Spiral balance

Hey Rich. This spiral balance had an outer plastic hull or sheath that has probably mostly eroded. It's that outer hull that you have to measure to get the diameter for. If any is still intact, that's what you want to try and measure for the diameter. This can be difficult since the plastic tends to be brittle from age. Do your best. That said, if the inner rod is 3/8" in diameter, it stands to reason that the overall diameter was greater, and I suspect you need the Series 600, which has a 5/8" diameter outer surface. 

If the rod, up to but not including the colored tip, is 31-1/2" long, then round up to 32". Therefore you'd need the S600-32. Finally, to choose the color tip, weigh the window sash these spiral balances support. You can then match that exact weight with one of the weight ranges listed alongside each color option under the product description on the S600-32 shop page. Let me know if you have further questions. 

My apologies! If the outer tube is indeed 3/8", then go with the S900-32 like you were originally aiming towards (To reiterate, if it's 5/8", then you'd use the S600-32 instead). If the sash weighs 14.8 pounds, the Red Tip option is fine; there's some overlap with the color tip ranges. 

Let me know if you need anything else--and thanks for the kind words. That's exactly why we're in business--to offer those parts that let you fix your window without replacing it outright!