Bicycle Mechanics - ERD for older rim

I acquired a rim from my Brother in law the other day for use on a Tandem rear wheel build. It is a Sun 40 hole Chinook with a wheel smith decal on it but no other markings or numbers. It is in great shape but when I contact Sun/Ringle for the ERD, they have no specs on the older Sun stuff. Does anyone know how i can get this ERD so I can calculate the spoke length? Thanks in advance!

Measure it (http://www.damonrinard.com/spocalc.htm#MeasureHubAndRim).

Measure it

Cut the elbows off of two spokes so they are exactly 250mm long. Locktite nipples to each spoke such that the end of the spoke exactly matches the end of the nipple.

Now insert the two truncated spokes in opposite holes in the rim. Measure the distance between the truncated spoke ends to get ERD.

Cut the elbows off of two spokes so they are exactly 250mm long. Locktite nipples to each spoke such that the end of the spoke exactly matches the end of the nipple.

I've seen the exactly same recommendation but the cut spokes were to be threaded into the nipples until they came exactly to the bottom of the screwdriver slot in the nipple head.

I've seen the exactly same recommendation but the cut spokes were to be threaded into the nipples until they came exactly to the bottom of the screwdriver slot in the nipple head.
Yes; that is wheelbuilder Roger Musson's (http://www.wheelpro.co.uk/wheelbuilding/book.php) advice:

http://i32.photobucket.com/albums/d7/k4drd/Bicycles/Wheels/MeasuringERD.jpg

Cool. I was hoping for answers like these. This is very helpful. Thanks!:beer:

nm