Espresso machine have a wide range of sizes of pressure gauges, temp gauges, ports etc etc.
It can be a bit confusing.
They may connect to a boiler or the steam wand, or a pressure gauge.
There are a few fitting types to remember:
T & L
These are used to connect three or two tubes
of the same size
Adapter or Nipple
This is an adapter.
These are used to connect two tubes (for example) of different sizes
This is a hexagonal nipple
Straight 3/8 to 1/2 BSP.
It's nickel plated brass
Most espresso machines use BSP fittings and is sometimes called a pipe or G-type.
There are two versions: Parallel vs Tapered.
BSPP=Parallel / straight (this is the most common).
BSPT = Tapered.
These use imperial inches, not metric measurements.
The main difference between G and M threads is that G threads are a type of pipe thread, while M threads are a type of metric thread:
G threads
Also known as British Standard Pipe Parallel (BSPP) threads, G threads are a type of pipe thread with a 55 degree angle and a constant diameter. They are cylindrical on the inside and outside, and are suitable for pressure-tight connections that are not sealed by the thread. G threads are commonly used in the water and gas pipe industries.
M threads
Also known as ISO metric threads, M threads are specified in metric units (mm). The nominal size of an M thread is determined by the thread's major diameter and pitch.
The common sizes you will find in coffee machines are: G1/8", G1/4", G3/8", G1/2", M9, M12, M6, M10
Flair 58 - uses G1/8
Flair Pro2 - M9
E61 machines
(like the Lelit v3, Rocket R58, La Marzocco GS3 and Profitec Pro series) -
They use a M6 thread for the group head pressure/temp connector.
I use a BooKoo pressure sensor on my machine which uses a G1/8 thread. This makes it compatible with the Flair 58 or La Marzocco GS3.
The Bookoo came with an G1/8" to M6 adapter so it can work with my E61.
I've added a Pressure profiling Kit to my La Pavoni Europiccola.
This uses a G1/8 adapter
So the Bookoo pressure sensor fits onto it (without need for the G1/8 to M6 adapter).
Here are some other examples of adapters:
the M6 end is to the left.
G1/8 on the right end
Pavoni Pressure Gauge Steel Nut M11
This adapter nut allows you to add a boiler pressure gauge.
The Pressure gauge has a 1/8" fitting.
1/8" = (8.6mm) Female
This stainless steel nut will convert La Pavoni models made after 1990.
Models made before 1990 will need a 1/8" to M12 (12mm) adapter.