TX200 – Short Stroke Conversion

Short stroking the TX200 is a very common tune these days. In this post I’ll explain why and how it is done. And show you on my own HFT gun, with a bit of help from Julian at the Air Arms Owners Club forum, and Bond Gun Accessories.

Firstly I am really excited to say that Bond Gun Accessories are now sponsoring my Air Arms TX200. You can see the gun in a previous post, CLICK HERE. This is my HFT competition gun and will be benefiting from some parts from Bond Gun Accessories in this post.
Their website, and the Air Arms Owners Club forum are at the following links. Definitely check them both out if you are an Air Arms fan. Just click on the links to get there:

www.airarmsspares.com
www.airarmsownersclub.com


A Bit of History

When Air Arms first released the TX200 in the early 90’s, it’s design closely followed the very successful Weihrauch HW77. The HW77 had a Ø25mm diameter piston, with 81mm stroke. The TX200 also had a Ø25mm piston, but a slightly longer 84mm stroke. Speak to people for long enough about these guns and you’ll soon find they are very highly regarded. In fact they are often preferred to the modern versions.

The modern versions now have slightly different dimensions. This allows them to generate a bit more power, and makes them more attractive to other markets that don’t have the 12 ft-lb limit we have here in the UK. Weihrauch increased the piston diameter to Ø26mm but maintained their original stroke. Conversely Air Arms retained the Ø25mm piston diameter, but increased stroke to 98mm. 

TX200 Mk2 Piston

From these dimensions we can calculate the swept volume. This volume is the amount of air that the piston displaces when you fire the gun. This air is compressed and fires the pellet out the barrel. Roughly speaking, the more air you have to compress, the more power you can generate.

HW77 (early) = Ø25mm x 81mm = 39.76cc
TX200 (Mk1/Mk2) = Ø25mm x 84mm = 41.23cc

HW77 (modern) = Ø26mm x 81mm = 43.01cc
TX200 Mk3 = Ø25mm x 98mm = 48.11cc

So we can see that the modern versions of these airguns have the potential to generate more power than their predecessors. However for HFT this isn’t actually any use to us. You see, the original versions were perfectly capable of making the full UK limit of 12 ft-lb. In fact the newer versions use weaker springs to restrict the power down and keep them legal in this country.

These weaker springs mean the piston is accelerated slower, and gives rise to a very popular term amongst air gunners. Which is the ‘slow and lazy shot cycle’. Whereas the original versions are often described as having much faster shot cycles. This is most noticeable on the TX200 Mk3, which has the highest swept volume of the examples above. The flip side of this is that the weaker springs are easier to cock, and some people find the slower shot cycle easier to manage.

The big question this raises is, which shot cycle is better? Unfortunately I can’t answer this question for you. It’s down to personal taste. I know people on both sides of the fence on this one. To be honest there is nothing particularly wrong with either. I shoot TX200’s and my personal preference so far is the Mk1/Mk2 specification. 

If you decide to try a lower swept volume tune in your airgun, there are a few ways to go about it. There are pro’s and con’s to each and all are currently used by people to varying degrees.

Volume Reduction Options

Mk1/Mk2 piston
If you have a Mk3 TX200, the easiest way to reduce the swept volume is to fit the piston from the previous incarnation of the gun. The pistons are fully interchangeable so it’s a straight swap. The Mk2 spring and spring guides are still available as spares, however the piston itself is not. Air Arms no longer supply this part so they can be hard to find.

Pros
Easiest option to fit.
Replicates the very popular Mk1/Mk2 specification

Cons
Can be difficult to find the correct part
Only gives one stroke option, so not as adjustable for the tinkerers

New latch rod
If you can’t find an earlier piston, this is the next best thing. The TX200 latch rod (the rod down the middle of the piston that engages the trigger) is threaded into the piston, and held there with strong thread lock. By heating this up to roughly 300°C you can break the bonds and unscrew the rod. Then a replacement rod can be assembled in it’s place. If you can find someone to make one, you can ask for any length you want, which is perfect for the shed tuner who likes to tinker. However be careful to get one from a reputable source, as the rod needs to properly heat treated and hardened. Otherwise you could find your trigger becomes a bit unpredictable…

Pros
Can be set to any stroke length you like
Slightly easier to source than a whole piston

