There are a lot of selling points to the Gaggia Classic espresso machine, but one of the big ones is being able to adjust the pressure at which the espresso is brewed at. That is a big deal because most of these machines are set at a high pressure to be able to brew espresso pods in them. However, if you buying your own coffee and grinding it youself, you don't want it to be at such a high pressure because it makes your coffee preperation to have to be perfect so you don't get your espresso puck to channel (thats when the water just goes through one part and over extracts.) among other things.
When I first got my Gaggia Classic I had a dropped a good deal of money on this machine and didn't really have the money to test the pressure, as a result I was consistently getting bad espresso, bitter and harsh tasting. I was actually considering sending it back and just eating the shipping + restocking fee. However, I finally after a month of having the machine finally got around to adjusting the pressure and while it made a big difference! My espresso became more what I was expecting, actually drinkable straight! Now it isn't going to fix everything, but it will help.
So the problem is this with your Gaggia Classic out of the box it's set to 12 bar while brewing, you need to set it to 9 bar. To do this we are going to adjust the pressure down to a measurable 10 bar, why 10? because we are going to be measure static pressure, no water is moving or very little.
You will need a device to measure it with, I recommend picking up a 200 PSI gauge, and a 3/8" to 1/4" NPT brass fitting.
That's under 10 dollars without shipping, so if you have the other tools it will be a real cheap fix, you will also need a phillips head screw driver, a small adjustable wrench and a 5 mm hex key.
With those tools you are going to want to attatch the pressure device to the bottom of the portafilter, i think this is the hardest part. To do that you need to remove the spouts from your portafilter, I suggest finding a vice to use as they are put on there very tight and have some sort of glue on them. Once you have accomplished this just screw on the device and you are good to go. Be sure to do as tight as possible, you might get a little leaking but thats fine, we don't need to be super exact.
Unplug your machine first, and do this at your own risk.
Next, you need to remove the top of the Classic, with the screw driver, if you can't do this part without instructions you probably shouldn't be doing this fix.
Now, once you have it removed you are going to find the over pressure valve towards the back of the machine near the pour over. There will be a plastic tube that come out of it and drains into the water tank, remove the tube from the top of it. You just need to pull and coax it off, it's a pretty sturdy piece so don't be afraid to pull a little.
Once you have that off we need to get to the adjustable part of the pressure valve, which is underneath. So take your wrench and loosen the top of it until you can finish unscrewing with you hands. Be careful to only get the nut and not the rest of the piece.
Finally, we can adjust the pressure. Insert your hex key and turn. I found it took about One and half turns to get the pressure where it needed to be. However, you may need more or less turns and you might need to adjust it more than once.
Now reassemble everything, but you can leave the top off just in case you need to readjust.
To measure you just insert you portafilter like making a shot except you need to remove the bottom tray as the device will not fit otherwise, leave you filter in! Since this is PSI not bars we are looking for about 145 PSI. As you can see mine is set at 140, I wanted it a little lower as I think it helps just a tad at being more forgiving.