How to measure exhaust springs?

[Blue]

I picked up a set of high mount exhausts for my 2000 M900ie.  The cans are held to the mid-pipes with two springs, one top and one bottom.  The pipes came with 3 of 4 springs and I need to know how they are measured in order to order another spring.  Are they measure from coil to coil (~51mm) or from hook to hook (~80mm).  Thanks for the help.

[ducpainter]

Inside of hook to inside of hook.

[Blue]

Inside of hook to inside of hook.

Thanks!

Now, I might be dwelling on this too much but measuring the used spring as stated above came back at 78mm.  I found springs at 75mm and 80mm.  Should I assume that the spring has stretched and was originally 75mm?  Also, the spring gap from exhaust to midpipe is ~95mm top and bottom if this helps to determine the spring length.  Is there a rule for this? 
Thanks again.

[ducpainter]

I wouldn't assume that the spring has stretched. Chances are that's the length they were made.

The only 'guideline' I know of is you want the installed length not more than 20% more than the free length.

If you do the math 78 mm is 18%, 75 is 22%, and 80 is 16%. I think the numbers are close enough you could use either.

I think I'd go with the 80mm springs. They'll give you less trouble installing.

[Blue]

I wouldn't assume that the spring has stretched. Chances are that's the length they were made.

The only 'guideline' I know of is you want the installed length not more than 20% more than the free length.

If you do the math 78 mm is 18%, 75 is 22%, and 80 is 16%. I think the numbers are close enough you could use either.

I think I'd go with the 80mm springs. They'll give you less trouble installing.

Thanks for the help; I will get the springs ordered soon.  I will be going for the 80mm springs.