Rohm Kit Fitting

Tuning the Rohm Twinmaster

(CO2 and Pre-Charged Air Models)
Read All Sections Before Starting Work On Your Rohm!

Kit Contents

First check your parts kit, you should have top and bottom spring guide/spacers (now steel in the newest kits) and an assister spring, shown here assembled with the conical upper carrier in the foreground and the flat lower carrier in the background. 

with regulator spring stack shims for the PCP kit or a hammer weight for the CO2 Kit.
An over-length spring is supplied and should be trimmed to length with pliers or wire cutters as required when tuning and fitted with the cut end at the bottom. The last 1/4 turn of the cut end may require bending slightly to locate on the carrier.
For CO2 tuning more spring may be required to achieve the best power, for PCP tuning it is normally sufficient to have the spring with less compression, use as much spring as is comfortable to cock and fire then trim later in the tuning process.
Once the side cover has been modified to clear the assister spring and carriers the spring and rod assembly can be removed without stripping the gun; cock the hammer, insert a pin in the hole in the rod which will now be just under the bottom carrier, uncock the hammer and lift out the spring and rod assembly. Be careful to hold the rod securely and control the springs while removing the pin - the energy in the compressed springs can fire the rod or lower carrier at high speed if not handled carefully. 

Ignore reference to shims and regulator for the CO2 Kit; ignore reference to the hammer weight for the PCP kit, the sections on valve cap thinning and valve stem flowing are under the CO2 heading but should still be read by all, they are primarily of interest to CO2 shooters but may be relevant if you require optimum power with the lightest possible double action trigger pull.




General Requirements

Before commencing this procedure you will have access to a Chronograph, your pistol is capable of performing beyond the UK legal limit and should you inadvertently increase your pistol’s power beyond this limit you will be strictly and personally liable in law.

It is assumed that anyone attempting this procedure is familiar with basic use of hand tools and will seek experienced assistance where they are not; the provider of this document will not be responsible for any damage to any equipment worked on.

Read and understand all the following instructions before carrying out ant work on your pistol.

Stripping the Pistol

Use a 4mm hex key to remove the grip fixing screw from the bottom of the grip.

Hold the pistol upright whilst lifting the body out of the grip. Note the orientation of the plastic spacer in the top of the grip, or keep the grip upright; this spacer must be the right way around for the grip to be reassembled to the pistol correctly.
Stripping The Regulator
The regulator should only be stripped in a clean environment, dirt can cause wear and affect function.
Use a 2.5mm hex key to loosen the 3 regulator cover screws a quarter turn then return near to their previous position without tightening, when ready to clean & lube the regulator remove the cover and carefully remove the internals -
(i) a set of conical spring washers arranged thus ()()()()()
(ii) an outer guide tube to keep the spring stack aligned
(iii) a central double ended piston with an O ring on each head
If (iii) is dificult to remove gently grasp with needle nose pliers below the front piston head and lever up, avoid damage to the O ring or the head.
To reassemble
(a) clean the bore in the (black) base and the bore in the (silver) front and lightly lube with silicone grease
(b) clean the piston ends and lightly lube the O rings with silicone grease
(c) replace the guide tube and then the spring stack, ensure the conical washers are arranged in pairs thus - ()
Increasing Regulator Pressure
When installing  shims these can be placed under the first pair of conical spring washers thus - ()()()()|(), begin with just one shim , check the power output adding one at a time and testing output, variability in the power of the production hammer springs fitted makes guesssing the ideal start point for each pistol impossible so start with one and work up.
At some point output will drop when a shim is added, this indicates that the pressure in the firing valve has reached the point where the hammer strike is no longer able to overcome it, at this point Spacer Carriers or Spacer Carriers and Assister Spring will be needed, again variability in manufacturer's hammer springs makes guessing a start point impossible, start with just carriers and test, then add another shim if more power is required, if the power drops when a shim is added fit in the assister spring.
When the assister spring is used and you have the pistol in the area you require fine reduction of output is possible by removing 1/8 of a turn at a time from the bottom of the spring, the carrier will guide the spring so finishing the end is optional.
(Kits supplied prior to 10/02/2013 need the shim washers between the 3rd & 4th Spring washers - ()(|)()()
Only if access to adjust the air seal position for correct function is required will removal of the regulator be necessary; using a 1.5mm hex key to remove the regulator locking grubscrew,

Then unscrew the regulator, the regulator should unscrew fairly easily, rubber gloves, rubber strap wrench or a special tool to drive the two holes in the face of the regulator can be used for this. With care the shaft of a close fitting drill bit in one hole along with an adjustable spanner across the thread can be used without causing damage, ensuring the hole is chosen so that direction of drive is away from the adjacent hex screw head. Next remove the four side plate retaining screws on the right side of the pistol;

there are three screws of one head size and one of another; ensure the driver is a good fit prior to removing the screws to avoid unsightly cosmetic damage, a bit which is a good fit will stand in the screw without support, this picture shows a quality driver bit located in the small screw.


Using a magnet to keep the removed screws and parts together significantly reduces the chance of losing parts.


Use the trigger guard to wiggle the side plate loose, do this with the screw holes upwards and the gun inside a plastic bag with the front of the pistol pointing into the bag or in a large plain clean area, the magazine feed lever pressure rod and possibly it’s spring will pop out forward or through the magazine slot during this procedure.


