ARCO Marine's Tech Tip on Drive Gears

ARCO's Tech Tip on Drive Gears


Did you know the gear profile for the 9-tooth OEM gear reduction starter and the 11-tooth ARCO High Performance gear reduction starter are the same?

ARCO Drive Gear diagram 9-tooth and 11-tooth

The only difference is the diameter of the drive gear.  Since the 11-tooth drive gear has two more teeth it is naturally larger in diameter.  To compensate for its smaller diameter, the center of the 9-tooth drive gear is located closer to the flywheel.  Both starters are thus completely interchangeable.  These are powerful starters and we are able to take advantage of a better gear ratio using this 11-tooth gear.


Spinning the drive with a wire wheel in the direction it clicks will disengage the locks.  

arco marine engaged drive gear and disengaged drive gear




The Folo-Thru type drive currently used on many starting motors has brought about difficulty due to a misunderstanding of operating and lack of information on proper servicing.  This fact has been reflected by the number of drives returned for warranty which are fully operative.

The Folo-Thru drive is designed to lock and remain in the extended and engaged position until the engine starts and reaches approximately 400 to 500 RPM.  The drive to flywheel rotation is fifteen to one.  When the engine is turning at 400 RPM, the starter drive gear is turning 6,000 RPM.  If the drive is locked in the extended position it has to be reinstalled on the engine and the engine started or the drive must be turned in excess of 6,000 RPM by a wire wheel mounted on an electric bench grinder to make it disengage.

The reason the pinion locks in the engaged position is to assure the starter continues to crank until the engine has started, thus preventing false starts.  This is accomplished by using a spring loaded pin which rides on one of the pinion screw threads and drops into a hole when the pinion is in the fully engaged position.  This locks the pinion in the engaged position.  When the engine starts, the flywheel of the engine drives the starter pinion.  A clutch mechanism is built into the pinion to protect the starter from excessive RPM.

The clutch allows the pinion to turn faster or overrun the armature shaft.  When the engine reaches 400-500 RPM, the pinion spins fast enough to create the needed centrifugal force to throw the spring loaded pin out of the hole in the shaft and allow the pinion to disengage.


Download the full Tech Tip sheet HERE, or visit our Tech Tips Landing Page HERE.

You can also visit our YouTube channel for more useful information