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...because every day is MotoGP day when you own a CBR 125r...
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PostPosted: Tue Mar 25, 2014 7:07 pm 
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Racing ECU (!!)
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Joined: Sun Mar 23, 2014 9:21 pm
Posts: 535
Location: Parksville, BC, Canada
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A couple of weeks ago I thought it would be a good idea to do an engine freshening.

I got the bike with 700 kilometers on it and it's seen severe service since. It spends the majority of it's time upwards of 10,500 RPM and it's been on the freeway/highway a LOT with a lot of all-day racing.

After all that abuse I wasn't sure what the top end would like like at 22,000 kilometers so I decided to just do it :top: To that end I replaced;

  • Cylinder
  • Piston
  • Rings
  • Piston Pin
  • Piston Pin Circlips
  • Valve Seals
  • Valve Springs
  • Head Gasket & Base Gasket

I was surprised how little wear there was on the cylinder and the piston at 22K. The piston was scuffed on the load bearing side and the cylinder did have some marks on it but it was minor with a capital M.

Having some understanding of how the engine wears according to abuse and time if I were to 'freshen' it again I would go this route;

  • Rings
  • Valve Springs
  • Head & Base Gasket

and call it done...

For my purposes I wanted a 'factory-fresh' bike for accurate dyno-testing. I wanted 'as new' OEM performance so that's why I went the extra mile.

What did I notice? The engine was quieter and the torque was increased considerably. I mean, nothing major but from a riding perspective hills that I used to gear down and wind the beep out of it to climb I could now lug up in the same gear. So there was definitely more torque post-rebuild.

For the most part though, it was a learning experience on how this particular engine wears according to severe service and mileage. Btw, anyone ever tearing down the engine should replace both the inner & outer valve springs. This was VERY noticeable and reminiscent of the clutch springs.....


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Marvin Miller
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...because every day is MotoGP day when you own a CBR 125r...
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PostPosted: Tue Mar 25, 2014 7:08 pm 
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Racing ECU (!!)
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Location: Parksville, BC, Canada
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The engine is so small and light it's remarkable (for someone coming from the automotive industry). You are literally carrying an engine/transmission & starter with no real strain. It's size also means you can just put it on your desk and sit upright in comfort while working on it. This is no small benefit :) No engine stand required...


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...because every day is MotoGP day when you own a CBR 125r...
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PostPosted: Tue Mar 25, 2014 7:10 pm 
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A better pic of the new cylinder....

BTW, I've always been of the mindset that any new/modern engine does not need a break-in period. Certainly, in the modern automotive industry this is true. This is largely due to the fact that modern machining of the cylinders typically means they are precision bored and honed to a fine degree. The smoother you hone the cylinders the less break-in period is required.

To my surprise I found the OEM Honda cylinder to be fairly 'coarse' and sure enough, it was clear that there is a break-in period required before the rings & cylinder mate in.

The engine was noticeably 'tight' post-re-build until I put a few thousand K on it. In fact, on a cold start, it went back to OEM-style requiring two starts before it kept running. The bike had done this when new, so this was normal, but as it wore and the friction was reduced one start was sufficient. A poor man's compression test? Perhaps


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PostPosted: Tue Mar 25, 2014 7:10 pm 
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Cylinder head after a 'quick & dirty' cleaning job with new springs and seals.


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PostPosted: Tue Mar 25, 2014 7:10 pm 
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What I regret not doing is going after my 5th => 4th gear downshift 'crunch' that's plagued this bike since the day I bought it. It's intermittent and annoying. In fact, I suspect that's where the majority of the metal comes from that I find on the drain plug every oil change.

Thing is, to do that I need to split the case and I'm certain I'll need to order a few parts to clean up the transmission side of things. If I'm going that route I'd rather also change out the crank & balancer and bring it out to an OEM 150cc model.

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PostPosted: Tue Mar 25, 2014 7:11 pm 
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Is that a stock cylinder? Or did you go the BBK route...

Cheers!


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PostPosted: Tue Mar 25, 2014 7:11 pm 
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Grenadiers wrote:
Is that a stock cylinder? Or did you go the BBK route...

Cheers!

No, it was new OEM, as I wanted a true to factory test vehicle for dyno testing.

ie. I needed a fresh stock baseline to measure successive modifications against to highlight increases or gains and it had to be real-world accurate as compared to a regular rider with a CBR125

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


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