36 replies to this topic

[dklawson]

A vendor will typically not replace a broken tap as they are made of hard (sometimes) brittle steel.  They wear and become dull, particularly when used on hard materials.  Once they are dull or if there is an accidental side load applied (or too much torque) when tapping, the tap will break.  The vendors know this and do not typically replace taps unless there is an obvious defect when you take them out of the box the first time.  

 

The set you are buying may or may not have information on tap drills.  I have posted the following information previously but I'll post a succinct version here.

When tapping a new hole (not chasing existing threads) the hole has to be drilled the right diameter to create 75% of the thread profile.  On common metric and UNF/UNC thread profiles that is achieved by subtracting the thread PITCH from the thread MAJOR DIAMETER.  For example, a common metric thread is M6x1.  6mm is the major diameter and 1mm is the pitch.  Therefore, the tap drill size is 6 - 1 = 5mm.  The same applies to UNC and UNF threads.  You just have to know the pitch.  Choose the tap drill you have that is closest to what you calculate.

[WimpyMiniMan]

 UNC and UNF Tap and Drill Chart.pdf   174.04K   21 downloads

 

Here's a table I found from the company I bought the taps from. I ordered the corresponding drill bit at the same time. I have also just been doing some research and found that a well known formula for calculating tap drills is 'Major Diameter - 1/thread pitch' - does this sound about right?

 

Once again, thanks to everyone for the advice.

 

George

Edited by WimpyMiniMan, 18 January 2016 - 11:04 PM.

[Fast Ivan]

yes, as a rule of thumb, major diameter - pitch

[gazza82]

Worked out that buying all the taps will cost about £50 was just the wrench that was a little expensive.

 

You should have checked out Axminster ... http://www.axminster...renches-ax21329 ... £1 standard delivery too!

[dklawson]

Having a good tap handle is nice and worth the investment if you tap a lot of holes.  

 

I have Chinese and Indian budget tap handles in my tool box but when possible I prefer to use the much older Starrett brand wrench that I inherited in my youth.

[WimpyMiniMan]

The tap arrived today - looks nice, however only tapping will tell whether these are really any good.

Attached Files

•  IMG_2813.JPG   65.74K   4 downloads

[WimpyMiniMan]

Update : Drilled out the hole and re-tapped and it's good as new! re-assembled thermostat housing with new thermostat and gasket, using studs and nuts. Then proceeded to fit new radiator, with new rad cap (15lbs) and new top and bottom hose. Filled to just outside 12mm of the filling orifice as recommended by the Haynes Manual. After a fairly involved rebuild went to start the car and had a flat battery! Recharged the battery and 5 hours later started the car and after some of the antifreeze i spilt burning off along with large amounts of wd40 i used whilst drilling and tapping, the engine appears to now be smoke free(-ish). Next thing i should think is the carburettor idle setting which needs adjusting, and a new windscreen and to fix the leak in the  sunroof - once they're done the mini could even be mechanical problem free! - although rust is an underlying issue... always! Anyhow thank you ever so much for all the advice, and if you have any questions about the process feel free to ask! (NB: I thought i took photos, but my phone seems to have deleted them, and i dont intend on stripping it down again!)

 

Kind Regards

George