Do you chase a galvanized thread, or leave it alone?

[elowen01Participant @elowen01]

I have a question about galvanized bolts that I thought the workshop crowd here might have come across.

Say you have a bolt and nut that fit fine before galvanizing, but after hot-dip galvanizing the nut feels noticeably tighter. Not completely stuck, but tight enough that you are tempted to run a tap or thread chaser through it.

Would you normally clean the thread up, use a little wax or anti-seize, or leave it alone and just use a new nut if it is too tight?

I know chasing the thread can take some of the zinc off, so I am wondering where people draw the line. For a small workshop job, is there a sensible rule of thumb?

Interested in how people actually deal with this rather than the textbook answer.

[JasonBModerator @jasonb]

Commercial fixing suppliers take into account any plating and cut the threads to a suitable tolerance so that they are not tight after plating. They are usually towards the slack side in many cases.

 

If doing your own plating then follow the same example and allow for the thickness of any plating.

[old martParticipant @oldmart]

You can certainly go for lightly chasing the threads until the nuts fit more easily. Do not rub them with an abrasive as it will go straight through to the steel. When I was a fitter in aircraft engineering it was common for the cadmium plating on male threads to be tight when inspected and light chasing followed by passivation would get the size and passivation showed that there was still a coating. If you have a thread file the bolts will be easy to work on if the bolt is slowly rotated in a lathe using back gear.

[noel shelleyParticipant @noelshelley55608]

Jason comment refers to plating, this is a completely different process to hot dip. The OP asks about hot dip. Slightly undersize male threads before galvanizing and then die to size after. Nuts or tapped holes re tap after dipping. Un galvanised surfaces in close proximity to a galvanised surface will get some electrolytic protection. The process gives you 3 layers, Zinc, zinc/iron alloy and parent metal, steel or iron. The zinc/iron is brittle. Lubricate when fitting. If a die is not to hand a hacksaw will bring things to a fit.  Good luck.  Noel.

[Nicholas FarrParticipant @nicholasfarr14254]

Hi, I’ve erected many galvanised palisade steel railings in my day jobs, which used galvanised nuts and bolts, never had any problems with them, but if you do chase the the threads, you may find them rusting in a relatively short period of time.

Regards Nick.

[Clive FosterParticipant @clivefoster55965]

My experience suggests that the over tight fitting effect could be more due to surface finish than size per se.

Filing a flat on a correct size bolt and attending to the exposed end of the threads to give a smooth entry producing a sort of burnishing tool did the deed, with a appropriate care to catch the zinc dust. That said the whole batch of nuts had a rather rough and uneven surface finish. Both outside and, so far as could be seen, inside. Hence smoothing out the surface seemed a good thing to try.

One of my metal working hand books has (obsolete?) data on the change from nominal bolt and nut size needed for both plating and galvanising.

Clive

[JasonBModerator @jasonb]

Noel, I thought that was why they make plus size taps to give additional clearance for the thickness of the HDG or as we agree making the male thread undersize

 

https://www.newmantools.com/taps/oversized-taps.htm

You would have to make an allowances on internal screwcut threads where you would have to remount in a lathe and pick up the thread as you can’t just run a tap back into the hole.

[noel shelleyParticipant @noelshelley55608]

My experience has been with heavy structures where on return from the plant the female thread is often all but  full of zinc on the lower side and no amount of oversizing will solve this – just retap. The same is true for male threads but I’m talking 20mm,22 and 24, run a die down them. Clives comment on the surface being rough would seem to indicate spin galvanising – this is not really hot dip. This is a process for small components.

The reference to Newmans is interesting, oversizing is fine for plating or spin galvanising but as above no help for hot dip. Work carried out near or on the sea shows no sign of corrosion a decade later following retapping or dieing. Noel.

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