Well we'm damn excited.
It turns out that the springs I am considering are a straight fit - machining of the retainers will not be required.
To repeat and confirm part numbers, they are CROW CAMS part# 5840.
They are a replacment upgrade spring to suit a L series OHC 4CYL DATSUN / NISSAN engines.
There should be similar springs to suit these engines available from a few different manufacturers.
For the specs on these spring see this link; http://www.crowcams.com.au/media/cat...PRING%20OD.pdf
Here is a comparison picture of the CROW springs versus a stock one off my junk head, along with a stock retainer.
Theye are both very close in free length and outside diameter.
Though the stock spring has a slightly smaller inside diameter than the primary of the CROW springs.
Heres a picture of the secondary of the CROW springs sitting on the retainer.
And here one of both of the CROW springs sitting on the retainer.
Both springs seat well with only slightly
more wiggle room on the retainer than stock.
I have compressed the CROW springs by hand down to the stock installed height and there is a definite difference in pressure compared to the stock spring.
I don't have a strain gauge to test but I would estimate about 25-50% higher seat pressure.
Which roughly confirms my calculations posted previously.
About the only thing I need to get these springs into a F2T head is new spring seats to protect the raised face around which the stock springs usually sit.
It's not absolutely necessary, going without them would just cause some galling (chatter wear) on that face, but I might as well do it right.
I'm thinking that seats from an 8 valve Mazda F series (which came standard with double springs) might just do the trick!