I just finished up rebuilding the Showa 360 shocks on my MR175, largely due to help from a member of this board (thanks, Coll !
)There's a lot of information already out there about rebuilding these shocks, but maybe this post will add some addtional stuff to help out the first-time rebuilder.
First, I pulled the shocks off and removed the springs (it wasn't as easy as it sounds, the space between the body and plastic sleeve was packed with dirt and put up a pretty good fight). The springs and sleeves were cleaned and painted.
The shock caps were mega tight. I sprayed PB Blaster into the cap threads, heated the shock bodies fairly hot with a heat gun, then used a flat-tip punch and hammer to initially loosen the caps; a Motion Pro clutch center holder removed them the rest of the way (this tool also works well for turning the spring adjuster). I used a giant Crescent wrench to hold the bottom shock eye in a corner of one of my shop steps while loosening the caps with the punch, it worked out great.
I had four shocks to work with - two off the MR, and two from eBay - and ended up with one good pair. One eBay shock leaked badly, and had been run a long time with maybe 20cc of muddy oil, chewing up the piston, ring, and cylinder a bit. Two piston valve plate spring washers had broken in half, which meant that the shock would have greatly reduced compression damping and intermittent rebound damping. I stretched a small O-ring (Honda 12x1.5mm O-rings 91353-HB5-003 or 13x1.5mm O-rings 91301-MEH-003 should work well) to fit in the space under the valve plate and hold it up against the underside of the piston, which restored damping in both directions and seemed to work OK (the O-ring fix wasn't tested, but I can't see the O-rings failing in this simple spacer application). Check the valve plate in the cylinder base too, which is the main determinant of rebound damping (all of mine were fine). Blowing hard into the head of the piston should result in significant air resistance, and sucking in should be nearly impossible; if you get little or inconsistent resistance in either direction, the spring washers are toast. This same test also will work for the cylinder valves, but in reverse, i.e., less resistance blowing then pulling in air. I believe the cylinders are still available from Honda.
The seals are a story in themselves. The stock seals are Honda P/N 91256-381-000, which are NOK AR7221P 12.5 x 25 x 14mm seals. You can pretty much forget about finding these very unusual seals. Single Honda 12 x 25 x 7 oil seals 91201-362-000 should work, although ideally the caps should be machined to a depth of @ 16mm to accept two of these seals face-to-face (yeah, I know, who's gonna do that). If you wanted try spec'ing out some seals, this should help:
Double spring-lip seal (1st choice), or single spring-lip seal w/excluder
Rubber O.D.
20 - 25psi OK
1"/25mm O.D., .5"/12mm I.D., .375"/10mm thick
.965"/24.5mm I.D. mounting hole, .5"/12.5mm shaft
Standard Buna-N material OK
axial shaft motion
I removed the old seals with a large screwdriver, from the bottom. Three came right out, one is still in there.
Single seals should be installed face-up into the caps, flush with the bottom edge.The shocks contained the following amounts of oil: 85cc, 90cc, 95cc, and 20cc as mentioned earlier. The OEM oil appeared to be graphite-based, apparently 5W. I initially filled the shock with 125cc of 3W Bel-Ray shock oil (I prefer light damping), but I wasn't comfortable with the oil level. Here's a picture of the shock cylinder, marked with the oil heights for various amounts of oil:
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Judging by the oil line (top edge of the shiny metal), I'm assuming these shocks were originally filled with about 100cc of oil. With 125cc of oil, a sharp downward motion of the piston would completely fill the shock body with oil, over and above the top of the cylinder - which fits tightly against the bottom of the shock cap and cap O-ring seal. This had the potential to occasionally turn the shock into a hydraulic cylinder, which I really didn't want to do.
After some trial-and-error refilling and lots of oil on the benchtop
, I settled on 110cc, which just submerged the piston head at full extension, giving full damping while still leaving plenty of empty space in the shock body for hard-bottoming impacts. I used a large mug to hold the shock upright on the bench, placed the cylinder in the shock body, and poured in the oil. I then slid the rebound spring and cylinder top onto the shaft, greased the inside of the seal and shaft threads, and slid the cap onto the shaft (if the seal lips are really tight, the cap will have to be 'threaded' onto the shaft). I lubed the O-ring sealing surface in the body with a Q-tip and shock oil, and applied sealant to the cap threads. Then, I inserted the piston into the cylinder (don't push piston in too quickly or oil will shoot out), fully seated the cylinder top into the cylinder, and screwed the cap on, using the Motion Pro tool again to tighten the cap (it will bottom out on the cylinder top).
Below is a picture of the finished shocks. I wanted to try the black/aluminum look for a long time, and I think it came out OK.
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Ray