It was several years ago that I was doing the research. Seems like the factors were size of the piano wire (diameter), number of turns, and the length.

I remember contacting several spring manufactures and all of them wanted to know how many thousands did I want. Then I found the company above they said no job too small! They were perfect for want I needed.....at a great price! I sent them my spring and they sent me 10-12 new ones.

~ Victor
1941 3/4-Ton Pickup (in process). Read about it in the DITY Gallery
1955 Grumman Kurbside "Doughboy" 235/3 on tree w/ OD
1957 3100 - moved on
1959 C4500 Short Bus "Magic Bus" - moved on
1959 G3800 1 Ton Dually "Chief" - moved on
1958 C4400 Viking "Thor" ~ moved on to fellow Bolter