HEI distributors are still one of the best ever invented. The cost factor is one of the reasons they aren't readily available, but our motto here since 1959 is "Engines Love Us!" and we are truly HEI fans and use them in our 16' APBA racing runabout-Chev powered of course.
Early twin-engined Chev V6 and V8 marine engines ran the starboard (right) engine in the opposite direction. HEI distributors will operate in either direction just by turning the centrifugal advance plates and weights upside down. Vacuum advance isn't used on marine engines or the new for 2019 ChevProMarine distributors so that negates that ordeal.
If anyone wanted to change the advance curve, that is easily done by changing the combination of weights and springs. Although rarely used, we offer these kits as well as most any part of an HEI distributor. The reason this is considered unimportant is that marine engines typically are operated above 3000 RPMs which is where the advance curve usually stops.
ChevProMarine distributors differ mainly that they have a screened hole in the base. This is to allow fumes from getting in through a vented cap where all the fire is, and to keep spark from exiting. This is an absolute must for your insurance purposes and to keep those pesky marine and insurance inspectors from towing your boat away. Your distributor must have J1171 prominently displayed on your distributor, as is the ChevProMarine. We also offer reverse rotation gears for engines where the crankshafts turn the same direction, but a gear is meshing with the camshaft rather than the common chain drive which reverses the firing order and requires these gears.
The new ChevProMarine for 2019 features 65000 volts with matching modules, and uses .060 spark plug gaps which increases the engines' volumetric efficiency which in turn increases power greatly, and fuel economy if used in this respect.