Cons 
Harder to fit than the whole piston
Must be properly hardened

Piston nose extension
Easier than both of the above options, this reduces the stroke by fitting a ‘nose’ to the front of the piston. This is usually aluminium or delrin, and held on with grub screws. An additional benefit of this option is that the nose extension can also double as a piston seal adaptor. So you could use piston seals designed for other guns, such as Weihrauch of Walther, or even an O-ring. This opens up another avenue for tuners. The main drawback is that you lose spring space.
Let’s say in the standard gun there is 300mm of space for the spring to fit in (I’ve just plucked this number out of the air as an example). If you fit a longer latch rod, there is still 300mm of spring room, as the piston head is unchanged. However if you fit a 20mm nose extension, the whole piston moves 20mm back from it’s standard position. There is now only 280mm of space for the spring. Therefore you generally need a even stiffer aftermarket spring to get the power you want.

Pros
Very easy to fit
No modifications required
Can use a wider range of piston seals
Range of strokes can be tested

Cons
Needs a stiffer spring than the longer latch rod option

Compression chamber spacer
This is a new option that I haven’t had a chance to try yet, but you can see it in a recent edition of Airgun World (I think December 2017). The idea is from Jim Tyler and is quite simple. He has machined up a delrin spacer which fits inside the compression tube in front of the piston. This has a machined transfer port down the middle and seals in place with an O-ring. Simply take the piston out, pop the spacer in, then replace the piston again. Assembly is the easiest of the options so far. The spring room issue is the same as the piston nose extension above. And there is another possible issue of the longer transfer port. This has been proven to give slightly less efficiency, so a bit more spring pre-load may be required.
However Jim reckons this modification could cost as little as 50p! Making it by far the most cost effective solution here.
Really this information is Jim’s to give, so please see his very well written article in Airgun World. Jim has said I can try his TX200HC with this mod, so I will know more then.

Pros
Very easy to fit
By far the cheapest option
Range of strokes available

Cons 
Needs a stiffer spring than the longer latch rod option
Longer transfer port may be slightly less efficient
New idea and not fully proven off yet

New piston
On the other end of the scale from Option 4, this is the most expensive. If you speak to the right people you can have a completely new piston made to whatever specification you want. Common versions on this theme include O-ring pistons, titanium pistons and very light skirtless pistons. As well as being whatever stroke you want, they can also be reduced diameter if you modify the compression tube. For example Ø22mm and Ø23mm pistons are currently being used with varying stroke lengths. 

Pros
Any choice of swept volume you could want
Other tuning options available such as piston weight and seal choices

Cons
The most expensive option by far
Variables (weight, spring choice, pre-load, stroke, bore) must all be balanced to achieve the result you want 

That’s the most popular options covered. Hopefully my explanations were good enough. Personally I’ve tried all of the above except for Option 4, the cheap spacer. They all had their merits, but my favourite so far is Option 1, and this is currently what I use in my HFT gun. I find the shot cycle an improvement over the standard Mk3, but without being too snappy or hard to cock. I also like that it is basically a standard configuration, and is known to perform well over tens of thousands of shots. The spring is an OEM part and I have real faith in it. The last thing I want is something going wrong halfway through a competition, which I’ve seen recently from some super tunes.

As I mentioned, my current HFT setup uses a Mk1 piston. I borrowed this from my old 0.22 and swapped it into my HC, and now my full length Mk3. The problem is my Mk1 gun is now missing it’s piston. I tried for a while to find another one but, as I said above, this can be hard. So the plan is to fit a new latch rod to my Mk3 piston. Hence the title of this post…

TX200 Short Stroke Conversion

This is the method I have used this week to short stroke my TX200. Hopefully the steps and pictures below are easy enough to follow. However I take no responsibility for any issues if you attempt this conversion yourself. If you do not feel confident, or do not have the means to power test, try a registered firearms dealer or gunsmith instead.

To do this conversion you will need:

  • TX200 Mk2 Spring and guides (talk to Julian at Bond Gun Accessories). I’ve tried aftermarket springs but honestly the Air Arms ones are hard to beat.
  • Longer latch rod
  • Strong thread lock (I use Loctite 271)
  • Tap and die set (with M10x1.5 bits)
  • Allen keys
  • Spanner
  • Blow torch
  • Bench mounted vice
  • Hacksaw and bench grinder (optional) 
  • Chronograph

Take the action out of the stock
There are four screws holding the stock to the action. One each side at the front, and two in the trigger guard. Remove these, then the trigger guard will come off and the action will come out.

