Taps/dies Basics

[nelmo]

OK, I apologise for my lack of knowledge, so be gentle...

 

GBS told me to use a tap (die? which is which?) to clear out the powder coat from the thread for the seat belt fixings, saying I needed a 7/16 tap. So off I went and bought a basic set.

 

Today, I started on the thread and after a few turns, it felt too difficult for just clearing out powder coat. A close look reveals that I have started creating a lovely new thread with a different pitch (is that the right word? Thread is much tighter than the existing thread)

 

So what have I done wrong? When looking at tap sets, there is no mention of different thread types - I just bought one that said it contained a 7/16 tap (it has '7/16 BSF18' stamped on it).

Edited July 26, 2015 by nelmo

[fry61]

Wrong tap!! You need 7/16" UNF. Haven't got my Zuess book to hand so can't say with confidence what the pitch & included angle differences are. Depending on how far the wrong tap entered the UNF hole will determine if you have removed any thread crests & therefore created a smooth (ish ) bore. Maybe able to step up to M12 fine pitch if UNF damage too great for safety.

[richyb66]

Imperial seat belt bolts are 7/16" UNF 20TPI (thread per inch), not 7/16" BSF which is 18 TPI. Basically, you've got the wrong tap.

[nelmo]

Balls!! I've gone about 1/3rd of the way in - is that too far for safety? I'm guessing yes...

 

Do those sear bolts have to be a certain size/type for IVA? I was thinking of just finishing this one with this size and get one bolt to fit. Then I'll do the rest 'correctly'.

[Guest mcramsay]

Seat belt bolts either have to be unf( which is a high tensile thread) or a high tensile metric eqivalent ie an 8.8 grade bolt or above, I would not tap the nut out to 7/16 BSF, the thread measurements are too close together and you will not get correct thread engagement, you would have to drill up to a larger size ( completely remove the thread in the hold currently ) and retap to a new unf or metric thread. This will give you issues in that all the seatbelt mounting hardware is designed for a 7/16 diameter bolt, if you go up a size you will need new seatbelt a or to drill out the original fixings to accept a larger bolt. You could ( if access permits) tap the nut you have messed up back to 7/16 unf , and using a longer bolt thread through the nut with partially damaged threads and then fit another good nut behind it, effectively double nutting the bolt. This would again give you back to more than adequate thread engagement, the only thing with that is nornay these nuts are welded on to a securing plate because you cannot get access to fit a nut on the other side.

 

Other option is to cut the nut off and have a friend with a welder to weld a new nut on.

[femster87]

Get a 7/16 UNF tap, clear the damaged one out. On the damaged one, I will get a nyloc nut on the end of the seat belt bolt, Get a one longer if required. Will give you peace of mind then

[Guest 2b cruising]

The other alternative would be to fit the correct size thread back with a Worth type thread insert.

These are special inserts that replacer the original thread with HT thread.

Don't even attempt to use any other type as there is no alternative good enough.

They come as a set with the exact size dril and tap to do the job.

Don't forget to use the supplied finishing tool to give you the correct amount/shape of chamfer.

"WURTH" They have outlets nationwide.

[Guest peter2b2002]

If you have the bolt that goes into the mount your trying to clear just put the tap next to the bolt then you can see if the 2 line up (the ridges on tap and bolt)you have the correct tap if not you know you haven't ,when you have the correct tap just retap the mount hopefully the first one you used will have a taper lead end so I won't have cut too deep into the mount

Peter2b

Edited July 26, 2015 by peter2b2002

[nelmo]

OK, thanks all. I haven't actually got the seat belt kit yet so I'll wait until it arrives and see what my options are.

[agent_zed]

personally (i am not expert so whatever you do is up to you) i would tap out the correct thread and then add a nut as well as said above. Can't see that is any different to what i have on my car which is a hole and then a nut on the other side. If you can weld a nut to a plate which can be riveted to the other side then you could drill out the hole and just rely on that, which would be basically the same as what i have. Or just a nut if you can get to the rear with a spanner.

 

An engineer once told me roughly speaking a threaded hole that is 1.5 x the diameter of the bolt is as strong as it will ever be. Could drill a 4ft hole but it wouldn't be any stronger than 15mm deep for a 10mm bolt. Might shake loose quicker on 15mm but then that's what thread lock or nylocks are for