These 20 x 40 add an extra 30mm of Z movement for your MM3. Note that the vast majority of MM3's were shipped with these as a stock item. Only the very earliest kits will benefit from these. You can confirm you need these by measuring your existing vertical 20x40 mm extrusions. If they are 340mm long (about 13 3/8") they are the shorter ones. These are 370mm (14.6"). These are sold as a pair at our cost.
These are required for the MendelMax 3 Linear Rail Upgrade.