1976 240d glow plug reamer

[240Dude]

so it's time to change the glow plugs on my 1976 240d. i tested them and two seem to be bad. my question is about reaming. the reamers for the 616 engines seem to be pretty expensive so i thought i would check on mcmaster-carr's web site. i can get a 1.5mm hand reamer with four flutes for about $15. i was thinking maybe i could hog the center out of an old glow plug and weld in the $15 reamer. does anybody know how long one of the $95 reamers is? what about the dimension from the bottom of the threads to the tip of the reamer?
john