For older models, OEM parts are often available, but for models from the 1990s and earlier, there are many parts that are no longer available, especially exterior parts. In particular, it is best to assume that there are almost no exterior parts. Most of the parts other than the standard parts have already been discontinued. In this article, I will report on the applications and hints for finding parts when not available, through overhauling and restoring engines that disappeared in the early 1960s.
The basic principle of oil seals is inner diameter x outer diameter x thickness.
There are many cases where the oil seals that prevent oil leakage from crankshafts, transmission shafts, etc. have been discontinued and are no longer available. The same is probably true for bearings. There are a surprisingly large number of oil seals and bearings that are no longer supplied as OEM parts, but are still available as STD parts. However, not all standardized parts are available for all sizes, so you need to be prepared for that and be flexible. In this case, I used STD oil seal that was the same size. Of course, this is all at your own risk. The transmission system is relatively safe because it is oil, but in the case of the crankshaft, it is a fuel mixture, so it is necessary to consider fuel resistance before using the seal.
Using STD O-rings.
There are many cases where O-rings are missing as well as oil seals and bearings. Here, standard O-rings are checked in catalogs ( Web catalogs of oil seal manufacturers are a strong ally nowadays). What is important is the cross-sectional diameter and size of the O-ring. The O-rings were like long, thin rubber bands, so I bought several O-rings with the same cross-sectional diameter and as large a diameter as possible. I was prepared to make mistakes, so I made my own O-rings by gluing them together using a "2-in-1" or "3-in-1" method. In other countries, there was a product called "O-ring making kit" which was made by gluing together rubber strings of different thicknesses. Instant adhesives can be used for gluing, so if you can't manage it, you can try making your own. The O-rings for the outer circumference of the rotary disk cover were also made by myself. I applied a thin layer of fuel-resistant liquid gasket to the sealing surface on the crankcase side to increase slippage during assembly and to improve sealing after drying.
The gasket sheet can be cut out and made by yourself.
There are some engines that have a gasket in the center of the crankcase and some engines that do not, but the Honda 4 Mini has a center gasket. By design, the left and right crankcases are not machined in one piece, so gaskets can be used. If the engine does not allow gaskets, use liquid gaskets together. For 2-stroke engines, use a fuel-resistant liquid gasket for the primary compression chamber. 4-stroke engines and 2-stroke engines can also use a silicone-based liquid gasket for the transmission chamber. In this case, I used Permatex Moto Seal 1, a fuel-resistant liquid gasket for the 2-stroke crankcase (same for the transmission side). In some cases, such as clutch covers and tappet covers, where gasket sheets are specified by the factory and there are no gaskets available, silicon liquid gaskets alone can be used to seal (it is worth trying at your own risk). In this case, make sure to clean the contact surface and apply oil stone before assembly. I have a motorcycle with a clutch cover sealed only with a silicon liquid gasket, and it has not leaked oil for more than 5 years. It was just a last-ditch effort, but it turned out okay. This is a work method that should be done at your own risk.
Base sheet to keep in case of emergencies
Before the 1980s, sheet base gasket production kits of different thicknesses were useful as materials for engine tuning. Thankfully, Daytona still has the product lineup today. The trick to making your own gaskets is not to just cut out the gasket sheet with scissors or a cutter, but to make a pattern on a piece of copy paper or the like and then transfer it to the sheet.
The notch has a meaning.
I couldn't find a gasket for the crankshaft end cover, so I made my own gasket using 0.5mm thick of base gasket sheet. There was a notch on the inside of the cover, which served as a lubrication port to pour fuel mixture into the crank bearings. When making your own gasket, be sure to check the shape of both sides of the contact seating surface before cutting it out.
- Point 1: If you can't find an oil seal, try to find one from standard products based on dimensions.
- Point 2: Liquid gaskets can be used in lieu of gasket sheets.
- Point 3: By using the base gasket sheet, you can make your own DIY gasket sheet.
The Yamaha YA5 mixed-stroke engine that was being assembled here was designed in the early 1960s. This engine was equipped with the first rotary disc valve engine for a mass-produced vehicle in Japan. Some sources say that the original engine was made in Germany, but in any case, it was designed during a period of technological transition, and this engine never developed, but later (in the Yamaha YA6) evolved into a completely new rotary disc valve engine design. Because of this history, there were only a few parts in circulation, and even if there were compatible parts, there were few users who wanted it. During the overhaul of this engine, I was able to easily obtain the parts that I found in the distribution stock (privately owned, etc.), but I had the impression that the parts that were hard to find were "hard to find any way".
As for oil seals and gaskets, I could hardly find any, except for the gaskets that were in demand on the upper part of the engine. Since I was aware of this situation, when I disassembled the battered engine, I removed the oil seals as much as possible without destroying them, so that I could check the dimensions. Also, to avoid confusion when looking for parts later, I attached a packing tag to the removed oil seal, on which I clearly marked the part to be used and its dimensions (inner diameter x outer diameter x thickness, since it is an oil seal). If there was a parts list with illustrations, it would be possible to search for the parts by their layout and part numbers. Unfortunately, the part numbers have since been changed, so the parts list was almost useless as a reference.
For these reasons, the search for an oil seal was extremely difficult. This was because I had to find a compatible part based on the actual dimensions. Nevertheless, as a result of persistent searching, I was able to find a part that was compatible in terms of dimensions. However, since this was a 2-stroke engine, the primary compression chamber would be filled with a mixture of air, so the fuel resistance of the part was unknown. Even if it is OEM parts, it is not a lifetime part, so all oil seals are diverted for use based on personal judgment and experience. As I mentioned in the photo explanation, it is easy to buy STD oil seals from oil seal manufacturers (through machine parts dealers or online). I was also able to find oil seals of the same size across motorcycle manufacturers.
As for the gasket sheets, I decided to make almost all of the gaskets myself by cutting out the actual parts. When making the gaskets myself, I used a piece of copy paper and made the shape by stone-printing, then cut it out and attached it to the base gasket sheet with spray glue. The contour part was cut with a cutter (not the image of a blade cutting through, but the image of a blade pressing down hard to cut), and the bolt tightening holes were pulled out by hitting a punch with a hammer.
When it comes to overhauling an old motorcycle engine, where repair parts are hard to find, ideas and experience often come into play. It is precisely because of these hardships that tinkering with old motorcycles from the 1960s and before can be so enjoyable.
