1/4 32 spark plug tap drill size ?

[V 45]

Another question !! Sorry for asking soo many...but I looked up the drill bit size for 1/4 32 tap. I found either 7/32" or a #2 bit. Which is perferred ?
  Thanx again,
   Dave

[steamer]

Hey Dave,

Well, google is your friend.    I found that 7/32 is preferred  (.2188) while #2 (.221) is an alternate.

I would use the 7/32 for general use, and .221 for really tough to tap applications like stainless steel ect.

Dave

[gmac]

Dave;

From Little Machine Shop.....inch and metric

http://littlemachineshop.com/Reference/TapDrillSizes.pdf

Enjoying your build, you've got me chopping up material for a build again - thanks!

Cheers Garry

[Jo]

100% engagement = 5.4mm

80% engagement = 5.6mm

What % engagement you choose will depend on the material and the purpose.

Jo

[steamer]

That jives  Jo....  0.2188 ...OK  .219 is about 75% engagement...which is considered nominal....at least on this side of the pond.

Dave

[mklotz]

Without considering depth of thread...

TD = 0.25 - 1/32 = 7/32   (77% DOT)

For N % DOT...

TD = 0.25 - 0.013*N/32

N = 50 is a good choice for stainless in which case the above yields:

TD = 0.2296

A #1 (curse these stupid American designations) is 0.2280 which corresponds to 54 % DOT.  A 5.8 mm drill (0.2283) is slightly closer and yields a 53% DOT but that is all turd polishing - use the formulae as guides and pick your closest drill.  Keep the DOT below 75 % for tougher and intractable materials, e.g. bronzes, nickel silver, stainless.

[Dan Rowe]

I use an android app for calculations of tap drill size. There are several free ones I like Tap Drill Tool the best as it works for most of the threads I have in the shop. It works for both cutting taps and form taps.

Dan

[Jasonb]

When I've used 1/4x32 UNEF for glow plugs I had drilled 5.5mm.

As its likely to be something that is in & out quite a bit and into an aluminium head you want a decent depth of thread

J

[V 45]

Thank you everyone !! As Jason stated it will be going into aluminum and the thread depth will only be about .300" or a little less.
I don't own any metric drill bits !!! I do have fraction, letter, and wire bits. With that said is 7/32" the way to go? Or get a 5.5 mm bit? I could most likely borrow one to have the deeper thread.
  Thank you again,
  Dave

[Jasonb]

7/32" should do you fine, I doubt the extra 0.05mm ( 0.002") will make a lot of difference. If you drill 3/16" followed by the 7/32" it should eliminate any risk of it cutting oversize.

J

[steamer]

Jason,

I think you mean....drill for 3/16 and then 7/32".....right?

Dave

[Jasonb]

Thanks Dave, I've altered it to 3/16"

[Tennessee Whiskey]

We all know a drilled hole isn't all that round. So, should you ream before you tap or will the tap do that for you? Or does it make a big rat's butt?

Whiskey

[steamer]

I have never reamed a tap drill hole....but having said that....I would bet someone has to get a very good fit.

Then again...if the nut was that important...I've always single pointed it in the lathe anyway....after boring to size.

Dave

[Tennessee Whiskey]

I never have either. Just one of those things that make you wonder. I bet it would tap a little easier.

Whiskey