Op-Rod Material? - Page 1

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Posted: 10/8/2019 10:49:41 AM EDT

My dad has been getting into machining, recently bought a lathe and looking for a mill and I mentioned that a fun project we could try would be to make a backup Op-rod for my 516.

The part looks relatively simple and not too difficult to make, so I think we can do it, but my question is what are piston op-rods generally made of? The 516's looks like and I would assume is stainless because it's polished and doesn't rust, but I also have the thought that stainless could be too hard/brittle, and some kind of carbon steel might be better for an op-rod, to have decent hardness but better durability/strength. What's the consensus? Anybody know what the manufacturers use, or also have any theories for a "best" material, as well as heat treating processes?

Posted: 10/8/2019 10:57:27 AM EDT

[#1]

I would look at ejector pins. They are typically H13, case hardened or thru hardened.
I've made gas parts from ejector pins and ejector sleeves.

If the diameter is a standard size, you can order them nitrided.

Example:
https://www.mcmaster.com/ejectors

If you want a full variety, look at Progressive Components or DME mold mold components.

Posted: 10/8/2019 11:16:04 AM EDT

[#2]

Thanks I'll take a look at these. I don't think standard sized could apply here, there will definitely need to be some machining done, as the op-rod has different sizes and shapes all along it's length. Maybe we can DIY nitride after the shaping is done. Looking at this site it looks like the H13 only come already nitrided? Either way thanks this is a good place to start.

Posted: 10/8/2019 11:20:04 AM EDT

[#3]

I like to have the wear surfaces nitrided, it keeps things slick, but I have machined steps and notches in E-pins/sleeves before.
Just a head's up, you can use a dremel stone to break through the nitride to get started. Even with carbide, on a lathe it can be difficult to break through.
A carbide end mill doesn't typically have much issue.

Attached File

Posted: 10/8/2019 11:53:33 AM EDT

[#4]

Quote History

Quoted:
I like to have the wear surfaces nitrided, it keeps things slick, but I have machined steps and notches in E-pins/sleeves before.
Just a head's up, you can use a dremel stone to break through the nitride to get started. Even with carbide, on a lathe it can be difficult to break through.
A carbide end mill doesn't typically have much issue.

https://www.AR15.Com/media/mediaFiles/173511/DSCN0343_jpg-1116977.JPG

https://www.AR15.Com/media/mediaFiles/173511/IMG_20150121_204703_714_jpg-1116980.JPG

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I guess even dogs get their toes in the shot when they take a gun photo.

Posted: 10/8/2019 12:24:40 PM EDT

[#5]

Start with a piece of drill rod or O-1 steel round stock. Don't screw around hunting up exotic stainless alloys that someone thinks is a good candidate, use common materials that can be heat treated if necessary. Don't heat treat unless necessary, and if so, don't heat treat to the highest strength possible ending with poor ductility.

Discard your notion that stainless steel might be something or other. There are dozens of alloys and heat treated conditions for different purposes.

The correct way to approach this is to measure the length of the rod you have, then the minimum diameter, make an estimate of the port pressure, calculate the load applied to the end of the piston, apply a safety factor, then determine whether the rod is stable. The down stream end that contacts the carrier is the only place that needs "strength" against battering so it won't mushroom.

Make a nice rod out of common drill rod. H13 is high speed steel; I don't know that I would go that way, but it is available as round bar stock, and machining will be no issue. O-1 is available as drill rod and it will be straight with a dead nuts diameter. Then test it in your rifle. Be amazed that such a simple solution works without drama.

Posted: 10/8/2019 3:01:18 PM EDT

[#6]

Quote History

Quoted:
Start with a piece of drill rod or O-1 steel round stock. Don't screw around hunting up exotic stainless alloys that someone thinks is a good candidate, use common materials that can be heat treated if necessary. Don't heat treat unless necessary, and if so, don't heat treat to the highest strength possible ending with poor ductility.

Discard your notion that stainless steel might be something or other. There are dozens of alloys and heat treated conditions for different purposes.

