The seal under the mushroom-head screw
The hex screw with the flat, mushroom-shaped head closes off the brew group at a point where hot water is present under pressure. This Teflon washer sits between the head and the group body. It is the only thing that seals there – metal on metal would never hold reliably on a surface like this.
Teflon gives way when tightened and adapts to the surface. That is exactly what makes it seal, and exactly what wears it out: after a few openings the ring is flattened and has no reserve left. Part number C469900420 distinguishes it from the other Teflon seals in the same group, which can hardly be told apart with the naked eye.
When it is due
A fine damp ring appears under the screw head and comes back after wiping. A whitish limescale ring forms at that spot. The screw can be tightened further than last time without getting any tighter – a sure sign that the material under the head has no height left. And of course whenever you have the group open anyway.
At a glance
| Type |
Teflon seal |
| Location |
under the hex screw with mushroom head on the brew group |
| Fits |
Rocket machines with this brew group – the documentation does not list specific models |
| Scope of delivery |
one seal |
| Part number |
C469900420 |
| Design |
Teflon (PTFE); dimensions and thickness are not documented |
| Installation |
installation by a specialist recommended |
| Brand |
Rocket |
Important for installation
The screw sits in a pressurised line. Disconnect the machine from the mains beforehand and make sure it is depressurised and cooled down – on a group that is still hot, water is under pressure and will shoot out when loosened. Teflon is not retightened: anyone who turns the screw «another eighth» tighter squeezes the material aside and will have the same leak at the next shot. We deliberately do not give step-by-step instructions here.
Who it is for
For anyone who services the brew group of their Rocket themselves and consistently replaces every Teflon surface when opening it. If you are not sure which of the screws on your group is meant, write to us – a photo of the opened group is usually enough for us.
What goes with it
Often needed in the same job: the Teflon seal for the relief valve, the Teflon O-ring 36 mm, the flat gasket of the brew lever and the camshaft of the group. For an overview, see Rocket accessories and spare parts and the original spare parts.
The Teflon seals in the Rocket catalogue
We stock five Teflon seals individually. They look alike and sit in completely different places – the part number decides, not the shape:
From our workshop in Bern and Zürich
We never order the small Teflon seals one at a time, but keep two of each number in stock. The reason is unromantic: during reassembly a ring like this likes to fall on the floor, and then it has vanished in the workshop. That is our routine, not a manufacturer requirement. For breaking in afterwards we like the Cozy Chocolate or the Nutty Delight; if you want punch and body, go for the Very Nutty. Everything else is in our coffee beans.
Returns excluded
Original spare parts are excluded from exchange and return. They are procured specifically for each order and cannot be resold once they have been installed or test-fitted on the machine. So check the model, year of manufacture and serial number of your machine before ordering – or drop us a quick line and we will clarify the fit together. Your rights in the event of defective goods remain unaffected.
Frequently asked questions
Is this an electrical part?
No. It is purely a Teflon seal with no electrical function.
Is the screw included?
No. Only the seal is supplied. The mushroom-head screw itself is a separate item and not part of this article.
What are the dimensions of the seal?
For C469900420, dimensions and thickness are not stated in the documentation. Send us the serial number of your machine and we will check the fit.
Is this the same seal as C469900421?
No. C469900421 is the 36 mm Teflon O-ring of the group, a separate item. With Teflon seals, the part number decides, not the appearance.
Can I see the part in your showroom?
Yes. We are in Bern at Gerberngasse 44 and in Zürich at Hagenholzstrasse 50b. Let us know shortly beforehand and we will have the part ready for you.
Questions about the fit? Write to us at [email protected]. Delivery across Switzerland and to Liechtenstein in 2-4 working days.
