How do I best measure chainring inner size

[p38lightning]

I have an old Fuji that I want to restore, and the large 52t chainring is worn. I want to replace it, but I'm not sure how to best measure the inner size, which is required for correct fit. It is a 5 arm crank for sure, and the rings are still mounted. Any ideas would be appreciated.
Vinnie

[dbrk]

If you can identify the brand of the crank this might very well allow us to identify the bcd (bolt center diameter) and this, of course, will tell us which rings will work. The alternative is to measure, for which you may need a drawing (pictures conveying a thousand words and all that). Try harriscyclery.com or rivendellbicycleworks.com if you need further explanation.

dbrk

[Peter]

Measure from one hole to the next adjacent hole. It's difficult to locate the center of the hole accurately, so don't bother. Instead, measure from say, the left edge of one hole to the left edge of the next adjacent hole.

Post your dimension here in millimeters. I'll convert for you into standard bolt circle numbers using my Sutherland's Manual so you can order the correct chainrings.

Or you can send me a private message and I'll do it that way.

[Louis]

If you have a 5-arm crank some trig will prove that the BCD (bolt circle diameter) = 1.7 x the distance between adjacent bolts (call that d). The BCD is the standard size used to define the attach pattern for chainrings (given the number of arms).

The details: BCD = d / sin(phi/2) where phi = 360 deg / # of arms

Louis