Bushnell TRS-25 main screw thread pitch

[Whiterabbit]

Hello all,

I am posting this here because there is misleading information on the internet on an "ar-15 website" that I am not affiliated with nor can easily register on to correct the information.

The Bushnell TRS25 has a screw for mounting to a weaver type base and it breaks easily if you use a warne torque wrench for scoperings and subject it to oversized handgun recoil.

That other site comes up high on a google search and is the only site that states the thread pitch so that replacements can be sourced: #6-40, 1.25" long.

That is NOT the thread pitch of the TRS-25!!

The true thread pitch is M3.5-.6. Yes, it is metric. Thread pitch is .6mm which loosely translates to 42.25 TPI. I can see how the other guy got confused. Also turns out an M3.5 major diameter is within .001" of the major diameter of a #6 screw.

Anyways, this mixup caused a week and a half delay in repair here and I'm the proud owner of an extra box of useless 6-40 screws. 1.25" is also too long, needs to be something like 25 or 28mm long. I have some correct screws on the way now, 25mm and 40mm to cut down if the 25mm is just a smidge too short. They had no 30's.

So hopefully now when people google "Bushnell TRS-25 thread pitch" they will be directed here instead, and see the true pitch of the screw.

Just an FYI for the archives.

[44man]

I broke a screw on one of the Ultra Dots, same problem. I just was snugging it when it snapped.

[Whiterabbit]

And you probably measured the screw, rather than trusting "the internet"

[44man]

Yes, but could not find one, A new one was sent free to replace it. They make the screw too hard.

[357Ruger]

Hello all,

I am posting this here because there is misleading information on the internet on an "ar-15 website" that I am not affiliated with nor can easily register on to correct the information.

The Bushnell TRS25 has a screw for mounting to a weaver type base and it breaks easily if you use a warne torque wrench for scoperings and subject it to oversized handgun recoil.

That other site comes up high on a google search and is the only site that states the thread pitch so that replacements can be sourced: #6-40, 1.25" long.

That is NOT the thread pitch of the TRS-25!!

The true thread pitch is M3.5-.6. Yes, it is metric. Thread pitch is .6mm which loosely translates to 42.25 TPI. I can see how the other guy got confused. Also turns out an M3.5 major diameter is within .001" of the major diameter of a #6 screw.

Anyways, this mixup caused a week and a half delay in repair here and I'm the proud owner of an extra box of useless 6-40 screws. 1.25" is also too long, needs to be something like 25 or 28mm long. I have some correct screws on the way now, 25mm and 40mm to cut down if the 25mm is just a smidge too short. They had no 30's.

So hopefully now when people google "Bushnell TRS-25 thread pitch" they will be directed here instead, and see the true pitch of the screw.

Just an FYI for the archives.

Would you mind sharing where you bought the M3.5-0.6 socket head screws?

[jmort]

https://www.fastenal.com/web/products/details/11507871

[Whiterabbit]

beat me to it.

FYI, 25 is a smidge too short, 40 is WAY too long.

[W.R.Buchanan]

Here's a hot tip for all of you non machinists out there.

If it wasn't made in the US, it is probably metric.

There are very few products manufactured here in the US nowadays, as such Metric Threads are the reality.

Also there are plenty of idiots who think the whole world should convert to the Metric system.(Usually young engineering types with no practical experience) From a machinists standpoint the metric system sucks outright. It is much easier to make mistakes when trying to utilize that system. I have 35 years of making them, so I feel qualified to judge.

When someone tells me that his girlfriend is 1.574 Meters tall (5'2") with a bust of 914mm (36") I am pretty sure he is an idiot!

Now if it is a gun part,,, and made in the US,,, it is probably NOT Metric.

You really need a good way to count threads,,, and by far the best way is to use an Optical Comparator so that you can get an exact measurement between the thread crests and determine the exact Thread Pitch.

A lot of Metric Threads are very close in both Diameter and Pitch to US threads.

Also Male threads will measure slightly smaller on the OD than the exact call out for that size. IE: a 1/2"-13 bolt will be around .485-.490 in actual diameter. This is because the actual crest of the thread is cut off so it will thread into a female threaded object and not bind up when the crest meets the valley on the female thread. This is called "Clearance." it is also the reason why many people cornfuse metric thread sizes with US thread sizes, as many are very close.

It is better to measure the shank of the bolt as that will be closer to the actual size than measuring the threads themselves, for the above reason.

You also need a Machinery's Handbook to nail down the exact thread size after you take the measurements.

Most can get by with the rule in bold type above.

Randy

[44man]

The world is crazy. I find US standard on my Smithy, made in China as well as some metric. Cars drive me nuts with 1/2 sockets for one bolt and 13mm for another.
I hate metric designations for ammo, What is wrong with .223 or .308?
Seeing a screw broken on a sight still makes my warnings about poor mounting systems on some reflex sights valid. One skinny, hard screw with recoil is silly.

[357Ruger]

https://www.fastenal.com/web/products/details/11507871

It states the length as 6mm and I believe it needs to be about 30mm. I only saw lengths in that thread pitch go from 6mm to 20mm.

