I was tapping out some 8mm and 6mm stud threads in an aluminium cylinder head today and decided to use the shiny new (albeit second hand) taps I picked up from Sert Tools the other weekend. No problem with the 8mm one which cleans the 15 or so threads in the head out quite normally and just removed a bit of crap and old thread sealant but no metal.
Moved on to the 6mm one and the first hole I tapped out had a bit of alloy corrosion in it due to that thread not being used on some engines depending on which ancilliaries are mounted. So I'm expecting to dredge out a bit of crap from it and I'm not surprised when the tap seems to be doing a bit of work and comes back out with a few ally shavings on it. The next two threads looked to be in good nick though but still the tap removed a bit of ally and was clearly 'working' very slightly as I wound it in.
So I stop and have a good ponder over things. First step is measure the o/d of the tap and it's 6.22mm. A bit big for a 6mm tap methinks so I get my old one out which is a good quality albeit second hand one also from Sert many years ago and that measures up at 6.06mm. Meshing the teeth of the two taps together gives a perect fit so the pitch and thread form is right.
So we seem to have a tap which is the right pitch but a bit big on the o/d. I find a 6mm bolt and it's certainly a good bit looser in the threads this new tap has been through but doing a tightening test I can snug it up as hard as I dare in ally and it doesn't strip any of them so no major harm done thankfully although it still bugs me that those threads are no longer perfect.
So I have a really good squint at the shank of this tap and found something I'd missed before. Under where it's etched 6mm x 1mm there's another line which says '+0.13m' (yes not mm, just m but maybe an m wore off or got missed off in production.
Well the 6.06mm of my old tap plus 0.13mm isn't a mile away from the 6.22mm o/d of this new thing. So here's the puzzle. Why would anyone make a 6mm tap that is 5 thou big on the o/d? To leave room for a thread sealant? To cut a thread to fit a special very slightly oversized bolt? To cut a thread in something like a threaded split bush which then gets tightened back up slightly with an adjusting bolt? I've never come across anything like this before. I now have to make sure it never finds its way back into my box of standard taps but I'm also loath to chuck it as it looks brand new and maybe one day it will 'come in for something'.
-- Dave Baker - Puma Race Engines