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1972 Honda Z50A, 1976 Honda 90, 2007 BMW R1200RT, 2012 Triumph Bonneville
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Discussion Starter · #1 ·
Hi Everyone
Relatively new to the forum, seeking a few tips and tricks with this restoration. I have a '72 K3 that I acquired in around 2002 and has been pretty much sitting ever since. It was in rough shape when I got it, but ran well until the mixture screw went missing from the carb. After that, I just sort of put it away, planning on restoration someday. Seems like a perfect Covid winter project. I've restored a Trail 90 a few years ago, so am familiar with the engine and much of the bike. I plan to turn this into a Nicky Hayden tribute bike, with some fancy paint. I'll try to keep it updated as we go.
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It was pretty dirty, but generally in good shape.
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Seat was in great shape, just really dirty. Tank has some minor surface rust, but is clean inside.
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The tires really did not want to come off of the rims!
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All apart, starting to clean and sort the bits.
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Need a new cover and sprocket. The rear sprocket was very pointy, quite worn out.
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I'm going to go up to an 88cc cylinder and piston kit. May need to replace the head too, given the chunk out of the exhaust port.
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Hole for the swing arm bolt will need to be replaced, as will the swing arm bushings (which came out fairly easily) and the bolt.
 

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1972 Honda Z50A, 1976 Honda 90, 2007 BMW R1200RT, 2012 Triumph Bonneville
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Discussion Starter · #2 ·
The rear fender and tail light assembly were missing. I'll replace them. The front fender is pretty rough, but might be serviceable.
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Polished the wheel hubs, working on the brake hubs.
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The exhaust guard cleaned up nicely. The pipe is still good, I think. I just need to get some of the rusted and broken screws sorted.
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Lots of rust on the rims, but no holes and I think they will be serviceable.

Next steps, clean the chrome handlebars, clean the brake hubs, then pull the top end off the engine. More to follow.
 

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The chunk missing from the head is normal...that's how they're made.

The swingarm mount damage is fairly common. You might just be able to doctor it up some. We could find a dimension for you if you need it. The inside diameter of the swingarm IS the dimension...I'm pretty sure. It should slip on with little clearance. Then when you torque the swingarm bolt...everything clamps together tight and solid.
Nice work so far. Lookin forward to following along.
 

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1972 Honda Z50A, 1976 Honda 90, 2007 BMW R1200RT, 2012 Triumph Bonneville
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Discussion Starter · #7 ·
The chunk missing from the head is normal...that's how they're made.

The swingarm mount damage is fairly common. You might just be able to doctor it up some. We could find a dimension for you if you need it. The inside diameter of the swingarm IS the dimension...I'm pretty sure. It should slip on with little clearance. Then when you torque the swingarm bolt...everything clamps together tight and solid.
Nice work so far. Lookin forward to following along.
Thanks. I didn't realize that the head was cast like that. However, I can see why, as it has the sleeve in there to take up that space. I think it should be easy to get a new bushing on the frame for the swingarm bushings. I'll just need to find a good welder.
 

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1972 Honda Z50A, 1976 Honda 90, 2007 BMW R1200RT, 2012 Triumph Bonneville
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Discussion Starter · #8 ·
All the chrome is cleaned up, and the wheel and brake hubs polished. Took the head off of the engine. Piston and cylinder look great, although the top ring was broken. I have yet to take the head apart, but it looks good. Now for a question. I was planning on going to the 88cc piston and cylinder. I'm not really worried about maximum performance, but just wanted to pep it up a bit. Do people recommend replacing the head as well? Carb? If so, am I better off just going to a replacement 110 or 125cc engine?
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Cylinder head removed. No marks or damage. Note the broken top ring (right side of pic).
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Honda 50 piston beside old Trail 90 piston for comparison :)
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Other than a lot of carbon, the valves look good. I'll know more when I get it apart.

I don't think I'll go into the bottom end. It was running well and all looks good there from the top. I did notice some tight spots in the cam chain though.
 

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This is cool! I got a free 72 Z50 from a shed and am doing a rebuild series where I try and get girls involved and show them how to refurb it. Then some of them will learn to ride with it so it will live on.
 

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1972 Honda Z50A, 1976 Honda 90, 2007 BMW R1200RT, 2012 Triumph Bonneville
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Discussion Starter · #10 ·
This is cool! I got a free 72 Z50 from a shed and am doing a rebuild series where I try and get girls involved and show them how to refurb it. Then some of them will earn to ride with it so it will live on.
Great idea! Are you posting that rebuild here?
 

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Discussion Starter · #13 ·
It worked for me, opened up to your playlist of all the videos...
Yes, that works great! Thanks. I'll go through them. I'm now looking at replacing the engine with a Lifan. I was going to go with the 88 cc piston/head option, but looking at costs for a few things in the bottom end, it's less expensive to go with a replacement engine. Any luck/experience with those?
 

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Yes, that works great! Thanks. I'll go through them. I'm now looking at replacing the engine with a Lifan. I was going to go with the 88 cc piston/head option, but looking at costs for a few things in the bottom end, it's less expensive to go with a replacement engine. Any luck/experience with those?
I avoid the china engines. What does your engine need? The one I am working on came with an 88cc piston and cylinder installed but never run like that. Only other thing I did was upgrade to the HD clutch (80 bucks) stock everything else. Your parts look salvageable to me pending skills and equipment. Also depends on how fast you want to go. The tb parts kits are great value and reliability.
 

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1972 Honda Z50A, 1976 Honda 90, 2007 BMW R1200RT, 2012 Triumph Bonneville
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Discussion Starter · #15 ·
I avoid the china engines. What does your engine need? The one I am working on came with an 88cc piston and cylinder installed but never run like that. Only other thing I did was upgrade to the HD clutch (80 bucks) stock everything else. Your parts look salvageable to me pending skills and equipment. Also depends on how fast you want to go. The tb parts kits are great value and reliability.
Well, I was going to put an 88cc head on. It will need a new timing chain and output shaft for sure, but I think that the rest of the bottom end is fine. I figured it would be about the same cost or less expensive to replace the engine. I don’t have any experience with those Chinese engines tho.
 

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Well, I was going to put an 88cc head on. It will need a new timing chain and output shaft for sure, but I think that the rest of the bottom end is fine. I figured it would be about the same cost or less expensive to replace the engine. I don’t have any experience with those Chinese engines tho.
What makes you think the output shaft is bad?? I've never seen a bad countershaft in a Honda engine.
 

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Hey kirrb the CT70H engine I currently have on my CT had the splines bashed about half through their width. Never had seen that before either. Wonder if some had not been heat treated properly.
 

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Discussion Starter · #18 ·
What makes you think the output shaft is bad?? I've never seen a bad countershaft in a Honda engine.
It's kind of hard to see in the pics, but the splines where the sprocket goes on are worn/chipped a bit. The sprocket was loose on the shaft. It would probably run again, but since everything is apart, I'm going to replace.
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Discussion Starter · #20 ·
Updates. Tore the bottom end of the engine down. Everything looks good, except for the cam chain and output shaft (see above pic of the output shaft). Had the frame, swingarm, exhaust, and engine cases glass beaded. Just finished the first coat of primer on the frame and swingarm. I'll do the exhaust and heat shield tomorrow with some high heat primer. Used my fancy spray booth! Have to complete my parts list, then get ordering.
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