# Anyone know what the perfect drill size bit is for an M8 threaded rod?



## Shesho (Jun 12, 2012)

I'm making some spool clamps using an M8 threaded rod. I'm trying to get the hole to match the rod as close as possible but at the moment I only have an 8mm and 8.5mm drill bit and neither of these give a perfect fit.

I was wondering if anyone had experience of what I need to use.

Thanks


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## Dave Paine (May 30, 2012)

I expect rod diameter can vary slightly based on whether this was made as a metric version of an imperial dye or DIN.

Since you have the rod, why not measure the diameter with a pair of calipers.

You do not state if the 8 or 8.5mm drill bits are making a hole which is too large or too small.


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## Fastback (Sep 2, 2012)

It would be helpful to know what the thread pitch is for example 1.0 or 1.25 etc. I will check when I get back into the machine shop and let you know. Do you have a tap? If I remember correctly M8 usually 1.25. You can get a set of metric taps at Lowes. HF has them, but they are crap. They are OK as long as you are doing something soft like aluminum, brass or hardwood.


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## FrankC (Aug 24, 2012)

According to my table for both thread pitches it is 9mm or a letter T bit, but this will depend on how your die cuts, especially if it is an adjustable die.


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## ponch37300 (Feb 27, 2008)

Lots of places sell calipers and mics for cheap, harbor freight is one. Even the cheap ones will get you a fairly close measurement, unless you need accurasy in the 1/1000s you will be fine with a 10 dollar caliper. Use it to measure the threaded rod and find the right drill bit.


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## Fastback (Sep 2, 2012)

OK, I just checked, the correct size drill for a M8 x 1.25 is 17/64 (.2656). One size smaller is 6.7 mm, one size larger is 6.75 mm.


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## FrankC (Aug 24, 2012)

If all else fails drill a slightly smaller diameter hole in the spool and use a round file to fit it to the rod. I made some up many years ago and made brass bushings for the spools, the threads will dig into the walls of the hole and loosen them up eventually.


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## Shesho (Jun 12, 2012)

Thanks. I have a depth measurer which just uses a ruler which that accurate but its saying 8mm or maybe 7.5.

The 8mm drill was too large.



Fastback said:


> OK, I just checked, the correct size drill for a M8 x 1.25 is 17/64 (.2656). One size smaller is 6.7 mm, one size larger is 6.75 mm.


I'm being stupid here but are you saying the right dize drill bit should be 6.75mm?


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## Shesho (Jun 12, 2012)

FrankC said:


> If all else fails drill a slightly smaller diameter hole in the spool and use a round file to fit it to the rod. I made some up many years ago and made brass bushings for the spools, the threads will dig into the walls of the hole and loosen them up eventually.


I might try this.


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## jschaben (Apr 1, 2010)

Metric or inch, I just take one of those plastic drill index measureing things and start sticking it in the holes until I find one I like:smile:


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## Shesho (Jun 12, 2012)

Ok I tried using a 7.5 then expanding it slightly till i could get the rod through. It seems it flexes or theres as much movement as with the 8mm, I thought I could eliminate it more but I guess its not possible to remove it entirely.


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## woodnthings (Jan 24, 2009)

*a threaded rod will be slightly less*

It will be slightly less than it's designated dimension...not much, a few thousands.
This conversion show a true 8.00 mm to equal 0.31 inches
http://www.sciencemadesimple.com/length_conversion.php
*Result of your conversion:*


*8.00* millimeters is equal to *0.31* inches
0.31 converts to 5/16"
http://www.seoconsultants.com/charts/inches-decimal/#conversion


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## Fastback (Sep 2, 2012)

I may have misunderstood your question. I thought you wanted to tap something. Oh, 5/16 = .312


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