I would like to provide an update regarding this kit which I fitted late last year. Recently I noticed a significant increase in hum at 200 hz and 400 hz. You will see from my previous posts that this was an issue as soon as I originally fitted the kit (and it was also an issue for Moon Unit, who fitted the kit to almost the same turntable / cartridge combination - RPM5 plus Ortofon Rondo - as me). It had never been an issue prior to fitting the kit, however the kit did dramatically reduce noise at other frequencies.
Anyway, I pulled off the platter, only to find that one of the new grommets had actually split. This seemed to have occured because I had to tighten the screws quite a lot in order to reduce the annoying hum at 200 hz and 400 hz (any lesser screw tension just made these humming noises too obtrusive).
After this, I installed the new grommets from the spare kit I purchased, but did not install the screws. As expected, the noise floor was about as perfect as one could reasonably expect - no audible hum at any frequency.
But then I realised that this configuration is mechanically flawed, because the tension of the drive belt pulls the motor at an angle towards the sub-platter, meaning that the pulley and drive belt angle are no longer perfectly aligned (tight screws facilitate this alignment). This caused the belt to "catch" very slightly every so often, producing a very soft clicking noise that you would likely never hear when music is playing. But it does mean the transport is compromised.
So I have found a different solution. I am now running the kit grommets, pulley and belt, but with no transit screws. But crucially, I have added a counterweight to the motor housing bracket on the opposite side to that of the sub-platter (i.e on the edge of the motor bracket nearest the edge of the plinth. The counter weight consists of 2 x 20 gram fishing sinkers, attached to the bracket by means of blu-tack (ugly, but it works perfectly). This counterweighting enables the transit screws to be left off, thereby significantly reducing the hum artifacts at 200 and 400 hz especially, whilst also letting the motor run at the correct angle relative to the sub-platter. The extra 40 grams of weight on the motor bracket also lets the motor bracket "settle" on the grommets - this is important since there is no longer any screws to hold it down.
I've been running this solution for a week now, and I am getting the best sound I have ever gotton from this turntable - even better than when the kit was first installed (and working properly).
Hopefuly this might help others who have come across this issue or would simply prefer to use the kit without having to re-fasten the transit screws (which are always going to re-couple the motor in some way to the chassis - something that I think should be avoided wherever possible).
In coming up with this solution I determined that the absolute correct counterweight needed to be 30 grams sitting directly on top of the upward-facing screw at the edge of the bracket, however I could not find any permanent 30 gram weight that could be fitted that close to the edge. Also, the 30 gram weight right on the edge did not settle the motor onto the grommets as much as I would have liked. Although there are 40 gram weights available, one of them is too large to fit (they will foul the platter). So I went for 2 x 20 gram weights and fitted them between the motor and that edge screw.
As for the ideal pulley height, with this setup it is such that the belt interfaces in height terms exactly at the midpoint of the sub-platter.