Message-ID: 
Newsgroups: AIRGUN.LIST
In the course of playing with and tuning the Tau 200 rifles, I have come
across some quirks
that I will share with you.
The standard exhaust valve return spring is a bit too weak and a bit too
large in diameter for its
own good.  I have seen stock and PC converted guns exhibit the following
problem:  The
valve spring gets out of postion and unwinds a bit of the end coil
beyond the front of the valve head
to hold the valve open, dumping the entire charge out, and,
unfortunately, damaging the plastic valve seat.
Once the valve seat is dented by the spring, it must be either fished
out (real hard to do) and replaced,
or recut to expose even, unblemished surface (real hard to do also, but
don't require new seat eh?).   I have
successfully recut the seat a couple times, and destroyed it in the
process a couple times as well ... it needs to be
done on the lathe in any case btw.  Be careful not to go too deep, there
is a hard steel bushing supporting the
seat from collapsing, so don't hit it!
Another related problem is the presence of a small shoulder about 2mm
above the seat in the valve chamber wall.
this chamber is what apparently snags the spring to get it cocked out of
position to begin with. If the spring were a teeny bit
smaller in dia it might not snag, but it still might too!  I have found
it expediant to bore this shoulder away to level flush with the top of
the valve seat surface.   Boring this shoulder away increased power
output somewhat, but it can't be helped  (:>).
On guns converted to PC, I have found the Colt Slide return springs I
had earlier recommended have a tendency to restrict
smooth hammer movement, at they are a bit too small in diameter.  I
enlarge the end coils, but there is still some binding that
tries to rotate the hammer actuating piston.  If you cannot turn the
spring perches down in diameter, use a slightly larger diameter spring
to prevent twisting of the piston ot binding on the power screw.
When re-installing the power adjusting screw, take care the hammer strut
goes into its correct postion into the hole in the sliding piston
and that the piston ball-plunger is aligned in the downwards position so
it engages the slot in the reciever tube.  Cock the bolt so the rod end
of the slider protudes slightly, allowing the power adjuster/spring
perch to slide over its end as it threads in.  Never attempt to cock the
gun with the adjuster backed out so the threads are exposed, as the
slider rod may  come out of engagement with the  adjuster and bend the
hammer strut!  Even if it doesnt bend the strut, it may pop out of the
hole in the aduster opon firing and dump all the gas out of the
reservoir, then the strut will be bent when attempting to recock the
gun.
The hammer strut can be straightened when it bends, but frequent bending
will eventually cause it to fail.  When converting to PC I have had to
use a stronger hammer spring to obtain more power ... but care must be
take to not go overboard on this, as the hammer strut (the "Acchilles
Heel" of this design) can only take a moderate increase in pressure from
the spring.  Some guns will develop 12ftlbs with only
the simple measure of 1/4" additonal preload (via spacer) on the
original spring, while others seem to have much more restrictions in the
flow path, and cannot achieve 12ftlbs even with a spring so strong the
hammer strut bends after just a few shots!  If the gun cannot achieve
12ftlbs with approximately 30% more hammer spring pressure, the valve
must be "opened up" by boring the exhaust holes 30% larger and by boring
the shoulder off the id of the valve body til its just flush with the
seat surface as previously mentioned (a good idea anyway).
The above mods all pertain to my PC conversion, using "hydro-tested
components" and no more than 2250psi.  If you need help with any of the
work, or further explanations, email me at:  lhdurham@mindpsring.com
ld
--- QScan/PCB v1.19b / 01-0671
---------------