Airgun World – July 2019

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HOME TUNING

is loose enough, your mainspring can end up
banana shaped.
This is only half the story; the spring also
needs to be a fairly close fit inside the piston
because excess space here can also lead to
similar problems. The fit inside the piston does
not need to be overly tight, though. Remember,
the spring expands on compression, and
needs a little breathing space.

FANTASTIC PLASTIC
The best way forward in this situation is to
make or obtain a replacement guide that is a
close friction fit inside the spring, and add a
second, shorter guide to the front of the spring,
often termed a ‘top hat’ due to its shape. This
secondary guide helps to stabilise the spring
within the piston, and gives it a nice smooth
surface on which to rotate.
Most people use Delrin to make the guides,
an engineering-grade plastic that necessitates

the use of a lathe, which puts this process out

of reach for most DIY airgun tuners. All is not

lost, though. On the Internet there are many

people offering this service, so we can all

benefit from this fantastic plastic solution. If

you do take this route, I would advise you to

send your spring to the maker in order to

ensure a perfect fit; springs do vary in size,

even from the same manufacturer. Expect to

pay around £25, or so, for a guide set – it will

be money well spent.

Thankfully, the fitting of a piston liner is well

within reach of everyone because often a very

thin layer of insulating material twixt spring

and piston is all it takes. A bit of trial and error

is sometimes needed to get the correct

thickness, and this can range from 0.15 mm

• 0.5 mm. A quick scout in your household
  recycling will reveal drink cans - typically
  aluminium - or plastic bottles, all of can be
  cut to size and utilised.

FIT FOR PURPOSE

The length is easy to work out because this will

be the piston length, and the width will work as

roughly three times the inside diameter of the

piston. This will get you in the right ball park,

but some trimming will be required to ensure

perfect fit.

To anchor the liner in place, cut a series of

V-shaped notches at one end, and then bend

them to 90 degrees. After insertion, drop a

washer on top and then the top hat will hold

everything securely. The result will be a

twang-free discharge, achieved without

smothering everything in grease.

One final word on resonance and plastic,

but this time in relation to stocks. There are

now a fair few rifles available with synthetic

stocks, and all the ones that I have owned

have a hollow butt section, capped by the butt

pad. The hollow sections can help to magnify

mechanical noise, but the addition of some

Well-fitted guides are not too expensive, and will
transform your gun.

I won’t promise more power, but a soft drink can is often just thick enough to make a piston liner.

Some ‘teeth’ cut into your liner will help it to anchor.

Tailors’ pinking shears can be useful for this. These

ex-army ones were a snip at £1 in a boot sale.

Voids within the stock can magnify

noise. Try filling with suitable material.