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Old 04-03-2012, 05:58 AM   #1
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Default Correct drill bit for 3/8 npt sump fitting

Hey guys,

During a garage cleanup Ive lost the countersink style drill bit that was leant to me...(hurr durr).

The sump fitting Ive got is a 3/8th NPT.

Im Asutralian and (have a sense of humour guys) here in Oz we use a measuring system that makes sense.
I figured out that 3/8th npt tap has a diametre is like 16.8mm, or about 5/8th which is stupid.
And now Im trying to find the right sized drill bit.
It seems that a 37/64ths countersinking bit is about right, but where do I get one of those from for a decent price. And what a ridiculous size!

Anyways Ive searched and found that most people have 1/2npt return fittings so that information doesnt translate.

Thanks heaps.

TLR- i have a 3/8th npt fitting and tap for the sump, what drill bit do I use, preferabbly a metric size or an easy to get imperial size.

Thanks,
Dann

Last edited by nitrodann; 04-03-2012 at 07:33 AM.
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Old 04-03-2012, 07:35 AM   #2
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Ok, so can a 5/8th be used, because what I used last time was a countersinking bit like this..



And they seem to come in 1/2 (obviously wrong), and 5/8th.
I remember it looking like it would be too big but being fine in the end.

Dann
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Old 04-03-2012, 08:31 AM   #3
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Hey Dan,

Why don't you send Begi a quick email and ask what size drill bit is included with their 3/8" Oil pan drill/tap set?

I'm actually going to need the same bit soon to drill my sump, whatever size it is.
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Old 04-03-2012, 08:43 AM   #4
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According to this thread a 9/16" bit should do the job, so about 14mm (14.3mm).
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Old 04-03-2012, 09:31 AM   #5
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Everyone should have one of these hung up in their garage:

http://www.brokenbolt.com/images/sta...-tap-drill.pdf

Print it out or get a huge poster of it. Npt is down in the corner, obviously saying 3/8 is 37/64. 9/16 can be used, it's only .015 larger. Keep in mind though, it's a tapered thread you're not supposed to bottom out, so the slightly larger hole wont require the tap going as deep before you go too far.
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Old 04-03-2012, 09:33 AM   #6
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Thanks guys.

Honestly, imperial makes so little sense its crazy.

Ive purchased a 5/8th countersinking bit and Im going to turn it down to the correct size.

Thanks,
Dann
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Old 04-03-2012, 09:36 AM   #7
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Also don't forget that the Teflon goo or tape is a libricant to allow the fitting to thread in further. So you don't need a lot, AND, if you're checking the fitting depth as you tap, remember it'll go further during final installation.
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Old 04-03-2012, 10:18 AM   #8
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Thanks mate,

really appreciate your help.



dann
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Old 04-03-2012, 06:31 PM   #9
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http://www.toolbox.co.uk/silverline-...ep-15441-86908

I used the 14mm diameter of a stepped drill bit like the one in the link.
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