The correct way to approach this is to measure the length of the rod you have, then the minimum diameter, make an estimate of the port pressure, calculate the load applied to the end of the piston, apply a safety factor, then determine whether the rod is stable. The down stream end that contacts the carrier is the only place that needs "strength" against battering so it won't mushroom.

Make a nice rod out of common drill rod. H13 is high speed steel; I don't know that I would go that way, but it is available as round bar stock, and machining will be no issue. O-1 is available as drill rod and it will be straight with a dead nuts diameter. Then test it in your rifle. Be amazed that such a simple solution works without drama.

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In quick reading up on O-1 steel, it sounds like it's prone to rust. What could my factory piston/rod be made of that's polished steel and good properties for being a piston, but have zero hint of rust? That's why stainless came to mind.

Posted: 10/8/2019 5:40:19 PM EDT

[#7]

Scroll down.

Barrett uses 17-4 stainless

Some other manufacturers use the same, iirc.

Posted: 10/8/2019 6:00:24 PM EDT

[#8]

Quote History

Quoted:
In quick reading up on O-1 steel, it sounds like it's prone to rust. What could my factory piston/rod be made of that's polished steel and good properties for being a piston, but have zero hint of rust? That's why stainless came to mind.

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Quote History

Quoted:

Quoted:
Start with a piece of drill rod or O-1 steel round stock. Don't screw around hunting up exotic stainless alloys that someone thinks is a good candidate, use common materials that can be heat treated if necessary. Don't heat treat unless necessary, and if so, don't heat treat to the highest strength possible ending with poor ductility.

Discard your notion that stainless steel might be something or other. There are dozens of alloys and heat treated conditions for different purposes.

The correct way to approach this is to measure the length of the rod you have, then the minimum diameter, make an estimate of the port pressure, calculate the load applied to the end of the piston, apply a safety factor, then determine whether the rod is stable. The down stream end that contacts the carrier is the only place that needs "strength" against battering so it won't mushroom.

Make a nice rod out of common drill rod. H13 is high speed steel; I don't know that I would go that way, but it is available as round bar stock, and machining will be no issue. O-1 is available as drill rod and it will be straight with a dead nuts diameter. Then test it in your rifle. Be amazed that such a simple solution works without drama.

In quick reading up on O-1 steel, it sounds like it's prone to rust. What could my factory piston/rod be made of that's polished steel and good properties for being a piston, but have zero hint of rust? That's why stainless came to mind.

I'm giving you something to start with that will be successful. The material cost is not high, so you can ruin one and then finish out a good one, try it out, then have a spare while making a part from stainless. Here's a secret, maybe - a high polish is resistant to rust. In this case, the combustion products on the end of the rod might defeat that.

17-4PH, 15-5PH. or even 17-7PH are good candidates and can be heat treated to a nice moderate strength of 155 ksi ultimate strength. They'll still corrode, but not like carbon steels. I started to suggest one of those earlier, but decided to hold off.

McMaster Carr has a good assortment of 17-4PH round bar stock in a huge selection of diameters. A 3/8th's piece 24 inches long is priced at less than $20. Plus it is heat treated to 30 Rockwell C hardness. That would be a nice balance, I doubt additional heat treat would ever be required starting with this stock.

https://www.mcmaster.com/standard-stainless-steel-rods

http://www.carbidedepot.com/formulas-hardness.htm

Get a Machinist's Handbook for feeds and speeds to use with carbide or HSS lathe tools.

Posted: 10/9/2019 9:03:36 AM EDT

[#9]

Thank you this is all excellent info. Between all you guys I've definitely got a good starting place.

Posted: 12/30/2019 2:29:49 PM EDT

[#10]

Quote History

Quoted:

In quick reading up on O-1 steel, it sounds like it's prone to rust. What could my factory piston/rod be made of that's polished steel and good properties for being a piston, but have zero hint of rust? That's why stainless came to mind.

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It might be hard-chrome plated. Lots of gas-system components are chrome-plated because it is very hard and can be wire-brushed aggresively for cleaning. There aren't many stainless steel components used in military firearms for various reasons.

Heat treating is another world of esoteric knowledge, difficult-to-find substances and equipment, and frustration. I'm not trying to be a Debbie Downer, but perhaps it would be better to start with making a part that is less critical.

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