SmedSpeed XS650


oil system modifications

The Yamaha XS650 oil system utilises what is termed a wet sump design, this is when the oil for lubrication is retained within the confines of the crankcases. In the past older designs used a dry sump design where the oil is kept in a separate oil tank. Bikes like Harleys, Triumphs, and most other British bikes had a dry sump design.

The wet sump design has some good features and some negative features to it. On a positive note, it negates the requirement for a separate oil tank, oil lines, and a return rotor for the oil pump. The not so clever features are the oil is kept within the confines of hot crankcases, so cooling is not as good as a dry sump design and the oil level is usually relatively high in the cases.

On an XS650 the crankshaft runs in a depth of oil, and depending what the level is will dictate how much depth of oil it runs in.

Oil level window

To check exactly how much oil the crank is running in, I built up a bottom crankcase, with just the crank and rods assembled in the cases. I then filled the engine up with oil until the oil just touched the bottom of the flywheels. In the picture above the red line is the oil level just touching the very lower part of the circular flywheels. 

So if the engine is filled with oil to the lower mark on the sight window, the crank will already be running in around 18-20mm of oil, by the time the oil level is on the high mark the crank is around 36-37mm deep in oil.

So what, I hear you say, well there is a downside to running your crank in this much oil. Imagine if you will, running down a beach where the sand is just covered in an inch or two of water, no problem at all. However if the water on the beach was knee high, it would entail a whole lot of effort to keep running. The same is true of your crank running deep in oil, a lot more effort to move the crank through the oil. When the crank is turned at speed through the oil it causes foaming, excessive oil on the flywheels is also flung up the bore., Whilst a bore needs oil for lubrication, too much will overwhelm the oil control ring, causing slightly oily spark plugs and oil traces in the combustion chamber.

The veteran Yamaha tuner Shell Thuett knew this, and always filled his race engines (using early XS1/XS1B cases) to 2.5 litres NOT the 3 litres capacity Yamaha had suggested, the result less oil drag, less problems with oil breathing from the head, and no oil trace on the spark plugs. 
To keep as much oil volume as possible, and at the same time to reduce the actual oil level, I apply two different methods. The first is to convert the often redundant starter motor into a reservoir; from a wholly original idea Smedspeed/Guilford Motorsport Fabrications bring you the enlarged oil reservoir conversion. 

starter motor conversion

The starter motor ends are removed and a new 6082 billet aluminium body machined to suit the existing Hitachi starter ends. This adds 415 cc of additional oil, the starter end cap which is nearest the engine is bored in the lathe to the largest size possible to allow as much circulation as possible, the conversion is supplied with a drain bolt, and a bleed facility to allow any trapped air out. The extra oil capacity is over 16% increasing the oil cooling of the engine.

The kit is on an exchange basis, all I require is your starter ends to modify.

The second of the oil level reduction methods is the addition of a 20mm spacer between the sump plate and the engine. The spacer allows for a further 200cc of additional oil, as the spacer is directly in the cooling airstream it aids cooling both by virtue of increase oil capacity and surface area.  

oil spacer 
A total of an extra 615cc of oil is available through adding these two “reservoirs”, the oil level can then be confidently set with a measuring jug at 3 litres, the static oil level will be just beneath the flywheels. (I have noticed that the oil level window and dipstick do not always show the same oil level; it appears that the oil level sight glass is higher than the dip stick reading, not by much though). 

The sump spacer doesn’t down extend too far either, above is my 880cc with a Smedspeed sump filter and the spacer, it can be seen it doesn’t hang down a long way, it’s not much below the centre- stand brackets in actuality.  If you live in an area where you do off-roading, it might not be a good idea, BUT I ride my bikes on dirt roads sometimes, and have not punctured a filter yet.

One of our competitors in sidecar road race ran his XS650 on his dyno with the oil level well onto the top of the dipstick/inspection window mark. His engine was flat and unresponsive. He realised quickly what had happened and lowered the level right down, so the crank was completely free of oil (under the lower dipstick mark) and picked up 4 hp !!!

All the modern Japanese engines still utilise a wet sump, BUT now they put all the components much higher in the cases and the oil much lower, every little bit of hp gain counts.


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These are experimental parts intended for racing and as such no warranty is given or implied. If you decide to buy it is on the understanding that you assume complete responsibility for any possible injury, loss or damage that may occur as a result of using these parts and that you accept these conditions of sale prior to purchase.