An engine refresh is underway on the 1970 Honda CT90. The top end is being rebuilt with a new piston and bored cylinder. With the covers polished out, the engine didn’t look too bad, but there was a good amount of grime on the back and underside of the engine. The rest of the bike is coming together and looking pretty fresh. The engine needs some attention to get it to match. After a thorough cleaning of the entire engine, the side covers will be painted, the head will be bead blasted, the rebuilt cylinder will be installed, and freshly plated hardware will be used.
The engine side covers show the wear up close. The re-plated hardware really makes the difference in a sympathetic rebuild. The sidestand and center stand were painted. A lot of welding spatter from the factory welds was removed on the center stand prior to painting.
With the side covers and top end removed, the engine is pretty easy to remove from the frame. It is said the frame is the best engine holder. In order to bust loose Phillips-type screws that in some cases have been in place for fifty years, an impact driver is used. If the engine is not held in place tightly, it makes it hard to get the down force necessary to keep from stripping the heads of the screws.
A lot of time was spent on just degreasing the parts. The head was completely disassembled, bead blasted, and the valves lightly lapped in prior to reassembly with all new seals and gaskets.
The stock cylinder was bored .075 and a new piston and rings will be used. The cylinder was bead blasted and painted with medium gloss black engine paint.
The small side covers were polished out. Although the high-low gear cover is painted from the factory, it is often worn off by the foot of the rider, and looks like its half polished, so polishing it out the rest of the way seems like a better solution.
The oil slinger was pretty clean, making me think someone serviced the clutch not that long ago. The similarities between the 90cc engine and 70cc platform are many, but there are still slight differences that had me pulling out the manual.
Waiting on a set of engine shaft seals to get the engine buttoned up. A new sprocket went in to replace the worn original one. The valve inspection covers, points cover and points base were also polished out. Even though the engine block didn’t look too bad from above, there was a good layer of dried grime just about everywhere aside from the small exposed square just below the carb.
The exhaust is in excellent shape. There were small areas of surface rust that were removed, and the backsides of the heat shields were de-rusted. Because the chrome plating process does not deposit as much metal on concave surfaces as it does on convex or flat surfaces, the backside of the shields are often rusted. The rust was light enough on this bike that chrome polish and some elbow grease removed it. Not a super big detail, but the reflectivity of the muffler under the guard will reflect the rust, and the pipe will not look quite as nice as it could.
After the cleaning and a couple insulating washers were replaced, the muffler is back on, with a fresh exhaust seal at the head. Waiting on the left side of the engine to get buttoned up before the footpeg assembly can be installed.
The intake manifold was oxidized on the front, so it was bead blasted and wet sanded to reproduce the original finish. The air filter and the plastic parts are being re-painted, and will be the last big hurdle. There are some cracks that need to be repaired in the down tube cover before it can be painted. Another reason to de-rust the backside of the exhaust heat shields can be seen from this angle.