query

[brickie]

hi, i have a bolt we will call it, with a threaded end which measures 9.3mm width ( outside diameter of threads), i have bored out a hole between 8.5mm to 9mm, and tapped it but the bolt is to loose in the 9mm hole and to tight in the 8.5 mm, how do i work out what size hole to bore before tapping to accept the bolt(9.3mm) or how do i measure the depth of the actual thread ie in the valley of the thread, i hope this explains what im trying to say! ,thanks

 

[Jasonb]

Whats the tpi of the thread as its likely to be some form of 3/8" thread for which there are known tapping sizes, mass produced bolts are often a little under the nominal size

Jason

 

[robwilk]

Are you sure it is a metric bolt.
The more common size metric bolts are M8 and M10
But for a M9 a 8mm hole should give you a 75% thread which should be a good fit.
Unless it was not metric.
Just my opinion.

Rob......

 

[brickie]

thanks to both of the above for an answer yes it could be imperial will have a look tomorrow

 

[Lew Hartswick]

With ANY machine screw (bolt) the tap hole for an approximately 70 percent thread
is the nominal diameter of the screw MINUS the pitch. That is the way all metric
thread charts are shown and all you have to do for "imperial" ( UNC etc) is convert
the TPI to pitch. ie. a 3/8 - 16 screw, tap drill is 5/16" . (end of math lesson)
...lew...

 

[mklotz]

Lew,

The complete formula (for Imperial threads) is:

TD = MD - 0.013*DOT/TPI

TD = Tap Drill size
MD = Major Diameter
DOT = Depth Of Thread expressed as percentage
TPI = Threads Per Inch

Simplifying this to:

TD = MD - 1/TPI

is equivalent to setting:

0.013*DOT = 1

or:

DOT = 1/0.013 = 77%

which is actually a bit on the high side for many small engine applications. 55-60% is plenty strong enough for lightly stressed apps and will save lots and lots of taps.