IMCO Marine’s Revolutionary SCX Dual-Downshaft Lower Is a Game-Changer for High Performance Sterndrive Boaters 

By Bob Brown

Southern California’s IMCO Marine has introduced its new SCX “Dual Downshaft” lower for boats with 600 to 1,000 horsepower.

IMCO Marine, known for its proven SC, SCX, and SCX4 drive options, spent the last 18 months developing the SCX “Dual Downshift” to fill what the company sees as a gap in the drive market for high-performance boats.

Left: SCX Dual Downshaft with exposed spur gears. Right: SCX Dual Downshaft complete assembly.

The concept behind the “Dual Downshaft” is simple. As more boaters cram more horsepower into the engine compartment, conventional single vertical downshaft drives quickly kill the relatively small lower unit pinion gears. 

Boaters behind the wheel of their hot rods continue to find ways to routinely tear up new gear sets with the inevitable repair bill making for an expensive day on the water.

With only one standard downshaft and single gear to gear contact mandated by the tight confines of the slender and hydro-dynamically efficient gearcase however, installing bigger, more heavy-duty gears is not a feasible solution without sacrificing performance.

SCX 1-1/4” propshaft with two sets of gears ready for dual downshafts.

Recognizing the durability superiority of Mercury Racing’s Number Six and M8 drives, also with dual downshaft technology, the path was obvious. Allowing torque loads to be spread between two shafts and two gears instead of one, this was IMCO’s answer to achieving greater strength, more reliability and longer duty-cycles with higher output engines.

Here are some important things to know. First, the new IMCO “Dual Downshaft” is only compatible with an IMCO SCX upper.

Also, it is not intended to be applied as a ‘surfacing’ drive like the SCX4. For that reason, the “Dual Downshaft” is intended primarily for use on single-engine air entrapment/cat hulls (especially deckboats) with massive horsepower, or vee-bottoms with big output twin or single powerplant applications. 

The “Dual Downshaft” is two-inches shorter than other stock drives and retains an internal shifting cone-clutch (no external transmission needed) and utilizes conventional wet sump lubrication. The propshaft is 1-1/4-inch, however a 1-7/16-inch propshaft option may be offered in the future. All IMCO SCX gear ratios are offered including 1.50, 1.34 and 1.25.

Top view of SCX gearcase with machined holes for downshafts.

Internally, IMCO has integrated a number of solutions including the use of two very high priority custom spur gears manufactured to aircraft quality specs. It is also evident that the lower bullet portion of the gearcase incorporates IMCO’s unique SC forward extended skeg with a built-in low water pick-up inlet.

If you’re wondering how IMCO engineered this product, it should be noted that it was done in-house, aided by sophisticated CAD software and a 3D printer. A little over a year ago, IMCO invested a considerable sum of money in a high-end 3D printer. As the engineering unfolded, what was on the computer monitor was ‘printed-out’ as full-size actual parts in plastic before any of the machining or mold making began.

IMCO has manufactured an initial run of ten complete “Dual Downshaft” units, several of which are out on loan to specific boaters for field testing. So far, all indications are positive without any reported failures. Some of those units have already returned to IMCO for tear-down and inspection receiving a clean bill of health.

So, how much does the SCX “Dual Downshaft” lower upgrade cost? As listed on the IMCO website, a “Dual Downshaft” is approximately $4,500 to $5,000 more than a conventional SCX lower (MSRP $9,900). 

Backend of the SCX gearcase machined with two pockets and downshafts for pinion gears.

A little over $2,000 of that price tag is in custom gears alone. Although it is difficult to calculate at this time how much additional longevity the “Dual Downshaft” will deliver a boater, it should be significant, especially if you’re talking about boats approaching 1,000 horsepower or more. 

When you compute the cost of drive rebuilds, the “Dual Downshaft” begins to look like both a bargain and an insurance policy against inconvenient down-time during optimum boating seasons for repairs.

The production run of “Dual Downshafts” is projected to be available in late January or early February 2019.

The IMCO “Dual Downshaft” is not a replacement or substitute for a Mercury Racing Number Six or an M8 drive, but it might be just what high performance boaters have been wishing for.

For more information: contact IMCO Marine, 510 E. Arrow Highway, San Dimas, CA 91773, 909-592-6162 or check it out online at    

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