The main reason that I decided to restore a CB350 instead of any other bike is that it is a simple, uncluttered machine that has very little weirdness to its upkeep. One exception to this is the oil filter cleaning.
The oil filter on the Honda CB350 is not of the spin-on/cartridge type that you can find in pretty much every modern car/motorcycle application these days. It’s actually a steel cylinder whose centrifugal motion (driven directly by the crankshaft) pulls heavier particles to its outer walls while allowing oil through its central outlet. This is the first weird thing about the oil filter.
You can gain access to the oil filter by first draining the oil from the crankcase and then removing the oil filter cover on the right side of the crankcase. Do not lose the o-ring on this cover. My cover was actually pretty tight in its bore, and I had to wiggle it out using a long flathead screwdriver. Although the cover is cylindrical and unmarked, it can only be re-attached to the crank case in one of the three possible positions (three because of the three equidistant screws supporting it). That’s the second weird thing about the oil filter. Make a mark on the crank case and cover so you know how it will re-attach. The three screws are the same.
In order to remove this filter, Honda recommends that you use special oil filter removal tool. People on hondatwins.net recommend using a M6x1.0 bolt instead. This seemed to work fine. Just get the longest M6x1.0 machine screw that you can find. The fact that Honda didn’t just recommend that in the first place is the third weird thing about the oil filter.
First you’ve got to carefully remove the circlip holding the outer piece onto the filter barrel (not shown) by squeezing the two holes towards each other with a needlenose pliers. Honda recommends replacing this circlip with each oil change, but you can re-use it if you’re careful. Also, it’s not a cheap part. (I was not careful and broke one of the two circle tabs — still usable but about ten times more annoying to use.)
Once you’ve got the circlip off, simply thread the M6 bolt into the threads at the back of the filter barrel as shown, and then when it bottoms out, pull the barrel out using pliers.
All that’s left is to clean out all the muck in the filter barrel. It may look like there’s nothing in there, but believe me, once you start scraping a towel around with some WD-40 on it, there will be layers of gunk coming off.
Mine wasn’t too bad, but I’m planning to run some Marvel Mystery Oil in the next change of oil so I plan to see some more muck at that time. Luckily there were no metal shavings or rust.
Anyway, assemble in reverse order and re-fill the oil, and you’re good to go.