Architecture
Crankshaft architecture: 2-stroke vs 4-stroke
Two bottom-end designs, two logics of diagnosis and repair.
A bottom-end noise isn't diagnosed the same way depending on the engine's architecture, because the crankshaft and its lubrication are radically different. The two-stroke relies on a built-up crankshaft turning on needle roller cages, in a case that must stay sealed to air and oil alike. The four-stroke relies on a monolithic forged crankshaft turning on plain shells fed under pressure by an oil circuit. This split immediately steers what to suspect and check. The table below sets the two architectures side by side, point by point.
| 2-stroke — built-up crank | 4-stroke — monolithic forged crank | |
|---|---|---|
| Crankshaft type | Built-up: webs, crankpins and shafts press-fitted. Dismantlable and rebuildable. | Monolithic forged in one piece. Not dismantlable: reground or replaced. |
| Key geometric check | Alignment of the press-fitted webs: a crank that has 'opened up' or twisted at the webs is re-trued in a press. | Runout (TIR) of the forged part with a dial indicator: bend = regrind or replacement. |
| Main-bearing line | Ball / needle roller bearings (big-end on a needle cage). Play and cage condition are checked. | Plain shells (inserts) running on an oil film. Clearance is checked with Plastigage and the size selection. |
| Bottom-end lubrication | Premix (oil in the fuel) or oil-injection: the oil passes through and burns. No oil pressure to monitor. | Pressurised circuit: oil pump, internal galleries, maintained pressure. Oil pressure is a central indicator. |
| Critical sealing | Crankshaft oil seals: a leak lets air in (air leak, lean mixture, seizure) or oil out. Case sealing = the engine's life. | Sealing of the oil circuit and galleries: internal leaks, clogged strainers/jets, valve-cover gaskets. |
| Typical failure modes | Piston seizure (lack of oil / air leak), tired needle cages, leaking oil seals, crankshaft play. | Worn shells (rod knock, mains), oil-pressure drop, clogged galleries, big-end play. |
| Diagnostic priority | First look for an air leak and check the oil seals and bearings; check web alignment. | First measure oil pressure, then bearing clearances (Plastigage) and runout; flush/check the galleries. |
| Repair logic | Rebuildable crank: replacement of the rod/cage and re-truing in a press by a specialist. | Shells replaceable to size; crank reground undersize or replaced if out of tolerance. |
Crankshaft type
Built-up: webs, crankpins and shafts press-fitted. Dismantlable and rebuildable.
Monolithic forged in one piece. Not dismantlable: reground or replaced.
Key geometric check
Alignment of the press-fitted webs: a crank that has 'opened up' or twisted at the webs is re-trued in a press.
Runout (TIR) of the forged part with a dial indicator: bend = regrind or replacement.
Main-bearing line
Ball / needle roller bearings (big-end on a needle cage). Play and cage condition are checked.
Plain shells (inserts) running on an oil film. Clearance is checked with Plastigage and the size selection.
Bottom-end lubrication
Premix (oil in the fuel) or oil-injection: the oil passes through and burns. No oil pressure to monitor.
Pressurised circuit: oil pump, internal galleries, maintained pressure. Oil pressure is a central indicator.
Critical sealing
Crankshaft oil seals: a leak lets air in (air leak, lean mixture, seizure) or oil out. Case sealing = the engine's life.
Sealing of the oil circuit and galleries: internal leaks, clogged strainers/jets, valve-cover gaskets.
Typical failure modes
Piston seizure (lack of oil / air leak), tired needle cages, leaking oil seals, crankshaft play.
Worn shells (rod knock, mains), oil-pressure drop, clogged galleries, big-end play.
Diagnostic priority
First look for an air leak and check the oil seals and bearings; check web alignment.
First measure oil pressure, then bearing clearances (Plastigage) and runout; flush/check the galleries.
Repair logic
Rebuildable crank: replacement of the rod/cage and re-truing in a press by a specialist.
Shells replaceable to size; crank reground undersize or replaced if out of tolerance.