My self made CNC'd 6061 underdriven alt pulley
 

I decided that rather than pay $60-something and wait a week, I'd make my own pulley when my 4000 mile old alternator decided to take a shit and ruin my nice aluminum pulley. 3"OD, double sheave, Modeled in solid works, and made on a Haas SL-20 Lathe... Out of scrap metal. :)

http://img.photobucket.com/albums/v3.../pulley004.jpg
http://img.photobucket.com/albums/v3.../pulley015.jpg
http://img.photobucket.com/albums/v3.../pulley009.jpg
http://img.photobucket.com/albums/v3.../pulley011.jpg
http://img.photobucket.com/albums/v3.../pulley013.jpg
http://img.photobucket.com/albums/v3.../pulley019.jpg
http://img.photobucket.com/albums/v3.../pulley025.jpg
http://img.photobucket.com/albums/v3.../pulley018.jpg

nice!

what does that do besides the size?

nice work. looks really good.

reduces the alt speed at higher revs. hence the term under drive. better on the bearings.

CNC version of Double Sheave Alternator Pulley
 

Are you offering the CAD files for others to use?
May I have a STEP file, or an IGES file of the native CAD file?
Or may I have a DXF of the CAD drawing file so I may use it to create my own model?
Or, simply a PDF of the drawing?
What length belt do you suggest using? Is it the OEM 32 inch v-belt(s) - connecting the alternator to the water pump?

Jon Bixby
1987 Mazda RX-7 6-port T4 turbo with MegaSquirt 2 ECU
Johnson City, NY 13790

Do your lights dim at idle?

with the larger dia pulley it might make the oem replacement belt too short. just go with the same type belt but a couple inches longer.

I kept my original belts and it worked fine, but then again, I am using a ported FC water pump/housing now, so the bracket it a little... Different.

My lights do not dim at idle either.

Unfortunately, the flash drive that I had the Solidworks models and drawings on fell apart a few days ago. But these pulleys are stupidly easy to draw. If I remember correctly, the V-belt grooves are 30 degrees, and all your measurements will come from original pulleys. Use the back part of the pulley that contacts the alternator shaft spacer as a datum to place the grooves, and use the water pump pulleys to place the grooves relative to one another.