Rebuilding the Air Arms TDR

Previously we got the neglected TDR stripped down to its major components, giving an initial insight into its condition, and what may be needed. It was clear from the outset that something needed to be done with the cylinder, which had a considerable amount of external corrosion. With the gun springing an air leak from somewhere, a full reseal was always going to on the cards and the non-indexing magazine also needed some attention. Apart from some less serious rust on the barrel, the remainder of the gun was pretty good, this extending to the internal parts as well. It was time to make out a shopping list of essential parts, all of which are available from the good people at Chambers Gun Makers.

GENUINE PARTS

It would be possible to re-blue the cylinder, or if not clean off the worst of the rust, but would it really be worth it? Although not currently law, it’s recommended that cylinders should be inspected bi-annually, and replaced after 10 years. Helpfully, Air Arms do label their cylinders with a manufacturing date, although mine had a patch of rust over the area. Considering the likely age of the gun, not to mention the safety implications, and the relatively lost cost of the part, the solution is really a no-brainer. A cylinder must be bought face to face, or ordered through an RFD, although it might be worth asking the supplier if they will send a new cylinder on receipt of your old one. It’s a strange point of law, but must be adhered to.

From experience I would only source ‘O’ ring seals from the gun maker, or at least from a reputable source. This will save a lot of time and heartache. Factory seal sets supplied through Air Arms are very reasonably priced, and each seal is individually packed and labelled with a part number, which can be cross referenced with the parts diagram on the gun’s handbook. To accompany those seals, make sure you have some silicone grease to hand – just a little on each ‘O’ ring makes them easier to slide into position.

Astute readers will recall a problem with this gun’s ability to cycle the magazine, and many times this will be due to a worn or damaged magazine actuator. These have been upgraded on more recent guns, with the bent metal strip being exchanged for a spring-loaded lever. The good news is that these older parts are interchangeable with the replacements, giving a promise of longevity. That completes the list of known parts, so it’s time to strip down the main components, and start replacing the seals.

CYLINDER PARTS

Starting with the valve body, measure and take note how far the brass pot protrudes from the body. This will ensure that it can be returned to the correct position, giving the right amount of tension on the main valve.

Locate the locking grub on the side of the body ¬ if it seems particularly tight, get some heat on there because I have seen these thread-locked. After the grub screw is out, you should be able to unscrew the pot by hand to reveal the main valve, spring, and spring seat.

I like to treat the main valve as a service part, so a new one was sourced. Ensure that the valve seat and internal surfaces are clean and grit free; refit the valve and its spring, returning the pot to its correct height. It’s always good practice to push down on the valve, it should operate freely under finger pressure. Finish off by fitting a new ‘O’ ring to the valve body.

Perform a similar procedure to the filler mount on the other end of the cylinder, the ‘O’ ring here is the same size as the valve body. The filler valve with its internal mesh filter is next up, two ‘O’ rings to replace on the body, with a further much smaller one on the internal valve.

Using a 22mm socket or adjustable spanner unscrew the gauge from its dog-leg mount, there will be an ‘O’ ring to replace on the back. The mount itself has two further ‘O’ rings; one where it connects to the cylinder, the other at the opposite end.

Prepare the new cylinder by cleaning inside with a cloth and rod, aim to remove any oil residue or swarf following machining. Orientate the cylinder so the writing on it can be read, and replace the valve body – hand tight is just right. Likewise, the filler mount can be replaced, using the special double-pin tool. Fit a new ‘O’ ring to the base of the filler valve, and fit to the mount with the aid of a 10mm spanner.

Finally, replace the gauge and its mount, ensuring that it is centralised, and orientated in the right direction, otherwise it will be impossible to fit the stock. At this point, you should have an airtight cylinder.

BARREL AND BREECH

Now we get to strip down the breech barrel assembly, which as well as replacing seals, will involve sorting out the magazine indexing problem. To remove the barrel, remove the first grub screw on top of the breech. Remove the two small dome head hex screws from the magazine retaining spring, to show another grub screw to remove. Turning the breech over will reveal yet another small grub screw to remove. Remove the three screws from the magazine actuator cover, and take off the cover with the actuator, and the cam plate underneath.

