You'll need a set of small diameter drills to clean the pipe bore and measure the internal diameter. Ideally, your set should range from 4/64 to 9/64 of an inch but metric sizes from 0.5 to 3.0mm in 0.5mm steps will work.
Use one of your drills to carefully clean out any debris then using the smooth end of each drill, find the largest one which will fit into the bore. If using metric drills, choose the nearest value. For example, if a 1.5mm drill fits but a 2.0mm drill does not the bore is equivalent to 4/64".
Bore (1/64") Bore (mm) Year 4 1.58 1720-1750 5 1.98 1720 6 2.38 1680-1720 7 2.78 1680 8 3.18 1650-1680 9 3.57 1650
There are formulae for calculating the date from weighted bore sizes, in which the most frequent sizes contribute more to the result. This gives a better estimate of the most common date of occupation. The formula used by Binford and Hanson assumes there is a linear change of bore size with time, whereas Heighton uses a second order polynomial formula which gives a better fit to the data.
To calculate the weighted bore:-
Here's a worked example
Bore (1/64) Number of stems Product 4 22 88 5 42 210 6 6 36 Totals 70 334 Weighted bore (334/70) 4.77 Date (Binford) 1749 Date (Hanson) 1745 Date (Heighton) 1751
Use the weighted bore size with the calculator below to obtain the date using the forumlae by Hanson, Binford and Heighton.