FixB wrote:I just received the parts and changed the water cylinder and the seal.
And it really works better!
[...]
Here is a picture of my first shot [...]

Glad to hear! That shot does look like a lot of volume, though?
If it's bitter and long then my first guess would be to go for a more restricted shot? Best of luck, I really like the versatility of the machine.

If you just started pulling a shot...

IMHO, I suggest your first shot... have a higher than desired temperature... because the cylinder may absorb some heat...

The following shots.... i suggest should have the exact temp you want to brew at since the previous shot has preheated the metals

Just my advice... may not really be accurate...or even useful perhaps....

I have used a ROK and have had some good shots from it. I never had an issue with water back flowing past the seal while pulling a shot. The best trick I learnt was to evacuate the air from the top of the water column so you were pressing directly onto the water column without compressing any air. My preparation went something like this:
With the arms down, portafilter locked in and a cup placed underneath, I would add water then lift the arms and push down, then repeat. This would preheat the portafilter and the group.
While I ground the coffee in my Porlex I would leave some boiled water in the group to maintain the group temp and reboil the kettle for my shot.
Once the coffee was ground I would purge the water in the group, remove and dry the portafilter, fill with coffee and lock it into the group. Don't forget to put your espresso cup underneath the portafilter.
My water would have boiled and been allowed to cool slightly. I would pour this into the top of the ROK.
After lifting the arms I would gently lower and wiggle them to evacuate the air in the top of the water column. I would then add a bit more water to the column and repeat the process.
It was then a matter of pressing the arms down and watching the shot extract.
If the flow was too fast I wold either tamp harder or tighten the grind and if it was too slow I'd tamp lighter or loosen the grind. This method resulted in a relatively consistent shot. Once the first shot had been pulled there was no need to carry out the initial preheating routine.

Thank you very much for your feedback.
aecletec: the volume is ok. The cup is just very small

I've been pre-heating with hot water just like you proposed 'Double Shot'.
I've tried again and it's still bitter. But, hey, I'm just beginning. And, to tell the truth, I had an espresso at Costa today at work... And mine is already way better than what I got there

Bitterness is probably a sign of overextraction (see guides on this site) so if it's not too long in brewing then the temp will likely need decreasing. I don't preheat my rok with the coffee I use and only use 100-125ml of generally boiling water in the cylinder which leaves plenty of air in it... The process is quite user dependent!
I find measuring the variables helps problem solve. Best of luck!

My friend owns a presso... quite similar to the rok...

And he loves the presso... he's been using it for months now...

I believe all presso/rok users will be incredibly satisfied... the design is simple yet allows such control over everything....

He says it a nice budget machine for a straight up drinker...

Okay, so here is my process: I preheat the ROK, the portafilter and the cup by pumping boiling water through empty potafilter into the cup. While the ROK is heating, I grind coffee beans, not too fine, and then fill the portafilter and tamp quite hard. I don't know how much it is in grams. I've recorded a video of the rest of the process. So far this is my best attempt to get some crema, but this is still too far from what I get on a cheap electric espresso maker:
Advices on improvement are welcome!

The stream looks more like coffee than espresso.

I am listing a few likely causes:

i) Grind is not fine enough. What grinder are you using?
ii) What coffee are you using? And roast date? If possible, get a bag of coffee from local roaster/cafe with roast date less than 2 weeks old.
iii) It's hard to judge. But you may need to press harder to generate sufficient pressure. Doesn't look like it's enough in the vid.
iv) Preinfusion went way too long. For starter, i would just gradually increase the pressure slowly for preinfusion, rather than having a distinct pause (or eliminate the preinfusion all together).
v) Fills more water than you need. Will help with the temperature and air issue.

From start (filling hot water into the chamber) to finish (done pulling) should take 1 minute max (25-35 seconds actual shot pulling time). Shorter the better. On the Rok, temperature falls like a rock(as its name implies ), so if the process takes longer, you may be very well pulling at 160-180F instead of the ideal 200F.

The espresso maker you had most likely has a pressurized basket which creates fake 'crema'. It will produce crema regardless of the coffee you put in,even if it's stale. So that's not a good reference for comparison. Technique wise, you may have to relearn because pressurized basket works differently from the single-wall basket on the Rok. The reward will be worth it though.

i) Grind is not fine enough. What grinder are you using?

I'm using a simple electric grinder. That time I ground a bit coarser than usual, but it's capable of much finer grinds

ii) What coffee are you using? And roast date? If possible, get a bag of coffee from local roaster/cafe with roast date less than 2 weeks old.

It is Tomasso beans, not freshly roasted

iii) It's hard to judge. But you may need to press harder to generate sufficient pressure. Doesn't look like it's enough in the vid.
iv) Preinfusion went way too long. For starter, i would just gradually increase the pressure slowly for preinfusion, rather than having a distinct pause (or eliminate the preinfusion all together).
v) Fills more water than you need. Will help with the temperature and air issue.

From start (filling hot water into the chamber) to finish (done pulling) should take 1 minute max (25-35 seconds actual shot pulling time). Shorter the better. On the Rok, temperature falls like a rock(as its name implies ), so if the process takes longer, you may be very well pulling at 160-180F instead of the ideal 200F.

I tried the above and got much better results, and keep on practicing. I think the main reason of my continuous failures for two weeks after receiving my ROK was bad o-ring that I replaced recently. Air does not escape anymore and I feel much more resistance when pressing levers.

The espresso maker you had most likely has a pressurized basket which creates fake 'crema'. It will produce crema regardless of the coffee you put in,even if it's stale. So that's not a good reference for comparison. Technique wise, you may have to relearn because pressurized basket works differently from the single-wall basket on the Rok. The reward will be worth it though.

You are right, it can produce crema from 6 months old ground coffee. It's interesting, I did not realise the difference in construction and between fake and real crema, will educate myself on this.

Thanks again, I think I'm on the right way )

Blusbreaker, Samuellaw178 is right on the money. I use about 100ml of water in for about 30ml of water out - I stop pushing at that stage. The brewing is about 30s - your flow rate and end volume is far too high.
This cup holds about 70ml maximum and is looking about right