What tread size

can any one tell me what tap size i would need to rethread a French WW1 F1 grenade. threads are rusted but not bad. i know i need a carbon steel tap and may have to order one special made for me. so, need all the tech dimensions. thanks joe gatz CWO4 RT

If the threads are not completely gone, I’d try a thread restoring file. This will clean the threads without risking undercutting and destroying the ones that are there.

will look into that, i have done a good amount of machine work, but i will try anything new. still i would like the tech info about the threads on the WW1 french F1. thanks joe gatz

If you have done machine work then why dont you measure it yourself?
A dial caliper and a thread pitch gauge should put you in buisness ;-)

A carbon steel tap is the cheapest and (use this word carefully) softest of taps, a HSS (high speed steel) tap is harder and better quality and depending in the degradation of the threads and the base metals you are cutting would probably be the better option.
From your wording “tap” I have assumed an internal thread so a thread file is for outside threads so will be of no help (based on an internal thread).

true , i do have mm on my dial cal but i am in Tx for another week with my granddaughters- no tools as i live in New York. another source of info would just confirm what i measure when i get home next week. i do not have a metric thread gauge but i will find one. retired at 76 i have to travel 30 miles to my friend’s house to do any machine work. i have a US Civil War navy cannon brass fuze and i was trying to measure the treads, Ha ! went to my friends house and we both said lets try metric. looks like 1.5 mm pitch thread as the us gauge does not have a 17 TPI. always interesting thanks joe gatz CWO4 RT

correct -internal threads at the top of the French F1 grenade WW1. the fuze is a zinc so any heavy rust would damage the threads. i remember looking with a flash light and there is some medium rust. what i was thinking to do is to put some rust jelly on the grenade threads for a day. clean it and then run a tap thru it. joe gatz

Thread files are available with both external and internal “teeth”.

The problem with using a tap as opposed to a chaser, is the risk of undercutting the thread. This is especially important with fine threads. You can end up actually removing the original thread.

Yes, I have seen that sort of tool generally referred to as a thread chaser, personally I have never heard them called a file (which the other end certainly is). There major drawback is they can only be used on large diameter size threads with sufficient clearance to ensure that the other teeth do not foul the thread being worked, at best they do a scraping action, not a filing action.
Still it is definitely a personal choice but give me a tap any day.

Since you have done your own machining, make your own tap. It’s not hard to do. The easiest way would be to take something like 4140 steel rod (a bolt might do), machine it down to the correct diameter for your particular thread and then cut threads on it to the correct dimensions. You can also find thread dimensions in the Machinists’ Handbooks. After the threads are confirmed to be correct, machine flutes along the sides like all taps have. A minimum of two, but three or four flutes would probably be better and not be as likely to break. Make sure to make the flutes off-center to create some “hook” for each section of thread. Then take it and put it on a fire-proof surface like a metal plate (that’s not sitting on something flammable, like I did!), heat it up to cherry red and quench in water. You will then have a pretty hard tap, for a one-time use that will work very well, to your exact dimensions. I have made three of them like this. Two were for the 3-1/2" threads on the nose of an inert M-117 750 pound bomb. I don’t remember the thread pitch off hand. I literally broke the first one by torqueing it too hard during use. I had accidentally created a stress riser with a center drill too near the hole in the top used for an 18" piece of pipe to turn the tap. The second one worked well, but I was lazy so I only put two flutes in it. It drags too much when you use it and I’m pretty sure it’s too hard to put more flutes in without doing one or more additional heat treatings. This is NOT guaranteed for cutting new threads, just chasing old ones. I also had an old RCBS “A” press that 1-3/8" threads in it. I include this description in case someone wants to know more about how this system works. While they used to make some dies with those threads, they haven’t in a loooooog time. So I converted it to 1-1/4" threads for my dies of that size by taking a 1-3/8" bolt, drilling a cross-hole in it so that I could put a 1/4" square HSS lathe bit in it. I then cut a hole perpendicular to that that I threaded for a set screw to keep the 1/4" bit in the bolt. The cross-holes were more than 1" up the side. I then set the bit in the bolt to the correct depth to create a correct-size tap hole for a 1-1/2" thread. I then screwed the bolt down into the press. When the bit started to touch the cast iron, it started cutting (in one pass) the tap-drill size for 1-1/2". I then continued screwing the bolt down in and the 1-3/8" thread that was in the press aligned the new (for want of a better phrase) manual lathe until it was almost all the way through the press. If the last thread or so was off-center, no matter. I then took an 1-1/2" tap, drilled a hole in the end with a carbide drill and VERY CAREFULLY tapped it to at least 3/8" thread, about 1/2" deep. I then made a pilot from some soft steel I had around that I put 1/2" (outside) threads to go into the tap and the large diameter of the pilot was a few thousands of an inch smaller than the tap-hole diameter in the press. I then inserted the pilot into the hole until the tap’s threads contacted it. Then I tapped the hole to the 1-1/2" size. At this point the bit tip shattered. No flute at the cutting edge of the tap, dummy, and so no place for cast iron cuttings to go. I took it back out, cut one flute where the cutting edge was and put it back in. Note that I had never sharpened the bit, it was just raked at the end as it came from the manufacturer. It worked just fine this time. After I was done with that, I made a bushing with outside threads 1-1/2" and inside threads 1-1/4". Someday, if I ever see a need for it, I’ll make another bushing with 1-1/2" outside threads and 7/8" on the inside for normal dies. Maybe for the .30-.378 Weatherby cases that my Lee press doesn’t care much for. If anyone wants photos of the taps and pilot, just email me and I’ll find them and take some photos. My email is Mignuc@mediacombb.net. I’ll also post this as a new topic in case anyone wants it.

I checked my F1 and it is 16mmx2.0 so a standard tap size you can buy.
The zinc fuze stem seems undersized at about 15mm. Try carefully by turning the tap in your hand or use a holder if tight. Check each thread turn for damage.

super thanks for the info, saves me from trying to get it correct, i can do some machining but it is a limited shop i can use. joe gatz

An internet search would probably find a machine tool supply house which stocks unusual or uncommon sized taps and dies or will make them to your specifications. But they are not cheap if you want only one of them. I do know of one in Brooklyn.

For high quality metal working tools, I recommend mscdirect.com Great company!