Thursday, 20 February 2014

.22 BSA Meteor Mk IV dissasembly

A bit of history

The Mk III and all three models of the MkIV are identical air rifles, the difference being that the Mk IV's were contructed out of poorer quality metal. The bigger and altogether different Mk VII is the final model to be made and is rated at 12ft/lb full power, though Mk's I thru VI are pretty similar and all produced about 10.5 ft/lb on a good day with the wind behind you. They were very popular as a first air rifle for larger youths and is still large enough for adults to use very comfortably, first appearing in the late 50's they have sold millions over the decades but recently the desistion has been made to close production. This tit bit of information informs us that there is not a shortage of Meteor's out there lying hidden waiting to be found and refurbished, though finding an old one with the rear open sights sill attached is next to impossible.

The story so far...

I picked up this .22 BSA Meteor over a year ago for a tenner, and believe me it was in a right old state.

Meteor after a quick clean over a year ago.

So i cleaned the metal work and decided to strip the enameling and apply some oil to stop further rust, 

The scope rail on the Meteor are a little shallow compared to a lot of other air rifles.

Top of the barrel in front of the breech

i also gave the inside a quick clean and lube as the buffer washer had disintergrated. 

No rust after storing for well over a year.

The forks are bent out a little allowing side play, 

Length of the cocking stroke.

this is common with most early meteors as the fore stock screws attach to a tab below the forks and will force the forks out when tightened up to much as the screws press against the breech block.   

Chewed up stock screws chewing into the stock, and a bloody awkward rear sight fitting.

The breech moved back and forth also, on closer inspection the breech pin was fine so the problem lay in the hole through the breech wearing wider against the hardened breech pin. 

The old breech seal originally crumbled out so i experimented with turning one out of Delrin, it was crap and didn't seal properly.

God alone only knows what other problems there are hidden away with this rifle, and the only way to sort them is fix them as they appear.


To remove the stock, first the trigger guard has to be removed. 

Trigger guard and a nice wide set back trigger, very pleasant indeed.

The screw at the back is accessable though the slot in the guard, this also conncts the stock to the rear of the compression tube.

You really need the right screw driver for this screw, though you can loosen it outside the guard and finish off with your fingers.

When the screw is removed the the guard is tilted out and slid out, as a tab holds it against the stock.

Simple trigger guard design, i like it.
Secondly remove the two screws at the front of the stock, and all the washers that have been added -or haven't as the case maybe. 
Added washers used to stop the screw pressing against the breech block.

These are the buggers that can force the forks apart when they have started biting into the wood over many years, i would imagine these screws get over tightened is because of the long cocking slot on the stock.

The long cocking slot needs the side screws to fit firmly, or else.

Then the stock comes away and is put to one side so you now get to the trigger assembly, which has to come out so the piston can eventually be removed.

Trigger assembly looks like this, useful for putting it back together.

There are three pins, One larger that pivots the sear, One smaller one to pivot the trigger, and another small one to act as a trigger stop.

The pins can fall out but usually don't, the rear stop pin is the usual suspect in most cases.

The pins can some times fall out, so be careful. However the are certainly loose enough to drift out with slight pressure from a small screwdriver, and this leaves you with a sear, a trigger, and quite a strong spring between the two.

Sear contact adjustment hex grub screw on the trigger, it rests against the trigger stop pin.

This is where i discovered that a bit of the trigger has snapped off, the part that secures the spring in place though there is enough left to just hold it.

And this is how it all should go back together, another useful photo to have for reassembly.

The plastic end cap can come off the end of the compression tube now so you can access main spring which is held in place by a washer and shaped retaining pin, the washer has to be pushed forward either side of the retaining pin. So a special tool needs to made which is easily done, i used a piece of metal pipe and an angle grinder.

The useful homemade tool for removing the mainspring, it really is easy to make.

Secondly the cocking arm needs to be removed from the compression tube, and the barrel needs to be removed for this

Remove the pin where the cocking arm joins the breech, so the cocking arm can slide back to allow the tabs though the wider part of the slot.

