Stainless comes in various grades, some easier to work than others. You will need sharp tools to work it what ever. You could use a bronze, or even brass as the piston rod. Good luck. Noel.

I cut a lot of threads in stainless, mainly 304 grade.

I use spiral entry dies and Rocol RTD as cutting fluid. Always power feed, around 200 rpm on an M6 thread for example.

A split die will cut exactly the same as a solid die. When you but high end brands, they will spilt dies if you need a (very) small amount of adjustment.

In summary, best dies and cutting fluid you can afford.

If you need a “decent” M5 die, let me know as I have a few spares

Jimb

A high quality split die used as large as possible for the first cut, and 303 stainless as already mentioned, but you can use other metals if they are easier for you to work.

I have a metre length of 303 ultra turn and that is nice stuff to thread but not always easy to find.

You can always single point some material away and finish with a die for final thread form. With stainless do not try such a fine cut that you rub a bit of material away, you need to keep cutting all the time because some grades will work harden and cause further headaches. As others have said a good cutting compound is always a good bet. I have both the grease RTD compound and, for brushing on a thin coating, the fluid RTD.

Martin C

At one time there were still very good quality high carbon taps and dies available Steve. Possibly there’s a very few manufacturers left that might continue to offer them, but none as far as I know with the better and more well known industrial brands. Those now seem to be only produced in high speed steel. Overall the high carbon taps and dies available today are pretty much at the low priced consumer grade, poorly heat treated and ground end of the market. And those reasons are exactly why I no longer even consider buying them. If any high carbon tap isn’t already sharp and well heat treated out of a known high quality tool steel, the risk of breaking one off in the hole rises exponentially. And having that happen in an expensive casting or a part that already has a lot of hours invested in it can make those low cost high carbon tools extremely expensive. Those lower quality dies also won’t last very long, and in general produce much poorer quality threads because there tearing the metal more than cleanly cutting it. And unlike taps, a die has no real positive guidance. Yes they can produce usable threads good enough most times. But a slightly wandering or as some call it, drunken thread is usually the result. For something as critical as a piston to rod thread, it’s thread quality, pitch accuracy and condition, fit to the female piston thread, and properly aligned to the rods long axis. I think I’d always choose to single point thread that piston rod. And just like anything else, you only get half way good at that by doing it enough. Some hrs practicing on scrap isn’t wasted at all, and that should then allow you to be confident enough you can properly do so any time it’s required.

There’s also a perception that the common varieties of stainless meant to be machined are hard. They aren’t really, yes there tough or tougher to cut than the more common varieties of mild steel, and tough would probably be more accurate or a better descriptive term. And that work hardening can be a serious issue. Excess frictional heat from the cutting tool is what will cause that, so you can’t allow any tool to rub or dwell on the work at all. And as others have already said, a very good cutting oil meant for the job wouldn’t be optional to me. The best method for drilling or cutting stainless or any work hardening material is to stay below the maximum recommended cutting speeds. Those are for industrial use where time is money, and those recommended speeds assume that at least flood coolant would always be available. At most, I’d use about 80% of what my reference books recommend as the suggested maximum cutting speeds for stainless. 60%-70% might be a better choice to start with. Sharp, good quality drills are the minimum. When having to peck drill to clear the chips, never allow the drill to dwell at all in the bottom of that hole. It’s either producing a proper chip, or it’s instantly on it’s way back out. Being timid or too light with the feed pressure isn’t the method to be using. You want that drill or any cutting tool to always be producing a proper chip. The cutting oil and the frictional heat being produced should be getting removed both by the cutting fluid and in the chip itself. I suspect most issues with stainless are caused by using duller tools than they should be, cutting rpm too high for the equipment and tooling most of us have available, and not being aggressive enough on the feed rate. Once I finally understood that, I can’t recall having any more work hardening issues.