OK, so what is the function of an oil seal? Well, to stop oil or grease or some other fluid from getting from one place where it should be, to another where it shouldn't. Initially most mechanical devices didn't have or need any sort of seals to keep these fluids in. Most steam type piston engines and even early internal combustion engines worked on a constant loss principle. Oil was delivered either by some toiling pleb with a can or from small reservoirs mounted on various moving parts or their bearing housings. The oil was either gravity or drip fed into the place where it was needed and then it was either consumed by friction of simply ran off/out/away to be replaced by more from the toiling pleb's can or the reservoir.
Early internal combustion motors also used a constant loss principle with oil either being injected into the crankcases via a hand operated plunger pump from the tank, which was often incorporated in the fuel tank on early bikes, or drip fed by an adjustable feed pump. the type I'm familiar with is the 'Pilgrim' pump on some early pommy bikes. The oil pumped into or delivered to the motor would then get flung around until it either escaped past the barely adequate early piston rings into the combustion chamber and burnt or was blown out past the main bearings where it dribbled down the cases and blew all over the machine and it's unfortunate driver or rider. This was one of the reasons rear mounted motors tended to be popular early in motorcar development. Gearboxes were usually lubricated by a delightful amalgam called 'Groil', a mixture of grease and oil that tended to be so tacky and viscous that it had trouble leaking out of the small gaps betwixt shaft and casing on gearboxes. It's problem was that it was also not very good at lubricating gear teeth and once pushed out of the way of the teeth tended to stay away meaning that bits wore quickly, tended to overheat and break.
Leather and Hemp Rope Seals
As power outputs and loads increased new methods of keeping the oil and lubricants inside the cases were developed. First leather and then hemp rope were used in various guises in an uphill battle to stop fluids escaping, this became even more important when recycling the engine oil through a tank or into a sump became commonplace on engines. At this time some sort of seal on either end of the crankshaft was seen as pretty important but as crank speeds rose so did the wear rate of leather and rope seals. To combat this, especially on car and truck motors, a scroll was machined onto the end of the crank with a flinger ring outboard of it. as the crank span the scroll tried to wind the escaping oil back towards the bearing. Any that got past this was flung off the flinger by centrifugal force and then ran down a gallery back into the sump or to the bottom of the crankcase where it was scavenged by a return pump on dry sump motors. Separate gearboxes tended to depend on leather or rubber seals and the use of as few exit points as possible for oil, that's one of the reasons that most pommy bikes used a non crossover box, the power went in and out of the box through one hole, it meant that there was less chance of leakage, (ho, ho!!!)
Rubber Seal With A "V"
About 50 years ago, (at a guess, I'm not sure on dates.) the development of synthetic rubber compounds had reached a level where it was possible to develop decent products that could stand up to being spun or rubbed up and down a on a smooth shaft at speed. These were the precursors of the oil seal we are all familiar with today.
Essentially they consist of a steel ring that has a ' Rubber' seal with a 'V' shaped lip on the inside, the pointy edge of the 'V' rubbing against the shaft. The *outside* of the seal appears flat usually while the *inside* shows a gallery in which there is a 'Garter' spring, (so called because it resembles the garters that men used to use to hold up their shirt sleeves, you know the ones, like bank clerks always wear in westerns when they are being held up by robbers.). When the seal is in use, pressure from the inside of the sealed object, be it a crankcase, gearbox, or whatever assists the spring to press against the shaft to which it seals and the oil is prevented from escaping by the close contact between the lip and the shaft, almost !!! In fact all seals let through a tiny bit of the lubricant, they have to, otherwise the lip would run dry and it would tear and leak. The thing is that the leakage is so small that in most cases it is almost un-noticeable.
'Viton' Type Seals
Now there are various different sorts of seals for different uses, some are more resistant to heat, some are designed to work better sliding rather than rotating and some are designed to work better witha shaft that only rotates one way. Some good examples of these are the brown, 'Viton' type fluorocarbon seals used on Guzzi crankshafts for the rear main bearing and the alternator seal. Because these have to withstand both high radial velocities, (that's the speed that the metal of the crank moves past the seal.)but only in one direction usually and at high temperatures . These 'Viton' type seals are very resistant to heat and where an *ordinary* non fluorocarbon seal will quickly deteriorate and break up the viton one will remain soft and subtle for a long time. The newer front seals also have a wavy line on the outside of the lip. What the purpose of this is I'm not sure but I assume it's to spread load. While I have had no problem with earlier seals this is an update recommended by Guzzi so I use the new ones with Bosch and Ducati equipped bikes.
Rear Main Seal
Directional seals, ones that are designed to operate better in one direction of rotation than the other are usually recognisable by having a serrated inside to the outer lip. Once again a Guzzi rear main bearing is a good example of this sort of seal.
Likewise there are different types of seal for fork legs. These are specifically designed to work better in a situation where they are expected to slide rapidly up and down a shaft without rotation. While I've used ordinary seals with a degree of success in fork legs they never seem to last as long as the ones Guzzi specify as sold by their suppliers. While there have been suggestions that the fact that Guzzi 35mm legs aren't actually 35mm but a bee's dick under I can't see this alone accounting for the problem, one of the objectives of the garter spring is to overcome slight discrepancies in size.
With any seal it's important that the surface it runs on is clean and unblemished. Pits, dags, rust spots, scratches etc will cause a seal to fail very quickly. In many cases small blemishes can be polished out but if seals repeatedly fail there is no option but to repair, renovate or replace the part the seal runs on.
Sealing Gearshift Selector Pawl with an O-Ring
Some shafts, especially those working in a non pressurised, low speed area like the gearshift selector pawl shaft in the back of a gearbox or the clutch thrust mushroom can be sealed quite effectively with 'O' rings, these can also be used in situations where no movement is involved between relative parts of a mechanism. An example of this being the 'O' rings used to seal the clutch input boss on the gearbox and the speedo drive on it's output shaft, (big block 5 speeder.).
There are probably dozens of other types of seals but for our purposes on our motorbikes these are the ones that matter. Remember, most seals are installed from the outside, there are exceptions, and the flat, 'Outer' face should be facing away from the pressurised part, the garter spring side inside where the oil is. Also, in most cases there is a 'Register', some sort of ridge that the metal ring of the seal will seat on. If there is it's important that the seal be pushed fully home onto the register as this will help keep the seal square to the shaft reducing the stresses on the lip. Where there isn't a register there is often a special tool available from the manufacturer of the vehicle or apparatus that will ensure that the seal suits square. While not vital these can be very helpful and can save a lot of heartache, especially with things like the rear mainseal which is such a sod to get to if it does fail after replacement.
I haven't mentioned gaskets simply because most of these are static. One exception is the one under the distributor on big blocks. This has to be moved from time to time so it's important that it is greased before installation. Quite often you'll find that a new paper gasket has shrunk. if this is the case soaking it in hot water for a couple of minutes before greasing it and installing it will ensure it doesn't rip and will be able to be moved if and when the points are replaced. No, I probably haven't covered everything but I hope it's useful to people.