GR reports cup diameter at two distinct reference positions and publishes cup volume separately from cup depth. This selected 64/66-family reference preserves the exact model suffixes, units and unresolved source issues.
Sources checked: . Next scheduled review: . These are access dates, not manufacturer revision dates.
r2 is not the measurement .032 inches below the rim
GR defines r2 where the bite radius ends and the cup begins. The series number corresponds to this diameter in thousandths of an inch. The r4 measurement is taken .032 in below the rim plane. They are different measurements, not interchangeable names for the same point.
For 66M, r2 is published as .660 in / 16.76 mm, while r4 is .673 in. Do not replace one with the other when comparing rims. GR reports cup volume in cubic inches; a medium-volume designation is not a measured medium cup depth in millimeters.
Original sources: GR: cup diameter, r2 and r4 · GR: rim geometry and measuring positions · GR: cup volume and depth
GR 66M: the checked specification
The 66M row lists a medium rim contour, medium bite and medium-high alpha category. Alpha describes the rim-to-first-cup relationship in GR’s geometry; this categorical entry is not an angle in degrees. The page states that the Classic blank is standard for this family, with exceptions identified for C variants.
The checked row does not publish a numerical throat diameter, throat length or shank taper. A presumed industry-standard drill size must not fill that gap. GR’s player-fit recommendations are manufacturer guidance, not independent performance guarantees.
| Model | r2 (in / maker mm) | r4 (in) | Cup volume (in³) | Backbore |
|---|---|---|---|---|
| 66M | .660 / 16.76 | .673 | .079 · medium | No. 2 |
Original sources: GR: 66-series specification table · GR: alpha-angle terminology
“GR 64” identifies a family, not a complete variant
Choose the exact suffix before assigning cup volume, rim or backbore. Preserve asterisks: 64Z*, 64Z** and 64Z*W are separate model names. This is the checked table’s selection of models, not a declaration that every variant is in stock or still manufactured.
| Exact model | r2 (in / maker mm) | r4 (in) | Volume (in³) | Backbore |
|---|---|---|---|---|
| 64ES | .640 / 16.26 | .651 | .059 | 1 |
| 64E-5 | .641 / 16.26 · source conflict | .656 | .0658 | 1 |
| 64Z* | .640 / 16.26 | .649 | .0625 | 1 |
| 64Z** | .640 / 16.26 | .653 | .063 | 1 |
| 64Z*W | .640 / 16.26 | .647 | .0623 | 1 |
| 64SZ | .640 / 16.26 | .651 | .0641 | 1 |
| 64S | .640 / 16.26 | .651 | .067 | 1 |
| 64MS | .640 / 16.26 | .651 | .071 | 2 |
| 64M | .640 / 16.26 | .651 | .074 | 2 |
Original sources: GR: 64-series specification table
Source discrepancies and comparison limits
The 64E-5 row pairs .641 in with 16.26 mm. Multiplication by 25.4 gives 16.2814 mm, or 16.28 mm rounded. That calculation does not establish which published value is intended. The 66Z* row prints .664 in³, conspicuously unlike neighboring volumes; the intended value needs maker confirmation and is not silently changed to .0664.
GR’s broad Bach comparisons are starting points for fitting, not identical geometry or universal size conversions. We do not assign undocumented specifications to selector-only variants, infer a definition for “buffer,” or turn company-status rumors into catalog facts.
Original sources: GR: 64-series specification table · GR: 66-series specification table · GR: rim geometry and measuring positions
Independent reference, not a performance promise
This is a selected source-checked reference, not an exhaustive stock list or a laboratory measurement database. Unknown specifications remain unknown. Manufacturer fitting advice, player reports and measured geometry are different kinds of evidence.
Commercial arrangements do not determine specifications. These are ordinary reference links; BrassFit has not activated a commission agreement with GR. Product prices are left to sellers’ websites.
Original source register
- GR: 66-series specification table · accessed 2026-09-30
- GR: 64-series specification table · accessed 2026-09-30
- GR: cup diameter, r2 and r4 · accessed 2026-09-30
- GR: rim geometry and measuring positions · accessed 2026-09-30
- GR: alpha-angle terminology · accessed 2026-09-30
- GR: cup volume and depth · accessed 2026-09-30