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CB350 Top End Rebuild, or Old Faithful Goes under the Knife

So, it turns out I should have listened to my inner mechanic back on the ride from Birmingham to Lake Wales when the motor would make a funny whirring noise whenever the tach hit about 6,200-6,500 rpm. Instead, I passed it off as harmless because the little motor ran as strong as ever with no performance issues at all. Maybe it was coming from my helmet or a kind of harmonic vibration of all the parts whirring down there under the tank. Apparently, it was the sound of a cam chain roller falling to pieces, and the chain running on nothing but metal roller hub.

I would have expected big sounds of chattering, bumping, and rattling exhuding unmistakeably from the CB motor, but the end of the tensioner came with a whimper instead of a roar—a cautionary tale for my fellow CB riders. Plastic parts built in the mid '60s and whirring about at six or seven thousand revolutions per minute are prone to failure. On the other hand, I have to hand it to the old girl, there were 24,288 miles on the clock before anything serious had failed.

Because the cam chain guide is nestled inside the cylinder assembly, the motor had to be disassembled down to the top case. I pulled out what was left of the tensioner and guide.

Old Faithful Sans Motor


Here is the tensioner restored using new parts next to what remains of the old parts. Main roller and guide came from Western Hills Honda in Cincinatti, and new and improved rear roller from a fellow in Poland that I found on eBay, built of better material and cogged to receive the chain.


The guide had slipped down into the top case, and the pin was completely missing. Either it is laying in the bottom of the engine or is in the oil that was drained out. It will have to be found; it could be catastrophic if it bounced around and into some moving internal part of the motor. I will pour the used oil through a screen and see if the vagabond part has been taking an oil nap. Otherwise, the hope is that it will be found when I flush all the debris from the case with diesel fuel. If that all fails, then I will get out the floor sweep magnet and try to pull it out through the drain hole. Finding that little metal pin is all that stands between an easy top end job and having to pull the engine cases apart, which I want to avoid if atall possible. Any debris left from the guide and tensioner, other than that pin, will be ground up plastic and less of a hazard to the motor, although it could still plug oil passages.

Now that the cylinders are off, I thought it best to give my cousin, Johnny, a call and see what he thought about the condition of the pistons and what he would recommend before sealing it all up again. I e-mailed him and sent a picture of the pistons. Johnny suggested replacing the rings. New rings means brush honing the cylinder walls. Even though they are in pristine condition, the new rings will not seat properly without a little microscopic cross-hatching on the walls.

When a local search for a suitable (2½ inch) brush hone, also known as a "Flex Hone," turned up nothing, I started searching on the Internet. Not very many places had that size and the ones that did sold them in the neighborhood of forty dollars plus shipping and up. I decided to check with local machine shops to see if the cost of having this done might be less than the cost of a hone. This would also save me the wait for the hone to arrive. I went to John, a local bike mechanic I trust, and he gave me the name of a machinist at a shop about 25 miles away, between Auburndale and Lakeland, that he had used and who he thought was an excellent machinist. I made a call to Lesnett's Machinery and asked for Harry as instructed by John. As soon as I told Harry that I needed a twin's cylinders brush honed and that it was a CB350, Harry told me he knew all about them as he had learned to ride on one and had worked on them before and in fact now owned a 1968 Superhawk. Twenty dollars would get both cylinders honed. As much as I would have liked to learn to do this myself, I could not justify spending more on the tool than on the job.

The next day I arrived at Harry's shop, cylinders in hand. The picture on the Internet I had seen of a little white, almost dilapidated, building didn't do the shop justice. Inside any machine one might require to make anything out of metal was there and in great condition. I handed Harry the cylinders, mentioning the cost of a hone and that I would have liked to learned to do it myself, but it was not worth the cost of the tool that would likely be used one time only. Harry took the cylinders, found a suitable hone, then said, "How 'bout I do the first one and you watch, then you can do the second?" as he chucked in the hone, walked over to the lube tub, inserted the hone and started spinning it while he making a gentle up and down motion inside the cylinder. He was done in less than a minute and handed me the drill. I copied his motions, first wetting the cylinder and hone with mineral spirits, then spinning it as I pulled and pushed it through the cylinder. I handed the drill back to Harry and he rinsed the cylinder clean in more mineral spirits then wiped it dry with a cloth.

I asked Harry if the $20 we had discussed over the phone the day before was still OK and he said, "Nah, ten, no five is fine; you did half the work!" I reached into my pocket and pulled out a crumpled five and three ones and handed it to him, thanking him. Part of the deal was that Harry asked me to ride back on the CB when I had it all back together so he could see the old bike. I left with a big smile on my face, not just because it cost me so little but because it is rare these days to meet a fellow like Harry who is genuinely interested in these old bikes and is glad to help.

The cylinders brush honed and ready for reassembly.


Now that the cylinders are ready, other parts have been coming in. The parts diagram in the Honda microfiche show #3 as the rings, with an arrow pointing to both sets. It looked like they all came together so I ordered one of that part number from Bike Bandit in standard size, as the pistons and cylinder walls mic'ed out well within specs. The rings arrived a day ago but there were only rings for one piston. I ordered another set. This is the thing I hate the worst about working on old bikes; every time you forget something or discover you need something you hadn't anticipated, it is back on the phone and another week of waiting before the work can proceed.

Meanwhile, I had ordered a complete engine gasket and seal set from Sirius Consolidated, an eBay seller I have ordered from before. I have found them to be a very good source for old bike parts, either NOS or aftermarket. It is surprising what they have available and they are easy to work with and delivery is prompt, in spite of coming from Canada.

I had already disassembled the clutch and went to do the same to the oil filter, but it requires a special wrench. This is a kind of circular socket with four prongs, making it look a bit like a low-budget crown. I took a spare socket I had on hand and went to town on the grinder to try to make one, but my attempts were, well, let's just say, less than useful. But lo and behold, while looking for the gaskets, I noticed Sirius offered the special socket for ten bucks—it's on order now. I need to remove that nut so the oil pump can come out, enabling me to properly clean its screen and hopefully give me better access for flushing the engine case of debris.

While I wait for parts, I will continue removing the base gasket that has sat there and baked on since 1968; the others came off with little resistance as they had been recently replaced. There'll be time to chip carbon off the piston heads and clean the blow-by off the sides. Also, while waiting I have time to try and resolve conflicting methods for installation of the gaskets; my Honda service manual and Clymers disagreeing with each other, and Harry telling me to just "put them on dry."

I'll post more once the reassembly begins.

Road Dog

"Ride Your Own Ride"

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