Vertical Crankshaft Engine Features Explained


Crankshaft Diameter and Length – This refers to the dimensions of the bare crankshaft sticking out of the engine. Anything bolted to the crankshaft such as blade mount, belt pulley etc. must be removed to reveal the bare shaft when measuring.

Old 4HP and 5HP 4 bolt hole engines (see Mounting Bolt Holes) had a flat bottom rather than a convex “belly” sump. If these engines sat on a flat chassis with just a small hole for the crankshaft to fit through (Hayter 21 and Osprey are the most common), a longer crankshaft engine must be used together with the 995023 18mm spacer ring in order to raise the engine above the chassis whilst maintaining the effective crankshaft length



Mounting Bolt Holes – These are fairly standard. Smaller vertical crankshaft engines have 3 bolt holes, larger vertical engines have 4 bolt holes. The hole spacings and positions have been standard for many years. Some older (1980’s and before) 4HP and 5HP engines used 4 bolts. If replacing these engines new holes will be needed to be drilled in the chassis.


Starter – This will be one or a combination of either Rewind (pull) Start, Electric Start (needs separate battery) or “Instart” (Integral battery). Electric Start uses a battery and wiring loom provided by the equipment manufacturer so engines with this are generally only used to replace similar. “Instart” engines can be fitted where not present before however, a battery, charger and loom kit (part number 995039) is needed as the replacement engine is supplied without these items.


Throttle Control – This is either fixed speed where the engine runs at full speed when started and stopped using the brake, or variable speed where the speed can be lowered by use of a throttle cable. Most modern rotary mowers use fixed speed as the machine is either cutting grass at full running speed or stopped by releasing the brake lever to empty the box etc. Variable speed is usually preferred when replacing older engines that do not have a brake, as the stop wire can be re-routed to the throttle bracket in order to stop using the throttle cable.


Engine Brake – Most applications over the past few decades have required a brake in order to stop the engine when the operator is not holding a lever on the handlebars. If replacing an old engine or one on an application not employing a brake with a new brake complient engine, the brake can be disabled using a Lockout Link and the stop wire re-routed to a throttle control bracket or separate stop switch. Some engines need a longer stop wire in order to re-route to the throttle bracket.


Flywheel – Most rotary mower applications have their blade mounted directly to the crankshaft using a basic blade mount. The blade provides sufficient centrifugal weight in order to act as a flywheel preventing kickback when starting. A standard light weight flywheel is fine where this is the case. Some instances however have a clutch between the engine and the blade or in the case of rotovators and some other equipment, only have a small pulley mounted on the shaft. These applications neeed a heavy cast iron flywheel engine in order to prevent kickback when starting as there is no centrifugal weight on the crankshaft.


Cold Starting Method – For initial “cold” starting, Briggs provide either a rubber primer which floods fuel when pressed 3 times, a manual choke applied via the throttle cable (once started, the cable is dropped back a little to take the choke off) or “Ready Start” automatic choke. This uses a thermostat behind the exhaust to activate the correct amount of choke according to the engine temperature.