In the past year (after I published the LPE, LEGO pneumatic 3 cylinder inline engine on my web site) I received many questions about how to open and modify a technic pneumatic switches (part no: 4694) (valve). To let a LPE (LEGO pneumatic engine) run super fast it's nessesary to modify the pneumatic air switches and cylinders. Elsewhere on the internet (e.g at www.nicjasno.com/forum) you can find written instructions about how to do it, but many people have trouble modifing a switch successfully. I decided to make a Youtube video of the modification proces and to give you an instructions on this page.
Discalmer
Adjusting the switch using this video instruction will still be very difficult. The first pair of switches that I tried to modify went wrong. In several cases opening a switch went wrong and the switch was forever damaged (deformed). In another case I had not cut enough material (old glue residues and plastic particals) away. I was not able to glue the back plate properly and it resulted in a air leaking switch. And I used in some cases to much glue, which resulted in a switch that clould no longer move smoothly. It is a risky operation and I am not responsible if you damage your switches while your using my instruction! Good luck any way!
Removing the back plate of the switch.
If you start to open a switch then use a very sharp knife. Try to cut around the edges of the back plate. Do not us to much force, otherwise the switch will be damaged. The shape of the plastic switch can deform permanently and later in the proces you won't be able to shut back plate properly.
Take your time to cut all the edge until you feel it got loose. You can place your knife into the switch body against the back plate. Then very carefully try to hop of the back plate (with the knife in serveral positions) until it goes of the switch body.
In the middle of the back plate there is a pin. Sometimes this pin breaks off while I remove the backplane and sometimes it doesn't. When it does breaks off I cut it away from the back plate, otherwise it prevents the back plate fitting well to the switch body.
Cuting away the switch position markers.
At the bottom of the switch body you can see three plastic cams. These mark the positions of the switch. If we remove them, the switch is able to move smoothly back and forward. Use the knife to cut them away.
Removing old glue residues and plastic particals.
The switch body and the back plate must fit well together, before (later on in the process) we can glue them back together. Original the switch body and the back plate were also glued together. You must remove these old glue residues together with plastic burrs and other particals.
Try after each time you have cut away material, if the back plate fits well. Certainly not cut too much material away, but make sure the back plate fits without exerting force.
Drilling the inlet and outlet nozzels bigger.
For an optimal air flow it is necessary that the air nozzles (inlet and outlets of the switch) are drilled up to a diameter of 2mm. Don't drill the holes without opening the switch. You will damage the inside and you can't remove the drilling dust.
Drilling the holes can be done with a small battery drive drill. The drill will center itself.
Cleaning the switch.
After drilling and cutting you must clean the switch parts. Otherwise small particals can damage your switch during operation. A good way to clean the switch is to blow with compressed air from an air compressor. Make sure it's very well cleaned: Any left partical in the switch can demage the switch ones it's in operation!
This is a picture of the compressor I use. It got a 4 liter tank and it can give up to 6 bar of pressure. The inlet handles 27 liter/minute:
Lubricating the moving parts of the switch.
Use some lubrication grees or silicone spray to lubricate the moving part. Don't forget this, because you can't do this ones the back plate is glued on again! But be carefull and don't mess any grees or silicone spray on parts which will be glued. Otherwise the glue won't stuck. Sealing the back plate to the body of the switch.
I've tried several types of Epoxy glues and many worked quite well. It is important to use a glue that useable for atleast a view minutes. Super glue (dry within one minute or less) is for me not workable. And glue that needs to be harden for aleast 24 hours takes to much time before you can see the result. Do not use any foam glue. That glue will expand and there may come glue on parts that you don't want to.
I use glue that is based on two components. Before you can use that glue, the two components must be well stirred until they have a uniform color. The processing time is 10 minutes. The glue is harden (and ready for use) within one hour.
Before you seal the backplate to the switch body be very sure that everthing is all right. Is it clean and lubricated? Isn't there any lubrication on the places where you need to put glue on? Does the switch moves smoothly? Don't put to much pressure on the back plate, otherwise there will be to much friction and the switch won't move back and forward. But if the back plate isn't tight enough mounted on the switch body, air will escape between the rubber valve and the bottom of the switch. Then your switch is worthless.
Failed switch modification.
If your switch does not work after you have performed the modification, then don't throw the switch away. Open the switch for an inspection and try to learn from what went wrong.
First (before opening the switch again) test if the switch leaks air. That can be done simply by connecting a compressor to the middlemost nozzle (air inlet) and place the switch valve in the middle position. Determine if any air is leaking away.
Does the switch moves back and forth too heavy? Try to find out what this caused. You can try to remove the back plate curefully for a close inspection. And you might try a second atemp to glue back plate to the switch body, before you distroy the next switch...
Instruction video.
The video demonstrates how to open and modify the switch:
My Pneumatic LEGO projects 
