S&S Cycle MK136B M8 Crate Engine for 2018-2024 Oil-Cooled Harley-Davidson Softail
Your stock M8 Softail has always been capable of more — the MK136B is the complete answer. Built on S&S’s MKB-Series crankcases with 136 cubic inches, high-flow heads, and a gear-drive cam, this crate engine drops into your oil-cooled Softail and transforms it into the most powerful and reliable version of itself that the platform can produce. Everything is included and ready to install.
Key Features
- Complete ready-to-install crate engine — no sourcing individual components; everything needed is included in the kit
- 136 cubic inch displacement from 4.320″ bore and 4.625″ stroke — one of the largest displacement crate engines available for the M8 Softail platform
- High-flow cylinder heads with +1mm oversized intake and exhaust valves, premium seats, guides, springs, and seals — fully assembled and ready to run
- Valves are .030″ longer than stock for improved valvetrain geometry at elevated lift
- .605″ max lift valve springs support aggressive camshaft profiles
- Enlarged cooling passages around exhaust seats for improved thermal management at high output
- MKB-Series crankcases with patented dual-spray piston cooling jets and precision-machined oil scraper
- Enlarged oil sump with contoured floor for reliable oil return under all riding conditions
- S&S performance oil pump and cam support plate included
- Premium tappets, tappet cuffs, billet aluminum covers, and sprocket shaft spacer included
- Champion spark plugs included
- Gloss black powder coat finish matches factory Harley-Davidson engine appearance
- Cam cover badge included
- Retains stock engine height — no frame modification required
- Accepts all OEM Harley-Davidson engine components
- Available in two cam variants — 475G (49-state compliant for 2018-2020 with factory ECM) or 550G (race only, full 2018-2024 range)
- 1-year warranty; made in the USA
Technical Specifications
- Displacement: 136 cubic inches
- Bore: 4.320″
- Stroke: 4.625″
- Compression Ratio: 11:1
- Valve Lift (Max): .605″
- Valve Oversize: +1mm intake and exhaust
- Valve Length: .030″ longer than stock
- Cam Options: 475G gear drive (49-state, 2018-2020 factory ECM) / 550G gear drive (race only, 2018-2024)
- Pistons: Forged
- Crankcase: MKB-Series with dual-spray piston cooling jets
- Oil Pump: S&S performance unit included
- Finish: Gloss black powder coat
- Warranty: 1 year
- Country of Origin: USA
Fitment Notes
- Fits oil-cooled 2018-2024 Harley-Davidson M8 Softail models only — does not fit water-cooled, liquid-cooled, or Touring models
- Does NOT fit 2025 or newer Softail models — no MK136B fitment confirmed for 2025+ platforms
- Does NOT work with VVT (Variable Valve Timing) or HELIX-equipped models
- 475G cam (310-1296) with factory ECM: 49-state emissions compliant for 2018-2020 Softail models only — S&S’s locked calibration does not support 2021-2024 models at this time
- 475G cam (310-1296) on 2021-2024 models: A ThunderMax ECM replacement bypasses the factory ECM and its calibration restrictions entirely, enabling full custom tuning on 2021-2024 Softail models — however, ThunderMax carries a race-only / not for street use on emissions-controlled vehicles designation, placing 2021-2024 installations in the same legal category as the 550G race variant
- 550G cam (310-1295): Race only — intended for closed-course competition use on all 2018-2024 oil-cooled Softail models; not street legal
- For 49-state compliance on 2018-2020 models with factory ECM, exhaust and air cleaner must match S&S-specified combinations — deviation may void calibration support
- Not legal for sale or use in California on any pollution-controlled motor vehicle
- Not intended for street use on any federally pollution-controlled motor vehicle under the Clean Air Act
- Review the Fitment tab for a complete year and model breakdown before purchasing
