I had to register here just to share my experience of fighting with these bearings
. Hope that this would help someone who wanders around internet searching info, like I did.
This all in all leaves me with a conclusion that the CBR125 just is not meant to be rebuilded. Honda's engineers have invented the most imaginative ways to annoy someone trying to rebuild this engine. First of all, the lock nuts holding the primary gear and clutch basket, not available anywhere but at Honda (not going to pay 60€ for a socket, made it myself out of a 20 mm one). Ok, got them out, want to get the flywheel off? Don't even think about getting a puller from a local store (even though they have 100's of different types). Why would Honda want to make a thread that would fit some common puller, when they can make their own dimensions and sell the f*cking tool for 50€? Ok, lets get to the bearings. As Marvin has mentioned, they are a 2mm thinner version of a 6207, made by NTN. Outer dimensions are 72 by 35 by 15 millimeter.
Left hand bearing, magneto side: 91002-KPP-901
Right hand bearing, crankcase side: 91001-KGH-901
I went to the local dealer to get the bearings (in Finland). Answer: crankcase side in stock, left side not available. "Why not?", I ask. "Just not available", he says. I then decided to buy 2 of the available ones (crankcase side). Some information of the bearings decoded:
Left hand, NTN part number (written on the bearing ring):
TM-SC07A87CS16PX18
Crankcase side, NTN part number (written on the bearing ring):
TM-SC07A87CS26PX18
Some of the essential info:
TM = specially heat treated steel
SC = special(!) single row deep groove ball bearing
07 = code for bore dia. (35mm)
A(87) = internal redesign (unclear for me, not relevant i think)
CS = special radial clearance(!), followed by the value in thousands of mm. For example, CS16 = radial clearance of 0,016 mm.
PX(18) = special tolerance, more precise than normal?
So the difference is that the magneto side bearing is tighter than the one in crankcase side. Magneto bearing has a radial clearance of
0,016 mm and the crankcase side has
0,026 mm. Most of the bikes I've seen use the same type of bearing for both sides, the most common clearance being C3 (C4 in some high-revving two-strokes). At this bore diameter, according to the NTN table, a C3 bearing would have a radial clearance of
0,015 to 0,033 mm (pretty big tolerance). As we see, both of the CBR125 bearings fall easily into this category.
Why would Honda still see it important to make special ones instead of using a standard C3?
That's a good question. I guess this has something to do with the different weights on the crankshaft ends (2 kg flywheel vs. 100 gram primary gear). Honda has probably done this to make the engine extremely reliable, and of course to drain the cash from customers
. Based on what I know about engines and stuff, I'd confidently use the crankcase side bearing on both sides.
To shuffle things a bit more, I got a hold of a pair of Honda Sonic 125 main bearings. The Sonic is for thailand domestic market and the engine is the same as CBR has, but with a kick-starter (fun fact, CBR125 engine actually has a empty place for the kick-starter: you'd just need the Sonic's clutch cover and the starter mechanism to install it). Sonic's engine is almost exactly the same, but the radial clearances are
0,042 mm for the left hand side (different part number than CBR's one) and
0,026 mm for crankcase side (same part number as CBR has). Why would Honda use a 3x clearance for a different bike with the same engine?
That's another good question. It's opposite to everything I've thought that could be the reason in the CBR125 bearing's clearances. Nobody will probably ever answer it. This is the most complicated and f*cked up bearing system I've ever seen. Some day I'll probably call Honda and ask. Or not. Until it I'm happily driving with 2 of the same bearings.