I decided to see how much play i had between the cocking arm and the piston when at rest, as it's possible to shorten the piston. The calipers gave me 7 mm to play with safely without the piston slamming into the cocking arm tabs.

Green tape marked where the end of the cocking arm rested with the breech closed, then open the breech till the cocking arm rests against the piston and measure the difference.

 The pin that holds the cocking arm to the breech drifts out easily enough, and is shaped so a rubber washer in the folded steel of the cocking arm pinches it in place and stops it working loose.

Note the shape of the pin and the rubber washer at the front of the cocking arm.

The breech pin drifts out easier one way than the other, and by breaking the barrel the detente bar and spring don't add to the pressure.

I placed the end cap there in case the detente bar shot out, there was really no need in this case as it turned out.

There is play between the breech and pin and it's the breech block, 

The shim i made over a year ago sat on top of the breech block, also the detente bar spring is a little worn but still useable.

though i had shimmed it before with a strip of Tango can and that fixed the problem.

The diameter of the breech pin hole...

The diameter of the breech pin which is the same as the hole in the forks, so that should tell me something.

Placing the prongs of the pre made metal tubing either side of the main spring retaining pin, the compression tube was put in the spring compressor and the retaining pin removed.

The retaining pin is machined to support the spring guide and washer, i had to prod the guide with a screwdriver to free it from the retaining pin.

  Then the preload is let off till the main spring is relaxed, and can then be removed.

A couple of inches of preload.

There was a couple of inches of preload, some of it was added by a delrin slip washer and a copper slip washer i had fitted before.

This is the spring it came with, it is a BSA spring though it may be a replacement for the original at some time.

Then the piston is removed.

Piston with hand made buffer washer and a worn O ring.

Usually at this point people can have a lot of trouble removing the piston because the original buffer washer on the piston has crumbled to dust, i had this very same problem when i first had the rifle apart.

Piston head assembly fits into a key hole slot on top of the piston body.

So i have already cleaned and polished the inside of the compression tube and made a replacement buffer washer, the rifle is also lightly lubricated already. Originally when i first took the internals out i used very fine grade wet and dry dipped in white spirits and cleaned the inside of the compression tube and piston assembly, this was clamped to the end of a rod so i could get right down to the transfer port. I followed this with OOOO wire wool then Autosol metal polish to give a highly smooth mirror finish, luckily there were no score or wear marks inside the tube.

Ali tubing with a slit down the end to act as a clamp, wire wool and white spirit cleaning a very dirty compression tube by the look of it.

A effective way to test the piston seal is to drop the piston into the upright compression tube while sealing the transfer port with your finger, if all is good the piston should stay put then slide down the compression tube when the transfer port is unblocked. If the piston needs a little extra weight to do this it's still a nigh on perfect seal, but if if it's a tight fit and needs a fair bit of effort to move the piston with an unblocked transfer port then the seal is over sized. One remedy is to put the piston and seal in a lathe and lightly wet and dry the seal till the fit is perfect, another is to put the rifle back together and put a tin or two of pellets through it till the seal sizes it self.

 With the Meteor the O ring piston seal was well worn and would slide freely halfway down the compression tube with a blocked transfer port, so in this state the air would only start to compress well after the piston was released severely lowering power.

Piston O ring outer diameter looks to be 26 mm.

O ring has a diameter of 3.5 mm.

Using the calipers i measured the head diameter and O ring diameter, when fitted the O ring is 26 mm and 3 mm in diameter so it's a good job i have a big red box of different size rubber O rings to hand. But this is where i shall leave this blog for now as it's gone on a fair bit, when i get all the photos in of fixing the problems there will be the next part.


Wing Commander Sir Nigel Tetlington-Smythe


  1. Just refurbing a Mk4, useful article & great pics.

  2. Thankyou, i do try to get as many useful pics as i can when working an a rifle. It's good to know people find some of my blogs useful, it gives me that all important boost i need to soldier on.