Silvia pressure mod

Silvia pressure mod

A couple of weeks ago my second hand espresso machine, a six year old Rancilio Silvia, started to act strange. There seemed to be little or no pressure on the portafilter. No pressure means no coffee. Bad news. This needed fixing, and fast. I only own a proper espresso machine for a couple of months now, but I love every single bit of coffee making and drinking. This is the setup I currently use:


My coffee setup

Notice the oh-so-useful naked portafilter. Get one immediately if you don't have one yet. Besides the fact that it is fascinating to be able see the coffee-stream emerging from the portafilter, it is an excellent learning tool. But back to the main story now...

At the time I didn't really know exactly how this espresso machine really worked. But I quickly found out. It's not that difficult. In short it is like this:

  • A pump pumps cold water from a reservoir into a preheated boiler. The pump pumps as hard as it can, which is 15 bar in this case.
  • An overpressure valve (OPV) limits the pressure on the boiler to a preset value (default: about 12 bar) by diverting excess cold water back to the reservoir.
  • The pressurized hot water from the boiler gets pressed through your coffee powder at the pressure determined by the OPV.

It's really as simple as that. The OPV is nothing more than a tube with holes, a piston and a spring. The spring presses against the piston. If the waterpressure against the piston is bigger than the spring pressure, the piston will move and create an opening, allowing water to flow back in to the reservoir and thus limiting the pressure on the boiler. Easy no?

Here is a link to a site that has a more detailed explanation:

Anyway, I decided to operate and make good use of this oppertunity to tweak my lovely little Miss Silvia while I'm at it. I was going to fix the problem and at the same time modify my machine to give off less than the usual 12 bar pressure. The default configuration of the Silvia gives off 12 bar of pressure. About 9 bar is optimal. If the pressure is higher, then the water will flow through the filter too fast and / or create tunnels in the coffee puck, thereby reducing the taste quality of the coffee. There are several tutorials around the web on how to modify this type of espresso machine to give off less pressure. Some newer machines do not need to be disassembled to adjust the pressure. I have an older machine without an adjustable OPV. But you can adjust it any way using a little trick. What you do is:

  • open the machine
  • remove the over pressure valve (OPV)
  • disassemble the OPV (which means: screw it open)
  • insert some extra spacers in the OPV
  • reassemble the OPV and the entire machine

So this is exactly what I did. By adding some extra spacers in the OPV, the spring does not get compressed as far as in the original configuration. This causes it to exert less pressure on the piston and thus will open at a lower pressure.

An interesting thing to note is that it took me a few hours of driving around and rummaging throuqh DIY stores before I found anything I could use as extra spacers. In the end I wound up at a shop called 'Brezan', which is a line of car-part stores here in the Netherlands. Believe it or not, but they selll copper rings of exactly the size that fits on to the OPV. They cost me 40 eurocents a piece and they needed no modification whatsoever. These rings are normally used as washers to seal oil drainage holes in combustion engines. I knew this and that is why I went to the Brezan in the first place. Here you can see the reassembled OPV in the machine. If you look closely you can see the extra rings that I inserted.

OPV with washers

After reassembling the machine, it worked again. But why? And what caused the machine to fail? At the time I disassembled the machine, I did not now about the overpressure valve and what its use is. I thought that either the pump was broken or that some water line was clogged. The last option seemed most probable because the pump was still making almost the normal noise as normal when operated. I tried to diagnose the situation as good as I could. I noticed that when I tried to operate the machine in coffee brewing mode (it can also make hot water and steam) the water would not come out of the portafilter, but instead run out of a return-tube back into the water reservoir. I now know this can mean two things: Either the OPV is permanently open, or the waterline running from the boiler is not allowing the flow of water. This last issue can be caused by either a clogged water line or the valve that regulates the water from the boiler is not opening. Well, when I disassembled the OPV, this is what I found:

Disassembled OPV

Yes indeed. It is a very, very dirty OPV. Limestone everywhere. It took me two and a half hours before I eventually managed to get this stoopid thing open in the first place. The limestone had completely fused the two halves of the OPV and because the thing is made almost entirely of brass, it was difficult to exert the amount of force necessary to unscrew the thing without tearing it to pieces. Brass is a very soft metal. Only when I jammed it in a BIG ugly vice, between two soft pieces of wood and using the longest wrench I could find and bashing the living coffee out of it, did it open. I kid you not. In the end it did get fairly dented, but the damage is purely esthetic, thank goodness. Also in the picture you see the two halves of my OPV. On the right you can see the original copper ring, which I later replaced with two bigger washers, as you can see in the second picture from the top.

I now knew fairly sure what the cause of my failing coffee machine was. All that limestone was probably jamming the OPV, preventing it from closing properly and causing all the water to flow back to the reservoir. I got rid of the limestone by removing the spring and piston from the OPV and boiling the rest of the OPV in household vinegar for an hour. Vinegar dissolves limestone. Household vinegar is even better than salad-dressing vinegar, because it is more concentrated. And hot vinegar is even more nasty. Be carefull with that stuff. I stuck my finger in the hot vinegar and it felt like it was eating away at it. Also try not to inhale the fumes; they sting your lungs and that is probably not good. A funny thing is that the OPV turned red like a lobster after boiling it. Probably because brass is an alloy of zinc and copper and the vinegar dissolves zinc faster than the copper, creating a red copper glow.

After reassembling my Silvia I found that I was left with two small and thick copper washers and two thin metal washers. Ooops; I had forgotten to replace these. Originally they were fit between the OPV and the boiler, but I had used teflon tape to seal the OPV to the boiler, which you can see in the second image from the top. The machine hasn't shown any sign of leakage yet, so apparently they're not needed anymore.

Anyway. The problem of my broken Silvia is now remedied and the machine is fully functional again. I do not know how much pressure my Silvia creates with the two rings inserted, but from what I've read on the Net, it should be something like 9 bar. The proof: The quality of the coffee it produces has increased significantly. Much more crema (AKA "yummy coffee foam") and the coffee-stream now looks tiger-striped, like it should, instead of like a blond gusher. The extraction time is now also in the 22 to 30 second range, instead of the default 15 to 20 seconds. For you coffee n00bs out there: Espresso should take between 25 to 30 seconds to brew for it to be really good. Anything shorter or longer is detrimental to the taste.

Hurray for OPV modifications!