Formerly made from brass, Michael's group head spacers are now machined from PEEK (Polyether Ether Ketone) material for the v1.0, v1.1 and v1.3 machines. Three thicknesses are available: 3.0mm, 3.4mm, and 3.8mm (marking dots on the rear of the spokes identify the spacer thickness).

Design:

PEEK is a hard plastic that is non-toxic and essentially impervious to the hot water and coffee present in espresso machines.  The material maintains its precision shape and dimensions over repeated high temperature cycles.

PEEK is not as hard as metal, but is sufficiently hard and stiff to maintain a tight seal against the lower brass.   The spokes transfer the compressive force from the central shower screen screw to the rim of the spacer.  A small gap between the center of the spacer and the center of the lower brass allows the spokes to be placed under a small amount of tension when the central screw is tightened.  In this way the seal of the spacer to the lower brass is independent of, and does not rely on, the tension of the shower screen.

The shape and dimensions of the outer face of the spacer are identical to the DE lower brass so any shower screen that can fit on a stock DE machine will fit with a PEEK spacer installed.

Dimensional tolerances are +/- .005 inch, surface roughness Ra 63 micro inch.

Michael's spacer in 3D

Availability:

The PEEK spacers are available as either a kit for the V1.1/v1.3 machines or a kit for the v1.0 machines.  Please send Michael an email if you are interested: mfs@tstorm.com.

The V1.1/v1.3 kit includes (1) a PEEK spacer of the chosen thickness, (2) an M5x16mm reduced profile flat head stainless steel screw, and (3) two brass washers (a primary and a spare, tested to be low lead).  The 3.4mm spacer will allow one to dose close to nominal basket ratings for dark to medium roasts. The 3.8mm spacer will allow one to dose close to nominal basket ratings for medium to light roasts.  The cost for the v1.1/v1.3 kit is $48 + shipping ($4 US domestic shipping or $15 for international shipping from the US).

The v1.0 kit includes (1) one each of the 3.0mm, 3.4mm, and 3.8mm PEEK spacers, an M5x20mm reduced profile flat head stainless steel screw, and (3) two brass washers (a primary and a spare, tested to be low lead). For the v1.0 the spacers are intended to be installed in pairs to achieve overall thicknesses of 6.4mm, 6.8mm, or 7.2mm. The cost for the v1.0 kit is $125 + shipping ($4 US domestic shipping or $15 for international shipping from the US).

Installation:

To install a PEEK spacer,  allow the group head to cool then loosen the central shower screen screw and remove the screen and the screw.  The DE lower brass may or may not fall as it is typically held by the close mechanical tolerances between the upper and lower brass.  Remove the lower brass if it does not come off by itself.  It is good practice to clean the lower brass and shower screen regularly. As a dry run for installing the PEEK spacer, place the PEEK spacer on top of the lower brass as shown below.  Note that the only restriction on the orientation of the spokes is that they clear the two large holes in the lower brass.  A lip on the back of the spacer centers the spacer in the screen recess of the lower brass.  Because there is clearance between the spokes and the lower brass surface, none of the flow from the nozzles will be blocked by the spokes regardless of the position of the spokes.

At this point one of the brass washers may be placed on the center of the PEEK spacer as shown below. This is optional. The purpose of the brass washer is to allow the screen to retain most of its curvature when the central screw is tightened. Without the washer the screen will be flat.

The last step of the dry run is to place the screen on top of the PEEK washer with the provided stainless steel screw as shown below. The reduced profile flat head screw allows the screw head to be recessed below the screen surface. Standard metric flat head screws can be used but can leave an imprint in the puck.

The spacer assembly can be readily placed up on the DE group head upper brass by either first installing the lower brass and then the spacer/screen/ screw as a unit, or all pieces as a single unit. The only tricky part is getting the two pegs on the rear of the lower brass to align with the corresponding holes in the upper brass while not being able to see what you're doing. Once the lower brass is placed correctly, ensure that the PEEK spacer is centered in the screen recess of the lower brass before tightening the central screw. Because of the precision fit of the DE upper and lower brass, the central screw does not need to be tightened really hard to achieve a seal; tighten the screw until resistance is felt, and then apply just a little more torque. Perform a Flush to verify that there is no leakage around the perimeter of the spacer or lower brass: leakage is very obvious with the gushing from around the spacer. You should see dripping from multiple locations more or less evenly distributed around the surface of the shower screen. Note that since the number and positions of the holes in the shower screen are different than the number and positions of the nozzles in the DE lower brass, the drips may be more concentrated in some areas.

Usage:

A spacer is most effective when used with "screen dosing", where the tamped coffee puck is just deep enough to not touch the shower screen when the portafilter is inserted into the group head.  The point is to have the coffee swell and touch the screen during extraction so that the screen helps support the integrity of the top of the puck.  This can be very effective in suppressing channeling and achieving uniform extractions.   Any shot that has channeling will suffer from a combination of over and under extraction, with the bad flavors of each assaulting one's taste buds and making it difficult to determine how to improve the shot.  In order to have the entire surface of the puck supported, the shape of the top of the puck should be a close match to the shape of the screen.  A flat tamper can be used with a flat screen, but better results are usually obtained with a convex tamper and a slightly curved screen.  The convex shape also helps suppress rim channels at the expense of somewhat reduced extraction around the perimeter of the puck.  Use of a tamper with ripples allows one to inspect the top of an extracted puck to see where contact with the screen has flattened the ripples.  The concentric ripples may also help get the water flow into the puck started in the concentric rings. 

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