Dedicated Subsystem WorkbenchPlatform Gen: 2014–2026

2021 Honda Grom (MSX125) Cylinder Compression & Leakdown Diagnostic Engine

Factory compression specs for 2021 Honda Grom (MSX125): standard pressure 145 - 185 PSI (1000 - 1276 kPa), service limit 120 PSI, dry vs wet test diagnostics and acoustic leak stethoscope.

Gen 2014–2026

Factory Platform Compatibility: All workshop fluid capacities, maintenance milestones, torque specifications, and consumables on this specification sheet are factory-identical across model years 2014 through 2026.

Select Model Year:(Active: 2021)

2014 2015 2016 2017 2018 2019 2020 2021 2022 2023 2024 2025 2026

Engine

124cc

Drive

420

Cooling

Air-Cooled

Cylinder Compression & Leakdown Diagnostic Engine

Multi-Cylinder Dry vs. Wet Test Bench, Differential Pressure Regulator & Acoustic Stethoscope Locator

Ratio: 10.0:1Std: 145-185 PSILimit: 120 PSIWhip Adapter: M10 x 1.0

Centrifugal Decompression Cam Weight

CRITICAL THUMPER NOTE: Engine features Centrifugal Decompression Cam Weight. At cranking speed (< 400 RPM), the decompressor holds an exhaust valve slightly cracked open. Normal cranking reading with active decompressor is 80 - 95 PSI. To verify true full cylinder compression (145-185 PSI), either temporarily deactivate the decompressor pin or verify via 100 PSI leakdown testing at TDC.

Diagnostic Presets:

Cylinder #1 (Single)IN-SPEC

171PSI(1179 kPa)

Single Cylinder Pressure: 171 PSI (Service Limit: ≥ 120 PSI)

All cylinders demonstrate strong, balanced mechanical sealing. Piston ring packs, cylinder cross-hatch, and valve seats are within factory specification.

Official Workshop Manual Compression Test Protocol

Step 0 of 5 Completed

Step 1: Engine Operating Temperature

Run engine until coolant reaches 60°C - 80°C (140°F - 176°F). Cylinder expansion brings rings and pistons to true operating clearances.

Step 2: Spark Plug Removal & Port Clean

Blow compressed air around spark plug wells to eject grit, then remove all spark plugs. Thread in M10 x 1.0 whip adapter hand-tight with fresh O-ring.

Step 3: Wide-Open Throttle (WOT) Lock

Hold throttle grip fully wide open (or lock throttle bodies open) to eliminate intake butterfly vacuum restriction and ensure 100% volumetric cylinder filling.

Step 4: Battery Tender Voltage Stabilization

Connect a 12V shop battery tender or booster. Starter must sustain uninterrupted cranking speed (350 RPM) to build maximum dynamic stroke pressure.

Step 5: Crank 4-6 Compression Strokes

Engage starter until gauge needle ceases rising (typically 4 to 6 strokes). Record maximum peak pressure and compare against the OEM limits above.

Official NHTSA Safety Recalls & Technical Bulletins

0 Open Campaigns (Clean)

U.S. Department of Transportation compliance database (Title 49 U.S.C. Chapter 301).

Verify VIN on NHTSA.gov

No Active Federal Safety Recalls Reported

The U.S. DOT National Highway Traffic Safety Administration (NHTSA) records zero open safety recall campaigns for the 2021 Honda Grom (MSX125) platform. Always check your individual 17-digit VIN at an authorized dealership or via NHTSA.gov for individual recall completions.