The Bantam that I rebuilt developed two air leaks during the shakedown runs, so it was necessary to strip the top end and repair the problems. The leak on the exhaust side was caused by an incorrectly installed exhaust gasket (easily repaired). The leak on the intake side was caused by two factors: The rubber carburetor manifold had become dry-rotted, and the mating surface on the cylinder head was heavily pitted from the rust that I had sandblasted off. Replacing the manifold was easy, but smoothing out the mating surface required a bit more work.
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| Condition when purchased |
When I purchased the bike, the cylinder was heavily rusted, and required sandblasting and machine work to repair. It was rebored, and the head surface was skimmed. However, I never had the carburetor mount cleaned up, and thought I would just seal it with silicone when the time came.
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| Pitted surface |
To seal, the rubber manifold has a machined groove on its underside, and a rubber o-ring is glued into it. However, this system only really works on a perfectly flat surface, so I added HondaBond sealant to try to solve the problem. Eventually, this was sucked out by the engine vacuum, so it was necessary to find a more permanent solution.
I disassembled the top end of the engine, cleaned off all the oil and sealant from the mating surface, and applied a coating of MarineTex 2-part epoxy to build up the surface. I scraped it flat, allowed it to dry, and then repeated that step two more times. Once the epoxy had fully set, and sanded the surface flat with 600 grit wrapped around a block. This filled in all the pits, and gave me a perfectly flat surface to seal out the air. I then installed the new manifold, and went to the next step.
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| Sanding the surface flat |
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| Ready for reassembly |
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| Piston and rings |
Because of the air leak, the piston had suffered some minor scuffing on the skirt (soft seizure). Before I could reinstall it, I had to smooth the skirt to prevent further damage.
This can be fixed by lapping the piston. Using 1000 grit sandpaper soaked in motor oil, I gently sanded down the damaged section, and also smoothed out all the rough edges on the rest of the piston to cut down on friction.
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| Light scuffing |
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| Smoothing the entire piston with 1000 grit, with focus on the seizure marks. |
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| Fully smoothed and ready for reuse |
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| Rings reinstalled |
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| Back together |
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