I've also found out some other complimentary shock info...
All Showa 50mm piggybacks with independent high speed compression AND low speed compression adjusters on the reservoir:
2001-2004 RM125 shocks are common, DRZ400 or 98-00 RM125 lower spring seats are shorter and best used on these when lowering slightly, to allow full preload range. DRZ are same shock but body has different reservoir mounting position so cannot be used on the H series KDX.
2005-2008 RM125 and also RM250 (250 used KYB some years but these should be Showa) - same shock but 18mm shaft vs previous 16mm shaft, supposedly to allow more oil flow through the shaft damping passages for big hits.
2006-2018-ish KX250F - very similar to the 01-04 RM but longer clevis, the clevis needs the bottoms chopped off and the lower bolt hole redrilled to the same length as the RM shocks. around 2010 or so, a year before they switched to the SFF forks with one side spring one side damping, the KX250F shocks got a different shock reservoir cap that bulges out for a larger bladder capacity. Apparently these are a pain to remove and everyone uses the aftermarket reservoir caps which also have the extended capacity and a standard schraeder valve instead of a needle fitting.
2007-2018-ish RM-Z250 - same as the KX250F shock but fatter 18mm shaft vs 16mm, and a better extra-capacity reservoir cap design, which is an absolute necessity on these due to the extra oil volume displaced by the larger shafts. Same clevis as the KX250F, needs shortened in the same way.
RM-Z 250 shock 2007-2018-ish:
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One thing of note - the Kawasaki model shocks in the OEM parts fiches break down every single part. The Suzuki shock OEM parts fiches only show the shaft and piston and all attached pieces available as one complete assembly.
The aftermarket sealheads between the 18mm shaft 2005-2008 RM shocks vs the 2007-2018 RM-Z250 shocks don't show the same part number interchange despite being the same style of Showa 50mm shock body/cylinder AND the same oversized 18mm shaft, and no Kawasaki models use the 18mm shaft, so I'm not certain on any of the parts breakdowns for the larger shaft versions since they are Suzuki-only. the 16mm shaft is still significantly fatter than the stock KDX shock shaft, but the hollow shaft bores do have a lot to do with the damping abilities as far as flow capacity for the rebound especially. I believe the compression may or may not use the shaft flow as well, I wish I had a better knowledge base on that at this point prior to doing a teardown of these shocks.
All of these shocks in addition to the clevis shortening on the KX250F/RMZ250 versions, will be best to be lowered 4mm or so internally, or at least have the preload backed out, as the KDX linkage rocker will top out on the frame before the shock is fully extended, so the shock will never see the top end of travel otherwise.
I have some KLX300R dog bone suspension links, both the 117mm steel KLX300R/KLX250 links (like the KDX's 112.5mm steel links) that are a direct swap onto the KDX, and a set of identical length 117mm aluminum+brass bushings KLX300R links (thicker) with their longer mounting bolt hardware.
These will lower the KDX around 15/16" or 1-1/16" in the rear, but since these shocks are 1" taller physical shock length, multiplied by the linkage leverage ratio, I'm thinking that these longer links will be very useful in dropping the height down to match the taller forks at around 1" taller. Lowering links generally advertise that 1" lowering won't alter the suspension rate much at all (slightly alters the progressive / rising rate curve however), but the longer links for more lowering will really start to alter the geometry of the linkage significantly.
Being that the KDX came with 5.0kg shock springs and the RM125/250 came with longer links and softer springs at 4.6kg to 4.8kg, I think that the shocks will already be fairly plush mounted to the KDX (longer links and the KDX rocker will perhaps soften the MX valving some?), but wonder if it will soften them up too much with the longer KLX dog bone links, or throw off the rebound? I may fabricate my own links as well if I determine something around 115mm would be more ideal due to this and height reasons. The bushed alloy KLX300R dog bone links sure are nice though, but for my purple 96/97 plastics, I also have tracked down purple 1998 RM125 4.6kg and 1998 RM250 4.8kg/mm Showa 50mm shock springs, so I'm pretty dead set on being able to run those... I'll fab some links if needed...
If I send the shock off to have it professionally revalved, I may send a good working stock KDX shock with it, to mimic the valving traits of the KDX shock, but give me the taller Showa obviously, versus the now too short KDX shock (due to taller fork swap).
Racetech calculator says for B or C class enduro racer on singletrack terrain, age 30-44, 155lbs-ish no gear, who prefers softer suspension, to run 4.6kg rear shock spring and .38kg fork springs, so the shock spring on the KDX is not nearly as appropriate as the RM125 or RM250 springs according to that. Teknik Motorsports tells me I want .40kg fork springs, basing that just on holding up my bike + body weight and letting the valving do the rest. I'd lean towards the .38kg considering I'm trying to bring the height back down a bit and also allow more down-travel for dips and ruts in the terrain/rocks/etc to keep tire contact with the terrain over dips.