Sorry for dredging up a very old thread, but I am trying to do precisely the same thing with a Balmar 7 series 110A.
I'm not sure where to find the right part numbers for pulleys. How big a job is that to replace the pulleys? Is it as easy as it sounds? I could also just derate the alternator somewhat and keep the existing belt, but I'd obviously rather do it right.
Which Balmar fits? None have the 2" saddle of a mitsubishi alternator, so I assume you can use the saddle type 7 series and some shimming is required? Will I also need to replace the adjuster bar, as it looks like the bolt to bolt radius is different on the Balmar?
Any help is much appreciated. I'm going to try to do this in the middle of the cruising season, so I don't want to be down for weeks waiting for parts I didn't know I needed or discovering that it won't fit after all because I got the wrong Balmar.
The alts on the W27 are a saddle mount Mitsubishi. The Balmar 6 or 7 series as well as the Electromaax saddle mount alts will usually fit though may require a shim or two. It is a tight fit on those motors due to the manifold so the small case alts like the 6 or 7 series or the Electromaax units seem to fit a little better than an older Delco 10DN style etc..
The W-27 engines have a 3/8" belt which means with a perfect alignment you can squeak about 80A or possibly 90A but the higher you go the more belt dust.
Of course the size of your battery bank and its state of charge will determine if the alt can even put out 80 or 90A. A 110A Balmar will often work fine of a well aligned 3/8" belt because when hot it won't be pumping out 110A for very long.
I am not a big fan of "dual belts" systems and find them only marginally better than a single belt. They often stretch at differing rates and it has become quite hard to get a well matched pair these days.
For the price of a custom "dual belt" pulley set up you can have a true 10 cog serpentine belt that will handle as much as 200A on a single belt. Electromaax has serpentine kits but I don't know if they have one for the W27 yet. They do show a kit for the Westerbeke 27 but I am not sure if this is the W-27 or not. Call Rob or Darren at Electomaax and they could help.
The serpentine kits are worth every penny if you are trying to drive high loads. A dual belt system is a band-aid approach, in my's, GEL's or wets in excess of 400Ah.
You can always go with a larger alternator and de-tune a Balmar regulator in belt manager or small engine mode then upgrade the pulleys at a later date. I really prefer the Balmar MC-614 regulator and even when I buy Electromaax, Powermax or Leece-Neville alts I still use Balmar regulators when installing external regulation.