Yeah, a bit late, but just went through mine. Note that that is *NOT* a fill port, but rather the detent for the shift fork. You have the bolt, a crush washer, a spring, a spacer and then the detent ball. If you can get all that stuff out, they you could likely add fluid, but if not, nothing will flow. It would be far easier to back off the case bolts about 1/4",and separate the case - everything internal will stay in place, and you can drain the oil fluid and then refill through the gap with the correct amount and not have to guess.
While in there, pulling it apart to inspect is not difficult, although I did find a few quirks in reassembly, but once that was figured out, it's pretty simple.
With the case split and all out, first assemble the driven components on the output shaft. First set the shifter, change shaft boss, drive keys and springs and key holder in place on the drive shaft. Then add teh large gear, a washer, the midsize gear, another washer, and the small gear and the special washer (see part diagram) on place, and then insert the shaft and all that stuff into the large side of the case. You will likely need to pull the shaft back a bit to get the shift fork to engage. Get all that aligned, and push it in until it stops.
Take that side of the case, and put it on a bench, vise, or whatever that will allow the shaft to be below the case and pointing up.
Get the other half of the case, and gently push the plug #47 out of it (at the end of the countershaft bearing in the small case end). Put the pinion in the other half of the case, and then the large washer behind the bevel gear, and then slip the bevel gear in place. You might want to put a small dab of grease on the back of the bevel gear to keep the washer from moving around too much. Insert the clutch folk in the small end of the case as well.
Take the washer #40 that goes on the end of the countershaft, and place it in the end of the large case just above the bearing recess. (This washer is too large to clear the driven gear otherwise, and will prevent the countershaft from being inserted otherwise.)
Assemble the three driven gears on the countershaft, and insert it into the large case. Using a finger to hold the gears from dropping off the countershaft when inserting it is helpful.
Add the driven clutsh and washer atop the countershaft. At this point, there should be no remaining parts on the bench, other than the bolts that hold the housing together and the small rubber plug.
Since the bevel gear is retained by the pinion and clutch fork, take the small case and invert it to position it for mating it with the lower case. Look through the hold where the plug was and ensure that the washer is centered on the bevel gear and bearing recess, and start the small half of the case over the axle. Make sure that the clutch fork is hanging down, and when you get close to the driven clutch, gently guide it into the grooves on the clutch. Continue lowering the small case half onto the lower half, and look in to the countershaft bearing hole to ensure that things line up, and if needed, use a small took to get the bevel gear and washer to align with the countershaft, and ensure that the case goes together easily.
Insert a couple of case bolts a small amount - leave 3/16 to 1/4 of an inch clearance. If using a typical bottle of gear lube, clip the tip as long as you can. Pull the case apart (limited by the bolts so that nothing can fall lose or out of a bearing) and squirt the lube through the gap into the lower case. A full fill does not overflow the lower case, so as long as you keep it level, you won't spill. Use the markings on the bottle or whatever works for you to determine quantity.
When full, pull the bottle tip out, and let the case slip together. Run the bolts in and tighten in a pattern gradually to ensure a good seal. Put the small rubber plug in at the end of the countershaft.
Done!
On mine, for what it is worth, other than a small amount of tooth wear on the pinion, the transmission internals looked new after 30 years of use . . . I can't see why these should not hold up, and it looks like Honda still offers all the parts . . .
Were I just wanting to refresh the lube, I'd back the bolt out, split the case to drain the old, and refill. This is an O ring seal, so should be reusable, and if not, it's $3.48 (3/25/2019) to replace, so do as you please . . .
All in all, nothing to be afraid of, but the manual is grossly lacking in detail on reassembly, and it seems that the biggest issue I have read folks having is not getting the case to mate, and my above assembly sequence ensures that that will not happen!
