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1981 Honda XR80 Engine Rebuild

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Hi all, a little background on my progress with the bike. I got a 1981 Honda XR80 when I was about 11 years old, I rode the hell out of the thing. It stopped running when I was about 16 and due to lack of time and commitment (I was a 16yo with other things on his mind) it got stowed away in my dads garage and never fixed.

 

Well I am 25 now and about a year ago I pulled it from my dads garage with intents to get it running again (I have a basic understanding of simple engine mechanics and am learning more every day). Of course I didn't store it correctly, it had gas still in it, the carb had frozen, and the piston was seized (badly). I tore it apart and began looking into it. Got the top end cover off and all the top end items (cam shaft, cam gear, rocker arms, valve springs, etc...) looked to be in perfect shape, I disassembled the top end with ease and opened up to see a piston and cylinder totally rusted. In order to free the piston, I had to prop the cylinder up off the crankcase, put a 3/4" socket on top of the cylinder and hit it as hard as I could with a 2-1/2lbs sledge. After a few attempts I finally got it free. I threw away the rusted piston and put the cylinder in CLR solution (along with the valves and other misc. items with rust) to dissolve all the rust. It worked fine but when I cleaned it up there was severe pitting in the cylinder so I decided to try and find another. I found one in good condition on eBay and jumped on it, the cylinder wall is clean and freshly hatched.

 

So here is my question...

 

The ID of the cylinder is 47.42mm (factory specs per XR80 service manual say they are manufactured from 47.5mm-47.51mm?). So I am a little confused about how it is 47.42mm.. any help there would be helpful. Well trying to find a new piston for it and I can't seem to find what the STD piston size is, I found something on a forum thread stating that OEM STD piston is 47.25mm (factory specs per XR80 service manual  state OD of piston is manufactured between 47.465mm-47.490mm) So I am a little lost there as well. I would rather not mess with boring out the cylinder (if I have to I will) so...

 

My main question is...

 

1. What is the (OD) for part #13101-176-000 (STD) piston?

2. If it is indeed ~47.25mm am I in spec with a 47.42mm cylinder (service limit per XR80 manual is .2 so I would be about ~.17mm)?

 

Please let me know what y'all do. Any help is appreciative. Thanks in advance!

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The ID of the cylinder is 47.42mm (factory specs per XR80 service manual say they are manufactured from 47.5mm-47.51mm?). So I am a little confused about how it is 47.42mm.. any help there would be helpful. Well trying to find a new piston for it and I can't seem to find what the STD piston size is, I found something on a forum thread stating that OEM STD piston is 47.25mm (factory specs per XR80 service manual  state OD of piston is manufactured between 47.465mm-47.490mm) So I am a little lost there as well. I would rather not mess with boring out the cylinder (if I have to I will) so...

How do you know what the ID of the new cylinder is? Did you measure it? Did they measure it? You can't always trust someone else's measurements and measureing something with that small of tollerences is not a easy thing to do. You're talking about a .08mm (the averate thickness of a human hair) difference.

 

I'm not familiar with piston and cylinder measurements, but I know that the piston is to be smaller than the cylinder so oil gets up in there. I know I didn't give you any solid advice, just more food for thought.

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