The broad strokes: what the engine is, how hard it may be worked, and the compression numbers that tell you whether the rest of this book applies to a healthy motor or a tired one.
Worth knowing before you read your gauge: American-market 1000 cc bikes from 1980 were dropped to 8.2:1 compression to meet emissions (other markets and later models differ — check yours). Combined with the lean jetting of the same era, that shapes almost every decision you will make later about fuel, ignition and cylinder kits.
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 | Value | Alt | Applies | Notes | |
|---|---|---|---|---|---|
| Bore × stroke — 1000cc | 94.0 × 70.6 mm | 979.4 cc | 1000cc | ||
| Compression ratio | 8.2:1 | 9.0–9.5 earlier | US-market 1000 cc, 1980–early ’80s | US-market 1000 cc machines were dropped to 8.2:1 for emissions from 1980, and the 1981 US R100 is the canonical example. Not a blanket rule. BMW's own figures for the US /7 range four years earlier were R60/7 9.2:1 · R75/7 9.0:1 · R100/7 9.1:1 · R100S 9.5:1 · R100RS 9.5:1 BMW-TD77 — so the drop is a 1980-on, 1000 cc, US-market event, not a property of American bikes generally. Market is part of the specification here: European machines of the same model year did not all follow, and later 1000 cc airheads use other ratios again — the R100GS/R100R era is commonly quoted at 8.5:1. Read the ratio for your exact model, build date and market rather than assuming the year decides it. | |
| Compression test | good >10.0 bar | >145 psi | poor <8.5 bar / 123 psi | R60–R100, except R80 below | Normal 8.5–10.0 bar. Wet test discriminates rings vs valves. These are diagnostic ranges, not pass/fail limits — a compression figure is only comparable when the engine is at operating temperature, the throttle is held wide open, both plugs are out, the battery will spin it at full cranking speed for 4–6 kicks of the gauge, and the same gauge is used on both cylinders. Altitude lowers every reading. The difference between the two cylinders tells you more than either number: more than about 10% apart is the finding, even if both are “good”. |
| Compression test — R80 | good >9.0 bar | >131 psi | poor <7.5 bar / 109 psi | R80G/S · R80ST · R80RT · R80 '85+ | Lower thresholds. A healthy R80 mono reading 118 psi is condemned by the general figures above. |
| Idle / max rpm — R100 | 800–1100 | 7000 cont / 7400 max | later 1000cc | R100/7: 6800 / 7400. | |
| Valve timing — '78+ cam | In 16°BTDC–44°ABDC | Ex 56°BBDC–4°ATDC | incl. 1981 | Datum: 2.0 mm clearance, ±2.5°. Exhaust closes 4° ATDC — after top dead centre, giving the normal overlap with the intake that has already opened at 16° BTDC. (An earlier revision of this table printed the same event as 4° BTDC in the note; ATDC is correct, and a 4° BTDC exhaust close would leave no overlap at all.) This is the 1978 column (R80/7, R100/7, R100S, R100RS). The 1977 cam is In 10°BTDC–50°ABDC, Ex 50°BBDC–10°BTDC. |
| Spec | Metric | Inch | Limit | Applies | Notes |
|---|---|---|---|---|---|
| Stanchion (stand tube) outer diameter | 36 −0.050/−0.075 mm | 1.417 −.002/−.003" | /7 range | BMW-WSM7 §31 Hard chromed. The 36 mm tube is why the aftermarket fork-brace market exists. | |
| Stanchion maximum deflection (runout) | 0.1 mm | .004" | straighten or replace beyond | /7 range | BMW-WSM7 §31 Measured on the bare tube. |
| Slider (sliding tube) inside diameter | 36 +0.025/0 mm | 1.417 +.001/0" | /7 range | BMW-WSM7 §31 Light-metal casting. | |
| Running play, slider on stanchion | 0.24 mm maximum | .0094" | service limit | /7 range | BMW-WSM7 §31 Beyond this the front end clunks and the seals go quickly. |
| Spring travel, 75 kg load | 200 mm total | 120 up · 80 down | /7 range | BMW-WSM7 §31 7.87 in total (4.72 / 3.15). | |
| Front wheel castor | ≈ 95 mm | 3.74" | /7 range | BMW-WSM7 §31 No provision for adjustment. | |
| Steering / fork lock angle | ≈ 42° each side | — | /7 range except RS | BMW-WSM7 §31 | |
| Steering / fork lock angle | 35° each side | — | R100RS | BMW-WSM7 §31 The fairing costs seven degrees of lock — why the RS turns wide in a car park. | |
| Test fitting length | 160 mm | 6.30" | /7 range | BMW-WSM7 §31 Top of support tube to the machined face on the lower fork mount. |