I really like the look of Gilmer belt drives and found a kit at a good price that includes the crank, alternator and (water pump Pulley which I will not use).
Does anyone know if I would be able to get a belt that would fit just for the Crank and alternator or add a tensioner pulley 
You can get belts in different lengths without issue...
Just realize that these are quite noisy. I have a Gilmer driven fuel pump and our first race boat was dry sumped and had a Gilmer dry sump pump, with mufflers you could hear the whine and would drive you nuts without the helmets on...
Otherwise pretty much bullet proof drive system...
We had way more issues with the Jesel cam drive set up, they hate to get wet......
GT
