Yeah, so I'd like to cut some fresh new thread beyond the existing thread on my cambered bolts for reasons I will reveal later.

After Googling around I now know I will need a tap & die set to create new thread, but to get to the unthreaded section at the end of the bolt I must pass over existing thread.

OK, so I watched a self help Youtube video on how to do this but the guy in the video advises against using a die on existing thread. Therefore I'm guessing I will need both a die and a rethreader to repair the first section of thread I damaged using the die...if that makes any sense.

Are you still with me ?..... Somebody wake up Hicks. :alien:

I did a search on what tools I would need to do a die and re-thread but I'm a bit confused with their jargon. Here's what I'm looking at with regards to the tap & Die set (description below) > 45pc Jumbo Metric Tap & Die Set




Two things in the above description confuse me. 1st) What is meant by National Coarse and Fine Threads ? ...2nd) Which of the two M20 dies do I need, M20x 2.5 or M20x 1.5 and why are there different sizes ?

Any tips on tap & die + rethreading much appreciated. Thanks

I did a little search regarding tap and die designations in the metric arena. Here in the states we have designations for our tap and die sets that would say something like 1/4-20 which means the outer diameter (OD) of the bolt or inner diameter of the nut (ID) is 1/4 inch and the 20 is the thread count per inch (TPI) so a 1/4-20 die would cut 20 threads per inch on a 1/4 rod.

It appears from what I could find from a brief search is that it is a similar designation in the Metric arena. From what I can tell the second number in the designation for metric tap and die is the pitch of the thread, so a M20x1.5 would cut threads on a 20 mm rod at a shallower pitch than an M20x2.5 ... So it's just a guess that the M20x1.5 is a finer thread than the M20x1.5.

And I've only ever threaded mild steel and all I can tell you is lube, lube, lube.

Quote: Originally posted by soliver

I did a little search regarding tap and die designations in the metric arena. Here in the states we have designations for our tap and die sets that would say something like 1/4-20 which means the outer diameter (OD) of the bolt or inner diameter of the nut (ID) is 1/4 inch and the 20 is the thread count per inch (TPI) so a 1/4-20 die would cut 20 threads per inch on a 1/4 rod. It appears from what I could find from a brief search is that it is a similar designation in the Metric arena. From what I can tell the second number in the designation for metric tap and die is the pitch of the thread, so a M20x1.5 would cut threads on a 20 mm rod at a shallower pitch than an M20x2.5 ... So it's just a guess that the M20x1.5 is a finer thread than the M20x1.5. And I've only ever threaded mild steel and all I can tell you is lube, lube, lube.

Thanks for your time.
I had the idea that those last numbers might be the nut size to fit onto the rod. Your explanation makes more sense. I'll do a search along those lines, now I know what to search for.
Lube up, good tip.

Yeah, whenever I've threaded solid rod from nothing, the best thing to do is crank the die in small increments no more then 1/16 to 1/8 of a complete 360 deg turn then back it out to break any burr that comes from the threading and just constantly spray lube or squirt oil on it.

Like so: crank 1/16-1/8 back off,... crank 1/16-1/8 back off,... over and over and over in a constant forward then backward motion until you've threaded as far as you want. When in doubt or if it just feels like its binding up, back it off or you might damage the die. I've broken a ton of taps by pushing it too far.

Like I said I've only ever threaded mild steel which is relatively easy, but if the bolts you are threading are stainless or grade 8 or something else extra hard, you may have a really hard time.

I hope you have access to a bench vise of some sort?

Lots of erl, and go slow.

The 2.5 is the coarse thread like most everyone in the industry uses including PTW. The 1.5 is a fine thread and I've only seen it on Ivanpah buggy axle bolts. The fine thread I think can be tightened more securely since it takes more turns to thread the same distance with the finer thread compared to the coarse.

Ok thanks all. Go slow, back off and plenty of lubricant. Use coarse thread. M20x2.5...got it.
I don't have a workbench vice but the cambered bolt is tightly bolted to the bug so should be ok do you think ?

The cambered bolts are stainless. From what you say soliver, this will be tough going.

Here's a picture of the cambered bolts and what I want to do with them.

