Why is it that when ever I am tidying up internal threads ( various sizes ) that the correct sized tap does more than simply remove the damaged thread and locking compound , it actually removes metal right along the thread , so much so that there is a noticable difference in the size of the hole , IE thread a good bolt into a good thread and it feels nice and firm , run a plug tap ( correct size ) thru it and now the bolt feel noticably loose .Using a good quality tap ( not some cheap shit thing ) with a bit of thread cutting lube and cleaning hole and tap ( comp air )( and goggles ) every 5-6mm or so. Pissin me off !!

What class tap ? class has nothing to do with quality,but tolerance, sloppy/tight fit.Different lubricants have a small effect on tolerance as well. J

Class ? ..... dunno Jay, I just buy them from an engineering supply shop. Maybe I need to tell the guys at this shop that I want them to clean threads . I just thought that there would be an ( international ) standard for sizes , but then , thinking about it it probly depends weather im cutting threads in steel or ally ... whatever. Rocol aerosol thread cutting lube is what I used
Cheers
Hamish

Mctool
Also mention the threads you are cleaning are locktited, makes a difference

Thanks Higgy , Jay. Cost me a small fortune ( OCD ) ally V8 cyl head exhaust manifold mounts ( 16 holes ) , I only tapped 2 b4 The penny dropped , so I have to heli-coil them ...... but then they arnt all the same so I just had to do the rest . Things like that bug the shit outa me . I struggle with my Guzzi as I can see one cyl is forward than the other , the beemer is worse ( side x side con rods ), I know the paso is the same but I cant see it whilst on the bike you'd think I had better things to worry about

I'm guessing you are chasing threads in an aluminium of sorts from what you're experiencing. You have to be soo fussy going back to clean up any threads and they are often best left alone as it's so easy to 'tear' more material out rather than remove burrs loctite etc. I've had more success using a soft steel bolt with two junior hacksaw slots neatly sawn up it's length (like a cruciform end on) for 4 or 5 mm then thoroughly deburred using a rotary wire brush. The gaps caused by hacksawing can work to catch any irregularities or locking agent but sometimes the later can be almost impossible to remove without further damage. I remember as an apprentice cutting M3 threads in expensive aluminium castings. There was quite a knack to it, we used to use turps as a lubricant as anything thicker could cause the tap to bind and even seize when you went to break the swarf off. A lot used to get scrapped sadly.
Re: steel studs in aluminium, even if a stud is tight on removal it's good to carefully study the stud (NPI) and the hole. If there is loads of swarf/loose aluminium particles and the female thread is visibly damaged then yes, it's probably helicoil (or more easily available/cheaper australian version) time but if no swarf or damage then just clean up and re-use the same stud if that also isn't damaged.

A bolt with a hacksaw cut in it ( or even a slightly angled cut with a dremel ), seems kinda obvious now Chris, and I have done just that when making a piston stopper , with a length wise cut to release compression ...... just never thought of useing it to clean threads , and I have 4 liters of turps
leg is no better
regards
Hamish

Sorry to hear Hamish - hope you are managing to ride. Do you still have the Guzzi?

giocast is right, a loose thread within an Aluminium casting is best left alone. If it has been 'worn' the thread face will have moved and any attempt to clean this up with a tap will remove a lot of metal. Not got a tip save for replace the bolt with a new one and avoid cheapo one piece cast aluminium casings... Oh, wait a minute, we cant.