One number cannot describe the complete interface
Nominal 51, 54 and 58 mm systems describe a family of dimensions, not a universal fit. Portafilter lug count, width, thickness and angle must match the machine. Basket rim shape and overall depth also affect fit.
Check the complete stack
Compare the basket depth with the portafilter body, then check the coffee dose, puck screen thickness and group-head clearance. Do not assume that adding a screen to a familiar recipe leaves adequate clearance.
Use specific model references
Record the manufacturer, model, revision and relevant dimensions. For OEM sourcing, request a drawing and a sample check on the target machine. Avoid labeling a part universal unless the supported range has actually been verified.
Cleaning accessories need compatibility checks too
A blind basket or cleaning disc must only be used where the machine manufacturer’s instructions permit the relevant cleaning procedure. Do not attempt backflushing simply because a disc fits the basket.