The first thing that you need to determine is whether the current threads were rolled or cut. A rolled thread is larger in dia. than the stock it is rolled from. Since you will be cutting threads, this means that you will not have full thread depth when you cut the thread with a die. Keep that in mind when you are torqueing the nut as you may strip the threads. Using the die is relatively simple. Thread the die on to the existing threads until you reach the unthreaded portion, using lubrication ( Crisco shortening will work ) advance the die a quarter turn, then back it off a bit and do another
quarter turn etc. until you have the thread length you need. Note that the die has two sides, the stock feeds into the wider tapered side. Go slow and all should come out fine,

Considering all the guff you'd have to buy....and I've been told hand cutting 20mm threads is not much fun.....have you considered taking it to a machine shop and getting them to do it?

Quote: Originally posted by TEDWESLEY

The first thing that you need to determine is whether the current threads were rolled or cut. A rolled thread is larger in dia. than the stock it is rolled from. Since you will be cutting threads, this means that you will not have full thread depth when you cut the thread with a die. Keep that in mind when you are torqueing the nut as you may strip the threads. Using the die is relatively simple. Thread the die on to the existing threads until you reach the unthreaded portion, using lubrication ( Crisco shortening will work ) advance the die a quarter turn, then back it off a bit and do another quarter turn etc. until you have the thread length you need. Note that the die has two sides, the stock feeds into the wider tapered side. Go slow and all should come out fine,

Thanks! I appreciate your help. It seems like there's a good chance that I could end up ruining these bolts. I'm starting to think that John' advice above would be a whole lot simpler. I should get these done professionally or drop an email to ptw and ask him to make me another set. I was hoping to do this myself without bothering anyone but reading your advice makes me think that I may be out of my depth as I have zero experience.

...or on the other hand, a useful skill to learn. Need to sleep on it.

It is certainly handy to have a few dies & taps around the place. I often use the 1/4 BSW tap for camera mounts and the 5/16 & 3/8th gets a bit of use here and there but I've never tackled anything like 20mm.....methinks you'd need a really solid vice.

A few washers or a sleeve on the outside (or inside) of the wheel would fix your hassle maybe?

The hard work has been done (getting the die to start correctly/square) and adding more thread should be doable.

As TEDWESLEY said, the rolled thread will be slightly larger, than what you will cut. Your V in the thread you cut will be shallower.
I would be surprised if it would make a lot of difference though, as it is not caring a huge structural load like most 20mm bolts do.

Lots of lube and go for it, but in a vice as some serious elbow grease will be needed. A good quality die will be a given.
Most engineering shops would probably do the job, for the cost of the die alone.

Yea, Red... I think in the long run you'd probably save the most money having a machine shop or engineering shop thread them for you... I'm sure the tap & die set is slightly costly and I can only imagine what a new set from PTW would cost.

Quote: Originally posted by Chook

A few washers or a sleeve on the outside (or inside) of the wheel would fix your hassle maybe? The hard work has been done (getting the die to start correctly/square) and adding more thread should be doable. As TEDWESLEY said, the rolled thread will be slightly larger, than what you will cut. Your V in the thread you cut will be shallower. I would be surprised if it would make a lot of difference though, as it is not caring a huge structural load like most 20mm bolts do. Lots of lube and go for it, but in a vice as some serious elbow grease will be needed. A good quality die will be a given. Most engineering shops would probably do the job, for the cost of the die alone.

Thanks Chook, loving the avatar. I did try a spacer on the bolt but the wheel I wanted to use (oops, given away my secret) ended up being too far out and imagine the bolt could bend under the increased leverage.

soliver, I'm going to speak with a guy tonight who runs a machine shop close by and see if he or someone he knows else can help.

Thanks to everyone for your advice. I hope that someone braver than me at some future date will benefit from all the knowledge in this thread.
I would have liked to have given this a go but with no tools or vice and no experience the smart way as you're all suggesting is to get this done professionally and I agree.

Machine shop service would be the best choice , avoids experimenting with the good bolts you have and the cost of experiment gone wrong. Also there are a variety of lubes that have different qualities for different metals , well worth a professional opinion. I once a had access to a spray coolant/ lube , bad for the environment , bad for the lungs , cut treads like butter!