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I removed the smaller spring from my 740 and measured it as follows"

Wire diameter : 0.038"
Number of turns: 6
Length between inside of spring "eyes" : 0.650"

My spring may have elongated slightly over time , it is not under tension on the pegs when in the static position so I would make the new spring a slightly shorter.

The end result will no doubt not exactly match the original advance curve but I have found with my cars they run better with a little more advance than original so the final tune by ear when the car is running is probably more important than absolute precision with these springs.

Not at all uncommon for only one of the two springs to be under tension at zero centrifugal advance.

McMaster has a commercial spring that is very close to those dimensions. Wire size .037 and length inside the loop ends would be just a bit less for the suggested shorter length. If tight maybe it could be stretched a tiny bit. Thinner wire and shorter length might mimick the original reasonably well. There is a corrosion resistant spring with the same dimensions but is made of stainless wire and has slightly lower spring load characteristics.https://www.mcmaster.com/#9044k139/=1a2kop8

Thanks everyone for your help, particularly David M for his kindness in removing and taking apart his distributor for the spring measurements. Howard, thank you for the McMaster link. I shall order that spring. Should be a pretty close match.

By the way, on Halloween, the 740 started up and ran for the first time in the seven months I've owned it. I've been through the engine from stem to stern. It appears it was rebored and repoured and put into storage 30 years ago, never run enough even to break in the rings.

The engine was stuck when I got it owing to a small amount of moisture having entered. Thankfully, there was no damage to the cylinder walls and a bit of lapping cleaned up the valves nicely. Cold compression is 83 +/-1 across all 8 cylinders. At any rate the engine sounds great. It was a very exciting day indeed.

PS - Dave Czirr, the next day the very first thing I did was re-torque the head. A few nuts did indeed need a bit of snugging up.