Oil sight glass replacement.

Two stroke bikes require the addition of oil to the petrol to lubricate the working bits. In theory the oil is atomised with the fuel and in the compression stage the oil is squeezed out and lubricates the piston/bearings before the spark ignites the whole lot. In the old days the usual way the oil is introduced into the engine was by mixing it in the fuel tank (hence the term pre-mix). This required some mental gymnastics to ensure the correct ration of oil to fuel. Older bikes use something like 20:1, something like the YG would probably require 50:1.. eg 50ml fuel and 1ml oil. So for a litre of fuel you would require 20ml of 2-stroke oil.  I remember the BSA Bantam had a measure built into the cap.. for every gallon you would fill the cap with 2-stroke oil twice. A bit tedious and some of the 2-stroke oil usually found its way onto the tank, gloves etc.

This was very messy and not very accurate. Most of the riders were of the school of thought which went "..if a little bit is good, more must be better!.. " Unfortunately this is not true. A carburetor will only draw a fixed volume of fuel/oil thru the jet. If there is more oil, it is at the cost of lesser fuel.. resulting in a weak mixture. A weak mixture runs hotter and defeats the benefits of the increase oil. Another problem was the inconvenience of carrying a bottle of 2-stroke oil around.

The Yamaha YG1 was one of the early bikes to have an automatic lubrication in that the 2-stroke oil was metered out by a separate pump. A separate tank held the oil and this was connected to a pump which fed into the crankcase (other types of 2-stroke oiling deliver oil to the carb). This was an excellent move in the right direction and all you had to do was ensure that the 2-stroke oil tank was kept filled. Yamaha figured a normal owner wouldn't want to peer down the fill-hole of the tank and conveniently built-in a window in the side so you could just bend over and check that the level was still ok.

50 years on and the sight window on my bike has seen better days. It is distorted and looks like someone has used some glue on it to seal it up.

The sight window is secured by a slotted ring. This usually requires a peg spanner to remove. My other hobby is watch-repair and I had this tool handy. Its usually use to remove watch backs and I thought I'd try it.

Unfortunately this did not work so I had to resort to the tried and true method of using a screw-driver and hammer.

This is the old sight-window.

Looks like there was some heat damage to the plastic..or maybe it cracked and the dark discoloration was caused by reaction to the stuff used to seal it.

I've been hunting for a replacement and found this advertised on ebay (seller oldpartsshop)

I took a gamble and found that while the diameter of the sigh-window was the same, the height was too low.

I did a dry fit and below you can see how much lower the sight-glass now sits.

But I have a cunning solution..

I did mention my other hobby was repairing watches right? Here I have a replacement plastic lens (called a crystal in horology) for a watch. I measured the opening in the fixing ring and it was 28mm wide. I dug out a 282 size crystal which is 28.2mm. The additional 0.2mm will make it a friction fit. Since it is not supported on the inside a hard knock will push it deeper. If it becomes a problem I'll make a spacer up but for the moment its good.

And with the side-cover over the oil-tank.

One problem was the seal for the sight glass was too hard and leaked. Since I didn't have a spare I made one by cutting a piece of high grade rubber (inner-tube) to shape. Rubber-tube may not last but it'll do. If it leaks I use something else.

I also spotted this filter sock on the inside to keep bigger bits out .

The outlet unscrews with the filter mesh attached.  This was rinsed in petrol.

I also rinsed the oil tank with petrol as there was some sludgy material on the bottom.

Reinstalled everything and put some 2-stroke oil in the tank. Its been 2 days and so far no leak!

Anil