In this thread:
solution for D-series poppet seat DIY?
@Kupu came up with an ingenious solution to the variable quality/sealing performance of the D-400 seats: a Duro 50 silicone 2-104 oring.
I'm pleased to confirm that this works perfectly with the D-420 as well, as
@lexvil has already proved.
As discussed above, the orifice (housing) of the D420 is a little different in material and knife edge. But the silicone 104 sealed perfectly, and I was able to get cracking effort down to 0.5" easily (though as discussed above, you don't really want it there, but rather around 1.2").
Simply grasp the shaft of the existing poppet with pliers and unscrew the lock nut. Pull off the plastic cap and remove the seat and tiny oring.
As discussed
here the o-ring performs better with a 1x3mm oring between the shelf of the poppet shaft and the 2-104 oring.
So first put the 1x3 on, then the 2-104, then the tiny 1x2.5 that was under the cap. Put the cap back on and screw on the lock nut. Tighten it so that the 1x3 is visible to the extent in the picture below:
Too loose or too tight and the 104 o‐ring doesn't seal as consistently.
This is one solution to the high cost of the new kits. Add your own orings and you have a kit for a few dollars, instead of the $60 list of the Service kit, IF your knife edge hasn't been dinged. If it has, then the housing is part of the Service kit. Lay in a few kits for the housings and some orings, and your D420 will last the same 30+ years that the D-series has.