I have got a disassembled rear hub Shimano M785. I cleaned it and started to assemble back. On this step I can see the lack of ball bearings. It seems that there will be 35-36 balls on each line. I thought I can replace them with the M475 ones, but they are bigger. Seems like 3/32" for M475.
Can anyone suggest how to find out the size? Measuring might not so accurate.
Apparently the freehub body on that hub is FH-M770, though several others fit as well. I've worked on a couple of models of freehubs from that list and they used 25×1/8" balls each side. I've bought and fitted an M770 before, but I'm not sure if that's one of the ones I've stripped down.
Measuring is accurate with the right tools. If you're going to get into more intricate bike maintenance tasks like this, a set of calipers (whether digital or Vernier) is a very useful thing to have.
Here is the Exploded View of the Shimano M785 rear hub. Note that the right, drive side uses 13 loose ball bearings size 3/16 inch. The left, non-drive side also uses 3/16 inch bearings but they are contained in a plastic bearing retainer. You can certainly use loose balls on that side too. Simply fill the race to capacity and then remove one ball. You're likely to use 13 loose balls there or a count of 11 in the retainer.
If it the actual freehub body you wish to service, you should know that Shimano states that their freehub bodies are not serviceable but are sold as replacement parts. Googling "freehub body for FH-M785" yields many results from a variety of sellers for this particular freehub body. It's a part common to several medium to high tier Shimano hubs.
While Shimano states the body is not serviceable, it can, in fact be opened by taking the dust cap off and then removing the bearing race. The race for the axle bearings is the cone for one set of internal bearings. This race has 2 notches a special tool must interface with and the threading is left-handed so to remove this part to access the inner body you would turn the removal tool clockwise. This piece is typically very difficult to remove as it is very tight, the notches don't allow a good deal of purchase. It's very easy to damage both the dust seal and this notched race/come.
Once the notched piece is removed you'll encounter two levels of loose ball bearings and also the pawls, which engage a ring of teeth to propel you forward during a pedal stroke. The most common size of loose ball bearings in Shimano freehub bodies is 1/8 inch. There are 25 bearings in each level of the internal races. You can determine the size of the bearings from your hub by using a caliper. With many loose ball bearing systems, the appropriate number of bearings is completely full race minus one ball. This leaves room for the balls to rotate somewhat freely, grease to move between them and do its thing. My guess is you have 50, 1/8 inch balls to replace.
Here is a video showing the step by step overhaul of a Shimano freehub body. I don't believe it's stated what model of hub it's from, but it appears quite similar to the M785, which, again, is used on several other Shimano model numbers.