Am I missing something?

[Whiterabbit]

nope. There's a reason my screws are 25mm and 40mm long, no 30's. That's all my vendor had above 20mm long.

[W.R.Buchanan]

The actual screw for this sight is 3.5 x 28mm (.135 x 1.100) with about 10MM (.393) of threads. I just measured mine. This bolt was probably cut down from a longer bolt, and done this way as to increase the strength of the bolt.

I do this all the time in the shop when I want a bolt with a shank that bares against two holes with something hung in between. I my case I don't want the threads supporting only one end as that will allow movement that will develop into problems later on. I want the shank of the bolt to completely support the load, and usually the threads of the appropriate bolt are cut off so there is only room for a washer and nut. Any excess thread length is not needed,

This bolt size was probably chosen because this sight is designed to mount to a Picatinny Rail where the depth of each slot is .125. the 3.5MM (.136 bolt) is the biggest bolt you can get into that slot that will allow the center of the bolt to bear completely against the side of the slot so the device will not shift axially. They also didn't want threads in contact with the slot face, as there would be considerably less surface area of the bolt in contact with the slot. The Picatinny slots are .210 wide and .135 deep, and so a 4 or 5MM bolt would have a centerline that was very close to the top edge and weakest area of the slot.

Typically bolts have full length threads until a certain length,,, after that they have a shank with threads only on the end of the bolt. An Example is a 10-32 bolt having full threads up to 1.25" in length. 1.5" long and beyond 10-32's only have partial threads. This would equate to 6.66 times diameter for the length of the threads. Past 7x Length to Dia. only get partial threads.

There are exceptions to all of this and I even have doubts that my Length to Diameter Rule is accurate, as there is no hard and fast rule I know of. There is pretty much anything you could possibly want already out there somewhere, and if not you can have what you want made by a machine shop. That's kind of what we do.

Reverse Engineering is all about figuring out how and why someone built something the way they made it.

I do this everyday. After a while,,, you start to understand more of what you think you know. I think?

Randy

[Whiterabbit]

well now I feel educated.

Of course I still have put hundreds of rounds downrange using a cut down 40mm SHCS (threads up to the head) without issue before replacing the sight for a magnified optic. The bushnell is still ready to go right back onto a gun if I want it.

[W.R.Buchanan]

I don't see that much stress on this bolt in the first place.

Just my logic on why they did what they did.

Randy

[44man]

I don't see that much stress on this bolt in the first place.

Just my logic on why they did what they did.

Randy

Stress is severe and the heavier your scope, the more it wants to stay in place from inertia so all strain is on the screw in the base. So many are made for the M16 with no recoil, not for a big bore revolver. I would want two screws to hold to the base.

[06ackley]

Well im glad to know the screw size in case mine ever breaks,but so far I have been lucky and it has held up to several hundred heavy 50ae out of my FA.

[W.R.Buchanan]

Stress is severe and the heavier your scope, the more it wants to stay in place from inertia so all strain is on the screw in the base. So many are made for the M16 with no recoil, not for a big bore revolver. I would want two screws to hold to the base.

The deal with the screw in this case is that a screw with full threads would have the crests of the threads bearing against the Front of the Slot in the Pic Rail instead the unthreaded shank of the bolt. In the case of a 3.5 MM bolt the difference in strength would be minimal. The screw is clamping the sight to the rail but the screw itself is also acting as a "Shear Pin" to prevent fore or aft movement on the rail.

I have found that anytime you remove and replace a clamping type of scope mount on a Picatinny Rail you need to push the mount forward before you clamp it to remove any slop from mounting that would be affected by recoil.

If you don't recoil will move it for you.

Also your Zero will probably shift when you it does. The GG&G mount on my Scout Rifle has to be pushed forward everytime or the Zero is off an inch or so to the left. I was told that a .001 change in position will result in a 1" change in POI.

The GG&G mount is about as good as you can buy at this time for a Picatinny Rail application, but the deficiency is not in the mount, it is in the Aluminum Rail. The rail itself can be battered much easier than a Steel Rail would be and the place on the rail where the mount is placed can change easily as a result of repeated clamping.

For this reason the full shank bolt would be a better option. The sight itself weighs slightly less than 3 oz so there is not a lot of mass to generate much inertia on a rifle. Now on a big handgun where recoil is much more lively this might be a problem.

06ackley: about the only way you're ever going to break that screw is if you over tighten it. That's why they give you that little Allen Wrench, you almost can't over tighten it with just two fingers. If it does break using only the little Allen Wrench then it probably was a bad screw.

Randy

[Finster101]

Reviving an old thread. I just snapped the screw on one of my TRS-25 red dots. Contacted Bushnell by phone and was pleasantly surprised by them sending me a replacement and two spares since I have three of them at no charge. There was no place on the website that I could find to order or buy the screw. If you need one you will have to call the number listed on the site. Just thought someone else could maybe use this info.

[Catshooter]

You are right Finster, this is handy info, thanks.

[AndyC]

Thanks, Finster