At this point, you should be able to wiggle the barrel out, although there can be some thread-lock in there, so some heat might be needed to get it moving.

The breech end of the barrel will have a brass seal carrier, with two ‘O’ rings on the circumference, and a smaller breech seal inside which seals on the bolt. Replace all these seals; the bolt seal is a lot easier with the barrel out, although it can be changed without stripping the barrel out.

To remove the bolt, undo the hex screw, which acts as a cocking dog, and then withdraw the bolt through the rear of the breech. Examine the bolt and the corresponding surfaces inside the breech for signs of wear of damage, before degreasing and cleaning.

Take a look at the magazine actuator; the cam plate simply lifts off, the actuator is then free to slide out from the bottom of the cover. This particular one looked to be bent out of shape, which meant that it was making inconsistent contact with the side of the magazine, thus unable to advance it.

The new actuator exerts greater pressure on the side of the magazine, resulting in far more consistent function. Clean out the cover, applying some molybdenum grease to the side of the new actuator, and cam plate. Ensure that the pin on the actuator slides in the groove on the cam plate, and that the actuator rises as the cam plate is moved back and forth.

The barrel can now be refitted into the breach. Take care to line up the barrel, so that the transfer port is not occluded, easily seen by viewing it from the underside of the breech block. There will be a number of reference marks and holes visible – depression for the grub screw on top of the breech, and a hole for the small grub screw on the underside of the breech. When all the marks line up, reintroduce the screws, tightening each a little in turn, before tightening fully.

Replace the bolt after applying some grease at the contact points, snugging down the cocking dog.

Offer up the magazine actuator cover, ensuring that the pin at the back of the cam plate engages correctly with the bolt shaft. At this point, it can be useful to fit an empty magazine, to ensure that the magazine advances with each cycle of the bolt.

MARRIAGE OF BREECH AND CYLINDER

Before we reunite the breech and cylinder, the hammer and trigger block need to refitted.

To prepare, check the hammer rail is smooth, these can benefit from a little polish. Run the hammer along its length – it should move freely, without any binding. If the rail is clean, and some resistance is noted, it might be worth replacing the two split bushes inside the hammer. These can be drifted out, and replaced under hand pressure if required.

I like to fit the hammer spring, its base washer, and hammer before fitting the trigger block. Ensure that the base washer is the correct way round – flat surface toward the rear, convex surface toward the spring. Thread it onto a screwdriver and direct it toward the base of the trigger block – that way it’s easy to get into position. Fit the spring onto the hammer and push it into the trigger block, but not all the way in so that the trigger engages, just so that the hammer is flush to the trigger block.

Introduce the assembly to the rail and slide on to engage the two dowel pins with the action. Hold the trigger block in place and make sure the hammer is free to back, before refitting the breech.

Make sure a new transfer port ‘O’ ring is fitted in the depression on top of the main valve block. Some grease will be useful in keeping it from moving around when the breech and barrel is introduced.

Drop the barrel and breech assembly back onto the cylinder, being careful not to move things too much, upsetting the transfer port ‘O’ ring. Make sure the cocking dog will engage with the hammer.

Whilst holding its position locate the six screws to hold the breech, these should be tightened evenly to avoid any distortion of the breech. I like to tighten them diagonally to keep even tension.

FINISHING TOUCHES

We’re on the home run now. Replace the barrel band, tightening the two fixing grub screws evenly. Finally, replace stock, and refill with air. If all has gone to plan, the gun should be airtight, and always remember to test-fire over the chronograph. The only thing left to do for this gun is to treat the stock to some oil. Currently, I’m considering the options, nothing to darken the wood too much, just a refresh.

It’s good to have the TDR in full-operation condition, where for a very reasonable outlay, I have gun capable of top-level performance, whilst having the option to break it down to suitcase size if required.