In late 1980 BMW changed what an airhead cylinder fundamentally is. Before: a cast-iron liner in a finned aluminium jacket, honeable, reboreable, forgiving. After: an aluminium barrel with a hard plating bonded directly to the bore — lighter, better at shedding heat, almost immune to wear, and impossible to rebore.
If your bike is post-1980, that single sentence governs everything. A worn barrel is not a machine-shop problem; it is a new-parts problem, or a specialist replating job. It also changes your ring gaps, your piston clearance, and which honing you must never let anyone perform on it.
Everything below is split accordingly. Check your build date first.
Every figure on this page resolves to one model and year in the book. Pick your bike and the rows that apply to it come forward, with the rest marked.
Open this chapter in the book| Spec | Metric | Inch | Wear limit | Applies | Notes |
|---|---|---|---|---|---|
| Thru 1980 | Iron sleeve, Al-Fin jacket | — | to '80 | R100 liners thin — 0.5 mm overbore usually not possible; +0.25 offered. | |
| 1981+ — Galnikal bore | Plated alloy barrel | — | NO REBORE — renew piston + cylinder | 800 & 1000 cc, '81+ | Wrong ring type ruins plating. Re-ring without classical honing. Specialist replating possible. |
| 1981+ — Nikasil bore | Plated alloy barrel | — | NO REBORE — renew piston + cylinder | R45 · R65 · R65LS, '81+ | Same no-rebore consequence, different plating — specify Nikasil, not Galnikal, when quoting a replating job. |
| Spec | Metric | Inch | Wear limit | Applies | Notes |
|---|---|---|---|---|---|
| Bore ID codes A / B / C | 94.005 · 94.015 · 94.025 mm (+.010) | 3.7010–3.7022" | 1000cc | Match letter codes A-A / B-B / C-C, stamped on both parts. | |
| Ovality / taper max | 0.010 / 0.020 mm | .0004 / .0008" | beyond = renew | R100 | |
| Piston OD A / B / C | 93.960 · 93.970 · 93.980 mm | 3.6992–3.7000" | 1000cc | Weight group ± stamped in crown — pistons must match. | |
| Piston/cyl clearance '81+ | 0.030–0.040 mm | .0012–.0016" | 0.080 mm max | '81+ plated | ~.0015" practical for plated bores. Thru-'80 iron: 0.028–0.052 (~.002"). BMW-RA382 R45/R65 are their own case: 0.030–0.050 mm, wear limit 0.08, and BMW specifies where to measure — 14 mm up the skirt on an R45, 18 mm on an R65 (248 book, 5.80 sheet). That measurement point is the part machine shops guess at. This closes workbook item M-01. Insist on the tight BMW figure. |
| Piston orientation | big cutout REAR | small cutout FRONT | All | Intake cutout larger, faces rear. Fit each piston to the cylinder it was measured and matched to, and install it by the factory orientation mark or arrow for that piston design — the cutouts are not symmetrical, and a used piston has bedded to its own bore. That, not a blanket “never swap left and right”, is the actual assembly requirement. |
| Spec | Metric | Inch | Wear limit | Applies | Notes |
|---|---|---|---|---|---|
| Top & 2nd end gap — '81+ | 0.40–0.65 mm | .0158–.0256" | '81 R100 | Plated bores gap bigger by design. Thru-'80: 0.30–0.50 / 0.30–0.45. | |
| Oil scraper end gap | 0.25–0.40 mm | .0098–.0158" | /7 | One-piece scraper, thickness 4.010–4.022 mm. | |
| Top ring side clearance | 0.060–0.090 mm | .0024–.0035" | /7 | 2nd ring: stepped face — marking UP. | |
| Min-gap fitting rule (iron) | — | ≈.0035"/inch bore → ≈.012" @ 94 mm | iron liners | File to minimum on iron bores — gap widens fast bedding in. Never below minimum. Stagger gaps ~120° (4:30 / 10:30 / 1:30). |