feb 2, 2015)

Thanks for the link.  I have not seen that before. 

I am using an "L" bit which is 7.366mm.  It seems like the threads are just not deep enough.   I was doubting that .066 makes a difference but maybe it does.  

What I might also be dealing with is the fact that my bits for those sizes are Harbor Freight.  They might not be running true making the hole slightly bigger.

Last night I ordered 7.2 and 7.3 mm bits hoping that I can dial in some accuracy here.  I am hoping it is just my junk drill bits. 

Thanks for all the help everyone!


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## mredburn (Feb 2, 2015)

Here is the pdf version from the library
http://content.penturners.org/library/general_reference/conversions.pdf


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## mredburn (Feb 2, 2015)

Not all threads on things are cut or formed to full threads. They can be 75% or less of the stated pitch.  That means the depth of the thread is not fully formed and you will need a bigger hole.  That .066 is enough to make a difference. Especially on fine pitch threads. I use a die on the Jowo feeds when I use them. It will surprise you how much they trim back. THere can be small fins, out or round housings or shallow threads. 
If your feed seems to drag and then release and then drag again as you screw it in its probably out of round a few thousandths.  I suppose I should have some dies made up for the Bock feeds.
THe 8.5 x 1 threads on most Sierra clones are not fully formed for depth and need larger holes as well.


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## Carl Fisher (Feb 2, 2015)

I've noticed that with the OOR feed housings.  Seems to be fairly common.


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## mredburn (Feb 2, 2015)

And one more thing.  IF the tap is Taper tap you may not be tapping the hole deep enough to cut full depth threads. Try running the tap all the way in (if you can)and see if that makes a difference.


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## Paul in OKC (Feb 2, 2015)

Id threads generally will not be full formed.


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## yort81 (Feb 4, 2015)

brownsfn2 said:


> I have several Bock housings and I have the m7.9 x .6 tap that goes with them from Classic Nib.
> 
> Can someone recommend a drill bit size for this?  I have tried "L" and one other size but having trouble.
> 
> Thanks for any help.



*For my sections using the Bock 7.9 x .6 tap.... 

I use a "G" Bit.... "L" Bit.... and a "P" bit

The "G" Bit all the way through
The "L" Bit a bit more than 3/4 the way through (this one is for the threads)
The "P" Bit a bit more than 1/2 way through
*
*The Fit is exceptional!*


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