My dreadful quality original MMC springs that had gone soft in just 10,000 miles were replaced with the excellent performing BCC Ant-Tramp springs
Yes. The MMC leaf springs have been of uncertain and dubious quality for almost 20 years.

The MMC has tried many times to rectify the problem..constantly finding new suppliers who were great for a while and then not as the suppliers tried to lower costs with cheaper metal. I had a period of 3 years with my UK Morgan when I went through 5 sets of leaf springs..each sagging like hell within 3 months. Yet my old 5 leafers on my cdn Morgan (2002) have not sagged more than 4 mm in 21 years!
That being said, I take care of my suspension like a fretting grandmother.BTW, leaf springs, scientifically and in fact are actually much better for the Morgan trad. They can be infinitely designed and deal much better with a flexing frame than any other system.
Sadly, stiffening leaf springs too much, which is characteristic of all leaf springs used for "antitramp" issues, exacts a brutal price on the suspension the rear suspension, the area which governs comfort and comportment. I had a UK mate who used such a "anti-tramp" feature whose wife (who had a bad back) refused to ride in his Morgan thereafter, until he returned to the MMC fare, which required as many replacements as mine did
until I rectified it. .With a 50-50 suspension, your rear spring rate should be the same as your front coil springs. What are using in front?
Good to know about the Biltsteins. But dampers are related to leaf spring rate. I am a famous (infamous?) fan of the Rutherford AVOs, different from the Morgan dampers marketed directly by AVO since David first spec'ed them. He is specifically
Morgan savvy.
Nylocks used to secure them. I've recently started to get a slight rattle from the N/S rear shackle area, so with my damper bolt experience fresh in my mind I will be removing my rear vallance this weekend to inspect and replace the Nylocks with superior Aerotights.
Right again. (unfortunately) Nylocks (aka Nylocs), are used by the MMC in many key stress areas for decades though they make no sense in that usage. Nylocks cannot be torqued and they are too high profile. I use graded Stoffer nuts, which are PIA but are steadfast, very low profile and eliminate the need for much of the constant maintenance and retightening this area demands.
https://www.gomog.com/allmorgan/stover.html I also changed my ubolts to thicker graded metal with stronger graded saddle plates. This stops the characteristic stretching of the MMC bolts which also can be the source of your issues.
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Can someone confirm the spring shackle bolt threads so I can source my Aerotight nuts in advance of the weekend?"
Depends on the size/strength of the ubolts you are using. Are you going to stick with the less-than-robust Factory fare? The best Morgan suppliers will offer you the stuff I describe, though only dear John Worrall and the racing community use Stoffers.
L.
P.S. MMC ubolts should be tightened and/or regularly changed. That being said, between myself, dear Peter Mulberry and Jw, I have not needed to address this area in 20 years.
P.S. Peter Morgan would often remark that he gave his buyers a start. It was for the buyers to finish the car. It is good to see that you and DaveW are doing so