The short answer
STOH stands for Since Top Overhaul: engine hours since the cylinders were overhauled or replaced without splitting the crankcase. A top overhaul addresses the cylinders, pistons, and valves — not the crankshaft, camshaft, or bearings — so it does not reset SMOH.
Top vs. major overhaul
A top overhaul works on the engine's "top end": cylinders, pistons, rings, and valve components. The crankcase stays closed, so the bottom end — crankshaft, camshaft, main bearings — is untouched. It's commonly done when compressions trend down or a cylinder needs work mid-way through the engine's life.
Because the bottom end keeps running on its original clock, an engine can read "400 STOH, 1,600 SMOH" — fresh cylinders on a bottom end that is still 1,600 hours past its last full overhaul. Both numbers matter when judging the engine's condition and value.
Related terms and reading
Keep every hour and inspection straight
Maggneto tracks engine times, inspections, and ADs from your actual logbooks — so terms like these become numbers you can act on. Browse the full maintenance glossary.
Frequently asked questions
Does a top overhaul reset SMOH?
No. SMOH only resets at a major overhaul, when the whole engine including the bottom end is overhauled. A top overhaul starts a separate STOH clock while SMOH keeps counting.