Remove the rear block
The rear block on the TX200 does a couple of jobs. Firstly it acts as a base for the spring guide, and holds the spring in the cylinder. Secondly it holds the trigger assembly in place. The hole in the block allows the piston latch rod through to engage with the sears. To remove it, push down on the block and undo the bolt. 10mm spanner required for this. The pre-load is generally very low on these guns, so no spring compressor is required. You will know if you are pressing down hard enough because the bolt will be easy to undo. 

Trigger block and retaining screw in place
Trigger block removed

Remove the piston
Simple. Once the rear block is removed, the rear guide, spring and piston will just slide out. There is also a top hat inside the piston. Sometimes this will come out with the spring, but it can get a bit stuck inside. Give the piston a bang on a hard surface and it should drop out.

Internals removed

Remove the piston seal and front bearing
You will need to remove the piston seal and piston bearings now, otherwise they will melt later on. Using a flat bladed screwdriver, you can carefully pry the seal up and off. The bearings have a split in. Just find the gap and carefully pull the two sides apart. This will open it up enough to slip them off.

Seal and bearings removed

Remove the latch rod
This is the part that might seem difficult, but it really isn’t too bad. The latch rod thread needs to be heated up to 300°C to break the bonds of the thread lock. I used a regular propane torch, and applied the heat where shown in the photo below. Hold the latch rod in a vice with wooden blocks. This allows you apply a good amount of grip without damaging the metal. Once heated up, grip the piston with an appropriate glove and unscrew it. The metal doesn’t need to be glowing red hot for this. Mine took about 45 seconds with a blow torch before it was ready. It should unscrew without much force, and you will see all the old thread lock coming out with it. 

Piston in vice with wooden blocks
Heat applied
Ready to unscrew
Rod removed

Clean the threads
For best results you should really clean out the thread on the piston. This will ensure the new thread lock adheres properly. The only good way to do this is to run a tapping tool through it. As you aren’t trying to actually cut a new thread, even a cheap tap and die set will do this job. For reference the thread is M10x1.5. Use a bit of oil to lubricate the tool and run it through a couple of times.  I did the old rod at the same time just to be neat.

Cleaning internal threads
Cleaning external thread

Size the new latch rod
At this point you can size the new latch rod to whatever length you need. The one I received is long enough to reduce the stroke to 80mm. So cut 4mm off if you want to replicate the Mk1/Mk2 (84mm). The shorter the rod is, the longer the stroke will be. The rod is hardened but a new hacksaw blade will cut through it. The end should tidy up with a bench grinder or a good quality metal file.
However I’ve decided to try the latch rod at it’s current length. This will mean a stroke of only 80mm. But there is some logic here. See the calculations below:

HW77 (early) = Ø25mm x 81mm = 39.76cc
TX200 (Mk1/Mk2) = Ø25mm x 84mm = 41.23cc
Short Stroke Test = Ø25mm x 80mm = 39.27cc

At 80mm stroke this is 1.2% less swept volume than the HW77, and 4.8% less than the early TX. But both of these could make 12 ft-lb muzzle energy, and I only want 10.5 ft-lb. That’s my usual HFT power.
The only potential issue is the anti-beartrap mechanism might not function at this short a stroke. Not a problem for me. This is an HFT only airgun so I don’t need it. If I was doing this conversion on my hunting version I would make sure it worked properly.

Mk3 Latch Rod
80mm Stroke Latch Rod
Latch Rod Comparison

Fit the new rod
For this you will need a strong thread lock and some kind of jig for keeping the rod central whilst the thread lock cures. If you have access to a lathe you could make a simple turned part that slots into the piston and has a Ø10mm hole in the middle for the rod to go through.
I don’t have access to a lathe so I used a more makeshift method, using a roll of PFTE tape and a TX200 top hat. The top hat is already close to fitting in the piston, so I just wrapped the PTFE tape around it, until it was a very close fit. Then the same with the latch rod. Wrap the PTFE tape around it until it is a snug fit inside the top hat. Now when you screw the latch rod into the piston, it will be held straight and true. Simple. (At the end of this process I measured how central the rod was. It was a couple of tenths of a mm out at the trigger end. Not too bad. And after roughly 100 shots there have been no issues).

Ready to loctite

Always do a test run without any thread lock first, just to make sure it all lines up how you expect. It can start to cure quite quickly so don’t get caught out. When you are ready, apply a small amount to the thread on the latch rod and screw it in. I used Loctite 271, I’m sure other brands are available. Now just let it cure. Over night should do it.

