SL175 Part 6

Click Here for Part 5

Loyal readers will remember that part 5 ended with a blisful state of turn signals and changing fall leaves. Now the leaves are changing again a year later and I'm just now getting around to updating the story. The bad part is that I haven't ridden in over a year. Just after I finished writing part 5 I was riding around the yard and enjoying life. The idle still wasn't quite right and I ended up stalling the engine on a hill. I rolled the motorcycle down the hill to bump start it and thus save some wear on the fragile kickstart mechanism. The engine made an abnormal noise and didn't start. When I tried to kick start it, there was no compression in either cylinder. I figured that the only way I would lose compression in both cylinders at once was to have a disconnect between the cam and the crank, such that the crank was turning without turning the cam. I started by draining the oil into a clean bowl, then filtering the oil into my kitty-litter turned waste oil bucket.

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Picture number 3 below shows no more metal than usual, and a few carbon deposits that are on par with the perpetually rich operation. I removed the engine and started investigating. The problem was clear almost imediately, and my suspicion of a broken cam chain turned out to be correct. I'm not sure why it broke, but it looks like the spring clip came off and the master link worked it's way out. I'm just glad that it didn't come out while the engine was running! I was able to retrieve the parts of the cam chain and thus did not intend to split the case.

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I also found that the combustion chambers were very dark with carbon deposits. I had a chemical made for removing rust called "Must for Rust." I tried using it to clean some rusty valves and discovered that it also removes carbon deposits quite nicely. I used it to clean the combustion chamber and valves and it worked pretty well.

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I fished a new cam chain into place and put everything together. I wasn't able to find a replacement cam chain that was the right length, so I ordered one that was too long and cut off a couple of links.

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After I put everything together I figured I would check the compression before putting the engine back into the frame. I got around 100 on once cylinder and 70 on the other. This is lower than it should be, and could explain the poor running and the differences between the two cylinders. When I used my calibrated ear to find the source of the leaks I found that the exhaust valves and rings were the culprits. Since I already had the engine out, I didn't see the point in putting it back in without fixing those problems, so I took it apart again. Check out those dirty pistons!

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When I had the cylinder off I noticed quite a bit of play in the top of the connecting rod. By this point I realized that if I wanted to have a reliable motorcycle I would have to stop ignoring problems like this. Since the kick start shaft was still having problems, I decided to go ahead and split the case halves and fix everything that was broken or out of tolerance. Imagine that!

It turns out that the connecting rod was overheated and blackened, but I seem to remember it was that way last time I had the engine apart. There was some scoring and wear that seemed to be the cause of the play. The connecting rods are not replacable without a press and some other special stuff to put the crankshaft together, and used crankshafts are inexpensive, so I ordered a replacement. Meanwhile I had to take all of the removable parts off of the crankshaft. I had a great tip from a website reader after he saw how much trouble I had getting the oil filter cap off. He pointed out that the cap itself is threaded with the next larger size of threads so that you can use a bolt as a puller to remove the cap. This is a much easier way to remove a stuck oil filter cap! In the second picture below the black bolt is the one that holds the cap on, while the silver bolt is the puller to remove the cap.

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This time I also purchased the motion pro socket that is designed to fit the nut inside of the oil filter. I could have made a socket by grinding away on a conventional socket, but the motion pro version was about $10 and well worth it. The gears are marked with little dots and the first two pictures below are to remind me to look for the dots during reassembly. The next target was the kick start shaft. You may remember from earlier in the story that I replaced the stock kick starter with one from a CB/CL 175 engine. The ratchet mechanism was in great shape, but the shaft itself had rounded splines. For this repair I found an NOS set of ratchet parts, so I dug out the old original shaft to rebuild it.

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I took apart the shaft and took these pictures so that I could remember how to reassemble it. I was hoping to put my NOS parts onto the original shaft, but I knew that I would have to replace my lever. I was trying to resist the urge to go cheap, so I found an NOS shaft and started the delivery process for it. Meanwhile I worked on cleaning up the combustion chambers and replacing the exhaust valves.

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Below you can see the dirty exhaust valves and the home-made valve spring compressor that I have been using. The middle section is a piece of steel tube with a slot cut into the side for access.

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I cleaned up the combustion chambers with a dremel-mounted scotchbrite wheel. Then the NOS shaft arrived. Check out the differences! I almost had to wear sunglasses to keep from being blinded by the gleaming metal.

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With the new shaft assembled I put the crankcase halves back together and forged ahead. I was also hoping to find a way to keep the clutch from sticking. It was annoying to have it stick in the disengaged position, because there isn't much you can do with a motorcycle whose clutch won't engage. I replaced the clutch pushrod, which was ever so slightly bent, so hopefully that will improve the situation.

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Another little problem was the locking tab for the oil pump bolts. I didn't want to bother with finding an NOS version, so I just made one out of some thin steel plate that I had on hand. It's not cosmetically perfect, but I think it will hold up fine in service.

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Check out the fit of that NOS kick start lever on the NOS kick start shaft! I now have almost $250 in the kick start mechanism, so I sure hope it will last!

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To solve the leaky rings I found an NOS set of 2mm oversized pistons on ebay. These are much larger than the usual oversize pistons, making them perhaps more like a big bore kit. I had the local motorcycle machinist cut the cylinders to match and was ready to put it all together.

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All was going well until I failed to comply with an important step in the assembly of new rings and pistons- I didn't clean the preservative off of the rings and check the fit in the piston grooves. One was a little bit tight, so I tried to remove it so that I could clean it. This was a mistake- I should have just sprayed a bit of carb cleaner on it. You can see the result below.


When working on a 40 year old motorcycle, it can be hard to find replacement rings. When working with a somewhat rare aftermarket piston kit that was designed for such an old motorcycle and stocked so long ago, it is almost impossible. After a search for any 2mm oversized parts, I realized that I might not ever find a replacement ring. I broke down and purchased another piston and ring kit and another cylinder, and took the other used cylinder to the machine shop and paid for yet another rebore. Breaking that ring was a $120 mistake.

While I was waiting for those proceedings to proceed I started to reassemble the head. I soon realized that the exhaust valves that I had purchased a few months earlier on eBay had been incorrectly identified and did not fit. The seller was nice to replace them even though it had been such a long time. I'm glad he had a large supply of extras.

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While I waited to resolve the ring situation I spent some time cleaning the combustion chamber. The "before" picture is to the right above. I didn't take an "after" picture but it looked much better. I used a solution called "Must for Rust" that was quite effective at loosening up the carbon. The first picture below is a brand new set of pistons. I got these on ebay and also picked up a nother cylinder set and had the rebores done again. These are the first over size. I didn't take too many other pictures of putting the engine together, mostly because I have those steps documented pretty well on the other pages. The last picture below is actually the CB350 engine. I realized after I had put the end piece on that I wasnt' going to be able to get the pivot pins for the valves in.
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Once I put it all together, it ran, but the valve noise was terrible. I drained the oil (as is required on these hondas) to check the valve clearances, and sure enough, I had set the valve gaps incorrectly. When I put everything together, I set the gaps when the valves were open, not when they were closed. Once I had that resolved, the engine sounded much better. I've ridden around 200 miles with it now, and it is running great. There is a slight oil leak, but I'm not too worried about it. The compression is up, and the idle is much more reliable than it was before.

Since the manufacture date on the data plate is 2/70, we wanted to celebrate the motorcycle's 40th birthday. Tabitha took a picture of me starting it up, and the last picture is of the motorcycle with a little birthday hat and a plate of fake oil.

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