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...because every day is MotoGP day when you own a CBR 125r...
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PostPosted: Fri May 09, 2014 7:07 pm 
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Racing ECU (!!)
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Location: Parksville, BC, Canada
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New info on what the difference is between the two bearings....

If I've decoded my numbers properly, the crank side bearing is a very high precision bearing, specifically;

Case side crank bearing radial clearance = .026 mm
Crank side bearing radial clearance = .016 mm

THAT is the primary difference between the two bearings :biggrin: in other words, the bearing pressed onto the crank is more precise, apparently by 0.000393701 of an inch.......

The good news is that I now feel much better about installing the crankshaft case bearing on the crankshaft itself, in lieu of not being able to readily source the 'proper' bearing that is, apparently, 0.000393701 more precise in nature.

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Marvin Miller
cbr125world Store

...because every day is MotoGP day when you own a CBR 125r...


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PostPosted: Fri May 09, 2014 10:00 pm 
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Location: Parksville, BC, Canada
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And so...in the meantime, I got a call from Honda today saying that my tranny gear still hasn't come in :angry: So that kills off my plans of completing the build and doing the install this weekend :wacko:

And so, with too much time to kill, I went back in and measured everything. Installed spring height, all clearances, etc. So now the engine is essentially blueprinted. Normally I would never go to these lengths given the remarkable lack of any appreciable wear at 25,000 kilometers, which is both a testimony to Honda as well as to Amsoil Synthetic. In fact, the only real wear on anything was that every bearing except one was trashed and the rings. And even those (the rings) are just being replaced because I'm there.

So, I decided to sacrifice come much-needed compression by cleaning the head :ohmy: In order to kill even more time I decided to lap the valves :laugh: After spending a lot of time with the suction cup coming off the valve I decided to break out the drill :top: As a result I now have some perfectly lapped valves :biggrin: I measured the installed height while I was there along with the valve sizes, the stem sizes etc. It's all so far within spec that you wouldn't know it wasn't new. Even the cylinder, after I honed it and with 20K on it was well within spec :top:

Here's some pics of the results of having too much time on my hands and not enough parts!


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Marvin Miller
cbr125world Store

...because every day is MotoGP day when you own a CBR 125r...
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PostPosted: Mon May 19, 2014 3:47 pm 
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Location: Parksville, BC, Canada
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The engine is back in and it's been test-driven several times over the last few days.

Breaking it in has been very difficult due to the custom racing camshaft....more on that shortly but here's some assembly pics....

In the first shots you can see the crank, transmission & balance shaft installed.....the red goo that's covering everything is assembly lube...


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Marvin Miller
cbr125world Store

...because every day is MotoGP day when you own a CBR 125r...
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PostPosted: Sun May 25, 2014 3:49 pm 
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And a few more....


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Marvin Miller
cbr125world Store

...because every day is MotoGP day when you own a CBR 125r...
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PostPosted: Sun Jul 20, 2014 8:13 pm 
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...and back in the bike and in operation...(for some time now) :bike:


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...because every day is MotoGP day when you own a CBR 125r...
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PostPosted: Wed Jun 07, 2017 12:59 pm 
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I had to register here just to share my experience of fighting with these bearings :biggrin:. 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 :biggrin:. 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.


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