Honda SL175 Engine

Motorcycle Part 1
Motorcycle Part 2

Having ridden around for a little while, I figured it was time to do some engine work. There were several reasons for this decision, including the worn kick starter, the old gaskets, and the metal fragments from the spark plug thread repair. The old gaskets and seals led to a cumulative oil seepage that could be described as a "continuous corrosion protection program" or "ongoing oil change program." Either way, it made a puddle everywhere I parked it, and the resulting coat of oil on the parts made it easier for the sand and dirt from the road and trail to stick to things.

April 2008 Update: I also had a bent valve, which I bent in the process of attempting to repair the spark plug threads. Let me point out that I paid $50 for the tools to repair the threads, and subsequently $30 for a replacement head. Because of these factors and others, I wouldn't suggest attempting to repair stripped threads. Although, if you are interested, I would be glad to sell you my 12mm repair tools.

Since my new storage place doesn't have much room, I decided to remove the engine with the bike in the back of my new truck. It turned out to be a really handy way to work. It elevated the bike to a more ergonomic level, it gave me places to put parts and tools other than the ground, and the straps helped stabilize things. The engine on this bike weighs about 80 pounds, and the bike weighs about 250. Since that is such a large portion of the weight of the bike, having the bike solidly held in place by the straps made it much easier to wrestle the engine around without having to worry about the bike moving around.

R~P1010002.JPG R~P1010003.JPG R~P1010004.JPG R~P1010005.JPG

I followed the steps in the overhaul manual for preparing to remove the engine, with the exception of not removing the fuel tank. That involves draining the tank, which sounded troublesome. First, I disconnected the battery. I removed the side covers, the chain, the final drive cover, the exhaust, the carbs, the clutch cable, the ignition wires, and most of the bolts. It really isn't more than a 1 hour job or so to get the engine ready for removal.

R~P1010006.JPG R~P1010007.JPG R~P1010008.JPG R~P1010009.JPG

I found that I had to remove a few of the bolts that held together the mounts, and then I removed the engine from the right side.

R~P1010010.JPG R~P1010011.JPG R~P1010012.JPG R~P1010013.JPG

R~P1010014.JPG R~P1010015.JPG R~P1010016.JPG R~P1010017.JPG

With all of that done, it was up to the bathtub. As the book suggested, I plugged up the intake and exhaust ports and scrubbed the whole thing down with a little bit of degreaser and an old toothbrush.

R~P1010020.JPG R~P1010021.JPG R~P1010022.JPG R~P1010023.JPG

With most of the dirt out of the way, I started at the top by removing the bolts that hold on the cylinder head cover. On mine one of the studs came loose instead of the nut, but that wasn't the end of the world. The interior of the engine was in pretty good shape. There was just a little bit of rust on the rocker arms. You can see the difference in the two spark plugs. They are indicating the problem with the right cylinder running rich. You can see the black color of the right plug vs the nice brown color of the left.

R~P1010024.JPG R~P1010025.JPG R~P1010026.JPG R~P1010027.JPG

I had to remove the generator cover next so that I could get access to turn the crank over. This makes it much easier to find the master link in the cam chain. The last picture on this row shows the link in position for removal.

R~P1010028.JPG R~P1010029.JPG R~P1010030.JPG R~P1010031.JPG

With the cam chain disconnected, removing the head was just a matter of lifting the whole assembly away. I turned it over, set it down, and took the first picture in this row. The second picture is the view that was under the head. The third picture is after I lifted the cylinder block off of the pistons. I put a rag around the pistons to help pad them from damage from the crank case.

R~P1010032.JPG R~P1010033.JPG R~P1010034.JPG R~P1010035.JPG

I found a little bit of a surprise when I removed the cylinder assembly. I knew that the cam chain tensioner adjustment screw was gone, but I didn't expect the cam chain tensioner to be damaged. In fact it had essentially eaten itself and become a pile of plastic bits in the sump. I also knew about the crack in the rocker arm cover, and was able to find a cheap replacement on ebay. The last picture on this row shows a nice view of the metal shavings on the right piston. These are from when I replaced the spark plug threads on that side. Hopefully that will improve compression and make the right cylinder run better. The second row of pictures are more views of the same stage.

R~P1010036.JPG R~P1010037.JPG R~P1010038.JPG R~P1010039.JPG
R~P1010040.JPG R~P1010041.JPG R~P1010042.JPG R~P1010043.JPG

After I removed the little circlip on the end of the piston's pin, it was easy to remove the pistons. I also took off the rings so that I could clean them and look everything over. There are three rings all together, and the two thinner ones have markings to differentiate the top from the bottom. I was surprised at the level of carbon buildup on the pistons, and I'm looking forward to removing it.

R~P1010044.JPG R~P1010045.JPG R~P1010046.JPG R~P1010047.JPG

Next I took off the left side cover. The bolt pictures are to remind me which holes correspond with the longer bolts. This side cover is not part of the oil sump system. I removed the cover, then removed the coil from the cover. I also took apart the clutch adjustment parts and cleaned out the grease. A little bit of cleaning left all of these parts ready for setting aside. Since I'm also in the midst of a kitchen renovation, I have some old pieces of counter top that serve the need for a table very nicely. The last picture on the second row shows that the o-rings go on the back dowels of the cylinder block.

R~P1010048.JPG R~P1010049.JPG R~P1010050.JPG R~P1010051.JPG
R~P1010052.JPG R~P1010053.JPG R~P1010054.JPG R~P1010055.JPG

The cylinder block cleaned up well, just like most of the other parts that I've cleaned so far. The picture on the right is the bolt layout for the left crankcase cover, so that I can know which screws go where for reassembly.

R~P1010056.JPG R~P1010057.JPG R~P1010058.JPG R~P1010059.JPG

The right side cover is part of the oil sump, unlike the left. Click on one of the thumbnails to see little bits of the cam chain tensioner idler. Luckilly it was plastic, so that the little pieces don't hurt anything. I'm not sure how long they've been floating around in there, but it will be good to get them out. I guess they could be capable of plugging up some sort of little oil passage.

R~P1010060.JPG R~P1010061.JPG R~P1010062.JPG R~P1010063.JPG

The little black seal in the first picture on the left is an example of what has been leaking on the left side. Replacements are available, which is a good thing. The following are the sizes that I have come up with so far:

Kick Start Shaft: S 13.8-24-5 AUS B24
Gear Shift C11 SDO 12-25-4.5-1
Final Drive 20-52-9 5.10
Clutch 8-25-8 5.12
Neuteral Light KC 12-25-15 24
April 2008 Update: Note- I forgot to list the little seal that goes on the tachomter drive, and it leaked. Be sure not to forget that one.
Aftermarket kits seem to be available on ebay and such.

With the clutch plates out, there isn't much left to separating the crank case halves. Just 9 bolts, then 9 more bolts.

R~P1010064.JPG R~P1010065.JPG R~P1010066.JPG R~P1010067.JPG

R~P1010068.JPG R~P1010069.JPG R~P1010070.JPG R~P1010071.JPG

Here is a picture of the worn kick start parts. Look at the union where the bronze-colored pinion meets the cast piece. The teeth are rounded off, leading to occasional slipping. These parts may prove to be hard to find, though hopefully I'll get lucky and find some in good shape.


I found an extremely handy online Microfiche for the SL175 at that has been very helpful in finding parts. Supposedly parts in his list that have a price are available from him, though I haven't tested that yet, and because of the large quantity of parts listed, it does seem very hard to believe that they are all available.

Click here for part 4, reassembly.