nuts on engine feet

[LittleSister]

. . . I am thinking of doing away with the lower, height adjusting nyloc nut, replacing it with a thinner, plain nut. . .

As such an arrangement seems to have proved perfectly satisfactory on the old mounts, then why not?

Add Loctite for belt and braces.

There is also a split washer that I suppose will help to provide torque.

No, if I understand you correctly that's not how split washers work. The nut should be clamped down firmly enough (= way beyond the split washer's limited 'springiness') such that the washer is flat and not providing any 'spring'. See the following -

"Split ring washers use friction to prevent bolted joints from loosening. They feature a ring that has been split and twisted - creating two sharp edges. These washers are installed between the bolt head/nut and mating surface, the bolted joint is then tightened in the same way as an unsecured bolt. When the nut is tightened, the washer flattens down, pushing the sharp edges into the mating surface.

Any locking ability works best on softer materials as it is easier for the edges to dig into them. It is ineffective at locking bolted joints experiencing higher levels of dynamic loads. It is also ineffective when the bolted joint requires lubrication, which promotes sliding and rotation of the bolt."

(The above is from the website of an international supplier of 'bolting solutions', which also has a free (if you give them your address etc) downloadable ebook "Why do bolts loosen?"(A comparison between common bolt securing options).)
Split ring washers.

 

[rogerthebodger]

I use nylock nuts to lock the top M16 nut down onto the mounting feet to clamp the hight adjuster nut in place

 

[BabaYaga]

No, if I understand you correctly that's not how split washers work. The nut should be clamped down firmly enough (= way beyond the split washer's limited 'springiness') such that the washer is flat and not providing any 'spring'. See the following -

"Split ring washers use friction to prevent bolted joints from loosening. They feature a ring that has been split and twisted - creating two sharp edges. These washers are installed between the bolt head/nut and mating surface, the bolted joint is then tightened in the same way as an unsecured bolt. When the nut is tightened, the washer flattens down, pushing the sharp edges into the mating surface.

Any locking ability works best on softer materials as it is easier for the edges to dig into them. It is ineffective at locking bolted joints experiencing higher levels of dynamic loads. It is also ineffective when the bolted joint requires lubrication, which promotes sliding and rotation of the bolt."

(The above is from the website of an international supplier of 'bolting solutions', which also has a free (if you give them your address etc) downloadable ebook "Why do bolts loosen?"(A comparison between common bolt securing options).)
Split ring washers.

Thanks, interesting information.
Not quite sure that I fully understand the explanation, though. When the split washer is tightened down and becomes flat, then 'the two sharp edges' will retract and will no longer be able to dig into the mating surfaces – or so it seems to me.
Some further reading on the subject (Wikipedia) suggests that the efficiency of the split washer is debated.
Another aspect is that the replacement foot pictured in post #1 comes with both a split washer and a plain washer. If the split washer functions as you outline, then it must be essential not to stack these washers on top of one another. I guess the plain washer is meant to go under the engine bracket. Given the limited vertical space available under my brackets I will most likely not use plain washer at all.