Lunch hour rebuild: A new top end for Pat's CT70

Here at RevZilla, sometimes we eat lunch. Other times, we fix bikes. Today’s patient is a sweet Honda Trail 70 belonging to Product Research and Training Specialist Pat. He scooped up a pair of them on Craigslist last year, and we managed to haul both to Philly in a Chevy Cruze: one in the trunk, and the other in the back seat with the tires aired down so the doors would close! One bike quickly sold to another Zillan. Pat kept this desirable 1970 Trail 70 HK0 for himself. Unlike most Trail 70s, this early example has a fully manual transmission and four speeds, not three. 

The Honda’s been great, because it’s a Honda and of course it’s great, except for an oil leak. Unacceptable. Pat discovered a crack in the head and elected to solve it with a new top end kit. The job is so easy, we decided to fix his bike over our lunch break. One hour, end to end. Oh, and one other thing: this would be Pat’s first top end rebuild. We needed a plan.

Honda sold millions of their horizontal singles, so parts are very easy to find. They’re also affordable. A complete aftermarket top end kit, including piston, cylinder, head, gaskets, and rings, rang in at under $60 shipped to ZLA. Big bore setups, oil coolers, high-flow oil pumps, and other mods are also out there, but Pat opted to stick to the stock displacement for now. Luckily, RevZilla’s shop has most of the tools needed for a top-end rebuild. I brought a few other things just in case: a Trackside bolt kit, my favorite JIS screwdriver, a set of bent feeler gauges, and my own tool kit so we wouldn’t disturb Joe Zito’s immaculate workbench organization. Lunchtime was approaching. Could we do this in an hour?

Pat thoughtfully disassembled the top end before we started, which saved valuable time. Up first was preparing the piston. Pat got busy with his first piston ring installation, carefully snapping each into place with the touch of an amateur surgeon. He did not have to experience the defeat of snapping a piston ring. 

A pair of needlenose pliers helped pop out the wrist pin retaining clips. The old wrist pin and piston slid right out and the new ones took their place. Most aftermarket pistons for this engine have shorter skirts than Honda’s original ones. Make sure to orient the piston correctly!

With the piston in place, it was time to install the cylinder and gaskets. Pat’s kit didn’t include the idler wheel that sits in the camchain passage. We harvested the old one off the original cylinder, then bolted it up. Cleaning out the cylinder bore, even on a brand-new head, is always a good idea. I always find something on the rag. The piston popped into place, and Pat started looking excited. This was turning out to be a very satisfying lunch.

Next in the order of operations was the head. We removed the camshaft, rockers and other parts from the fresh head to clean them before installation. 

Pat chose Maxima’s assembly lube for his build, which has a pleasant cinnamon smell that stayed on our hands all afternoon after installing the head’s components. Due to our time crunch, I didn’t have much opportunity for sightseeing around the engine, but I did pick up on a few differences between this engine and my 1965 Honda S90, which uses the same basic architecture. One of the changes is the addition of threads to the inner edges of the rocker shafts. This makes it much easier to get a grip on them for removal using a bolt or a pick. My S90’s shafts are smoother than a fresh jar of Skippy, and pulling them out after years of neglect was a real chore. I liked the design on Pat’s Trail 70 much better.

With the head in position, we slipped the cam sprocket into position with its timing mark at top dead center. Pat torqued down the three cam sprocket retaining bolts while I looked through the hardware kit to find replacements for his mangled intake manifold bolts. Many Trail 70s lived a hard life in the hands of kids, which is why it’s increasingly difficult to find nice ones today. Older riders, nostalgic for their trusty childhood Hondas, are snapping up nice ones and driving prices through the roof. 

With lunch hour nearly over, Pat snugged up the head bolts and mounted the distinctive round cam sprocket cover. Our last job was setting the valve clearance, which Pat will re-check after breaking in his minibike. Just like that, lunch was over and Pat’s engine had a fresh top end.

You know what’s awesome? Fixing stuff with your friends, learning how to do new things, and getting a classic fun-machine back on the trail. I acknowledge that we’re really lucky to work at a place with tools, shop space, and employers who don’t mind oily motorcycle engines in the building. You might not be able to build engines in your lunchroom, and for that, I am deeply sorry. But if you have an hour or two and some buddies to help out, challenge yourself to move that languishing project forward. Motorcycles are for riding!