You're welcome. Actually, your post had intrigued me enough to conduct a little research to get down to the bottom of the 'unserviceable dashpot' issue and I think I've figured it out.
There are at least three generations or 'families' of SME dashpots.
Early Series I:
The
early Series I dashpots had a knurled top which looked like this:
It was sitted on a
different dashpot than what we know from later variants (I have no experience with it but I assume it's piston/cylinder principal based):
Later 3009 / 3012 ("factory sealed") dashpots
I figured the later dashpots were filled with a proprietary damping
fluid and the piston in cylinder motion was impeded by a nifty hydraulic principal where the transfer of fluid from the bottom of the chamber to it's top through the vent hole in the piston damped the vertical cue motion of the tonearm. For this to work properly, air bubbles and air traps had to be eliminated so the the entire assembly was vacuumed and sealed. This explains the 3rd screw (unmarked) in the dashpot cover - which I suspect was the point of suction. This also explains the seal and "O" ring.
It looks like this:
Sometimes the inside looks like this:
A round hole through which the cue rod emerges is visible from the top of the dashpot cover:
We hack this mechanism when we open up the cover, release the two mounting screws and replace the stock SME damping fluid with silicon oil around the piston. We basically replace the designed hydraulic damping with the silicon's friction damping. We also render the seal and "O" ring redundant and unnecessary - and naturally, we don't need any vacuuming . We achieve the same goal - but as far as SME is concerned, those dashpots are "unserviceable" except for in the factory where vacuum can be applied through that 3rd screw.
New 'improved' dashpots:
The new design is the only one SME calls 'user serviceable'. Their service manual shows an exploded view of the design. Note the cross-section hole in the dashpot cover - through which the arm lift cue 'banana' bar can be attached to the cue rod:
The inside of this dashpot is a little different:
It still uses the piston/cylinder impedance damping - except I'm not clear whether or not SME had changed the damping fluid to something more viscous, so the seals and "O" ring are now redundant by default. In any case we service it in the same way as the previous version and use a thin layer of 300KcSt silicon oil around the piston rim.
This is just my view of it - nothing official, certainly not an SME analysis but I hope this helps nonetheless.