Re-assemble
This is the reverse of the disassembly process. Except this time using the new Mk2 mainspring and guides from Julian at Air Arms Spares. The Mk3 spring is too soft to make much power at 80mm stroke. The Mk2 spring is a bit stiffer. One change I made was to use the 0.22 calibre top hat instead of the 0.177 top hat. It’s a bit lighter and I think better suited to sub 12 ft-lb power levels.

Mk3 on top vs Mk2 on bottom

With regards to grease, a little goes a long way. A light smear is all that is needed on the spring and the guides. Even less is needed on the piston bearings and the seal. A good method for these is to wipe on a small amount, then using a clean finger, wipe it off again. The tiny amount left is all that is needed.
Getting the seal back on the piston can be a bit of a pain, but I have a little trick that works very well.
Step 1: drop the seal into some hot water for a few minutes to soften it up a bit, that makes the next step even easier.
Step 2: If you’ve ever put a new grip on a cricket bat, this will look familiar. Push the now warm seal down onto a 3/4 inch extension (as shown below). This stretches it out to the same size as the cone on the top of the piston. Then place the extension on top of the piston and push the seal down onto the piston head. It might take a couple of tries to get the knack right.

Ready to go back in the gun

Putting the trigger block back on will need a bit more force than before, as the new spring is a bit stiffer, but should still be very easy to do by hand. If needed, use a bit of body weight to pre-load the spring. 

Total pre-load

Power Test
This step is very important. You absolutely MUST power test your airgun after you have made any changes that might affect the muzzle energy. So get a chronograph, or get friendly with someone who has got one.
For the initial rebuild, I only used one power washer, and I’m aiming for 755 fps. That’s the magic number for my HFT setup right now. It’s also worth testing with a range of pellet makes and designs as the power can vary.

10 shots to let it settle in, then 10 shots over the chrono with my HFT pellets gave an average of 779 fps. That’s 11.4 ft-lb and about perfect for some people.  A little hot for me though. So it all came apart again and that washer came out. Back over the chrono and the new average is 758 fps (10.8 ft-lb). This spring is brand new so I’m going to put a few hundred shots through it before the next competition as the power might fluctuate as it beds in.

Accuracy and final thoughts
Accuracy was never as issue with this TX so I’m expecting it to perform just as well as it always has. However the shot cycle definitely feels different. The 4mm stroke reduction and new spring has made it feel even quicker than it was before, but not in a ‘snappy’ way. Compared to the 98mm Mk3, the difference is huge. 
As predicted, the anti bear trap system doesn’t work anymore, but I’ve been reliably informed it will work fine at 84mm stroke, so that’s something to keep in mind if you want to go down this route.

The card below is three groups of 10 shots at 20 yards. They aren’t the one hole groups you would expect with a PCP, but they were shot from the prone HFT position and holding up the peg. No benches, no beanbags, and no resting the butt on the ground. I’m happy with that.
If ultimate accuracy and bench resting are your goal, I’m sure this would one hole group with ease.

10x shot groups at 20 yards. HFT prone, holding up the peg

Overall I couldn’t be more happy with how this has turned out. It has made the TX even nicer to shoot than it was before. The most noticeable difference is the reduction in sight picture movement. Meaning when you take a shot, the crosshairs don’t move as much as they used to. 
Best of all (in my opinion) it’s using OEM spring and guides. This should mean no reliability issues, and if anything does go wrong, it’s a simple fix. In fact I’m going to get some spares to keep with me just in case.

To finish, a final thank you to Julian at Bond Gun Accessories. The parts I got from him were spot on, packaged well and have done the job perfectly. 

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6 Comments on "TX200 – Short Stroke Conversion"

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Rich
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John Knibbs sell the mk1 pistons for the TX.

Joseph Hallare
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Thank you for a comprehensible explanation of the various methods of reducing the volume of air required to achieve what is wanted.

Joseph Hallare
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Unfortunately Jim Tyler’s article on his Compression chamber spacer is unavailable to me. Airgun World is not distributed locally in the Philippines. But I could understand and grasp the principle well enough. Instead of decreasing the volume by lengthening the piston you decrease the volume by lessening the distance the piston has to travel. An added benefit in doing this is that there is no added weight to the piston. Although the Transfer port will be lengthened I don’t think it will matter as much as a heavier piston. Besides the Transfer Port diameter of the spacer can be increased… Read more »