Modify the cover plate to clear the helper spring to the profile shown:



This can be done with a round file, sandpaper on a dowel or using a Dremel with the appropriate accessory tool.

Replace the rod and it’s spring in the cover plate and push fully home, use a bent paperclip or staple in the hole taped in place on the outside of the plate to retain the rod and place this to one side.





Increasing the power of the PCP Rohm is a process of increasing hammer strike and regulator output in conjunction with one another, increasing regulator output alone can give an initial power increase (if the standard spring is powerful enough), followed by a decrease as the regulator output is further increased and the firing valve becomes harder to knock open.
A good starting point is difficult to predict as there is much variation between pistols, one regulator shim with the standard hammer spring plus carriers and assister spring fitted often proves to be about right for the first step. If higher power is required add another regulator shim, when the power goes slightly above the required level reduction is achieved by trimming 1/8 of a turn at a time from the assister spring until the desired level is achieved
With CO2 there is no option to increase gas pressure so in addition to the assister spring a hammer weight is supplied to increase hammer mass and thus valve dwell (opening time). Material should be filed or sanded from the sides of the hammer weight until the power just begins to drop to ensure the optimum weight for your gun. Further power increase is possible with a flowed valve stem and thinned valve cap as illustrated below, if this is not within your engineering capabilities Magic 9 Design can offer a postal valve stem flowing and valve cap thinning service, email for details.
Removing and Refitting the Valve Cap
The valve cap threads are very fine and care should be taken not to damage them through cross threading or over tightening. When unscrewing the valve cap gentle forward pressure should be applied to take the valve spring load off the threads, when replacing the valve cap the same forward pressure should be applied and the cap should be screwed in with only fingers for at least two full turns after the threads catch to ensure the threads are not crossed prior to screwing the cap all the way in and tightening.
The cap should be very gently tightened, it is only necessary for the rear O ring to be seated and the cap to be tight enough not to unscrew, once gas pressure is applied in the gun the O ring will lock the cap. If using a spanner holding it's head with thumb and forefingers will provide plenty of force to tighten the cap adequately, similarly if using 1/4 drive socket tools hold close to the axis of the nut with thumb and forefingers - avoid holding the bar or ratchet handle
Flowing the Valve Stem
This is usually only required in CO2 pistols to make best use of the fixed CO2 pressure and the inefficiency of a short barrel, it can be used in PCP guns to improve flow and output whilst keeping a lighter double action trigger pull.
The object of flowing the valve stem is to improve gas flow by increasing gas path cross section and improving flow by rounding sharp corners in the gas path, do not drill the holes larger as in B below, this usually undermines the firing pin leading to collapse and failure. Instead elongate the holes around the circumference of the valve, as in C below, and round off sharp edges in the gas path.
Avoid any ctool ontact with the sealing surface at the circumference of the valve as this can lead to gas leaks and recutting the seat in a lathe may be required.
Thinning the Valve Cap
This is required where valve travel is insufficient and is sometimes required in PCP or CO2 Rohms, if your valve stem protrudes less than 2mm you may see a power improvement from increased valve travel, in the drawing below A shows insufficient valve stem stick-out, B shows ideal stick-out the job is best done in a lathe as this gives a flat surface and allows the correct shamfer and countersinks to be applied.
Installing the Spring Spacers / Assister Spring
Cock the hammer and place a locking pin in the cross hole in the the bottom of the hammer pressure rod, release the hammer and the rod will lift out with the spring, remove the spring then assemble the spacers, standard spring and assister spring back onto the rod ensuring the upper carrier's relieved side is to the rear of the pistol. The current kits do not require any specific orientation of the bottom carrier. (Note that on earlier kits [prior to 22/8/2013] the bottom spring carrier has a relieved surface at one side of it’s curved base; this side is inserted into the pistol first. With the pistol held firmly use a slotted dowel rod or similar tool to compress the spring(s) / spacers into position.)

Installing the carrier with the cutaway at the top rear (to clear the hammer when cocked) this also applies to the new steel Guide/Spacers;


Ensure all components are in their correct positions and drop the side plate back into position, squeeze back and front firmly to locate before removing the tape and magazine feed lever pressure rod retaining wire. Check hammer cocking magazine feed and trigger action before proceeding, if anything is wrong go back and correct the fault before proceeding.



Adjusting piston Valve Seat Position

If your regulator is adjusted far enough from its standard working range you may encounter a situation where it ceases to function, it may then be necessary to adjust the valve position to suit the regulator’s new working range. This is achieved by removing the regulator, taking out the brass flow control plug and using a small hex key to adjust the piston nose valve seat position.

Before doing this you should familiarise yourself with the adjustment with the piston out of the regulator.

Q. Do I need both springs fitted? 
A. Most often yes for HFT or Silhouette, probably not for 10m or action shooting but experimentation is the only way to get your Rohm just right for you.
Q. Do I need to strip the gun each time I want to trim 1/8 of a turn from the assister spring or remove the assister spring for 10m Target work?
A. No, once the side cover has been modified to clear the assister spring and carriers the spring and rod assembly can be removed without stripping the gun; cock the hammer, insert a pin in the hole in the rod which will now be just under the bottom carrier, uncock the hammer and lift out the spring and rod assembly. Be careful to hold the rod securely and control the springs while removing the pin - the energy in the compressed springs can fire the rod or lower carrier at high speed if not handled carefully.