Crankshaft Thrust Washer

[squirrel_hunter]

So trying to stick the engine back together today and hit another problem. It would appear that I've lost my Crankshaft Thrust Washer. Not sure how or where but thats not important right now, what is important is its no longer in my possession.

So just order another one? Not as simple as that. The Crankshaft Thrust Washer comes in 1 - 10 different thickness' ranging from 1.925 - 2.150 in 0.025 increments. So how do I find the thickness that I require?

[yorkie_chris]

Dozy pillock!

Bolt engine up and measure crank end float. Might be problematic. Perhaps better to get the smallest one and assemble it with that to get accurate measurement.

Reason being I don't know if anything else will stop the crank moving where the end float washer should and deny you an accurate measurement.

[squirrel_hunter]

Long shot here... anyone got a pic of one?

Finally got the engine back, but I'm not convinced at what I'm seeing.

[yorkie_chris]

No but it is a ring bright on one side, brass colour on other with a groove down it

[squirrel_hunter]

In which case thats what I've got.

Which side should go against which bit?

[yorkie_chris]

Brass side goes against crank

[squirrel_hunter]

Cool.

Purely out of interest... why?

[yorkie_chris]

reduce friction and avoid galling I reckon. groove appears to be to allow oil flow.

[keith_d]

Years ago I saw a car engine with a stuffed up crank end bearing. When it was idling, if you pushed the clutch pedal down it would stall because the whole crank was moving back and forth. Took us ages to find out what was wrong.

No idea if the same would apply to a bike engine.

Keith.

[yorkie_chris]

Nope, clutch isn't attached to crank :-P

[embee]

The grooves across the faces serve 2 functions.
It is a means for oil to be distributed evenly across the bearing surface (it gets entrained between the bearing surfaces by means of chamfered edges to the grooves usually), otherwise the oil film loading tends to be uneven and the ultimate load capacity is reduced.
Equally important it provides a means for contaminants to escape from the bearing, otherwise they can tend to accumulate within the bearing and can result in severe scoring.

[squirrel_hunter]

Now this might be a silly question, but if I don't ask it I'll never going to get an answer...

What do you mean by way of Bearing Surface in this context?

When I think of bearings, I normally think of the wheel bearing type of bearing, but I know that there are different types of bearing such as the bearings in the big end that I've seen and on the generator side if the engine. These bearings from my understanding are simply smooth metal built to tolerance, but the right hand side of the crank where the thrust washer sits between the crank and the crankcase doesn't have this like on the left hand side. So thats where I'm confused...

[embee]

Bearings are usually referred to as either "rolling element" bearings (ball/roller/needle) or "plain".

In plain bearings the surfaces are usually "conforming", i.e. basically the same shape, either round or flat.

Plain bearings rely on the oil film between the surfaces to carry the load such that the bearing surfaces never actually touch. This is usually achieved by using hydrodynamic oil film generation, in other words the oil is supplied into the bearing and the motion of the surfaces relative to each other and the very small changes in clearance result in a wedge of oil being formed which lifts the surfaces apart as they move. This is why the clearance of a plain big-end or main bearing is critical, too little clearance and the wedge won't form properly, too much and the wedge becomes too short to support the load. The oil supply pressure is required to ensure it gets into the bearing in sufficient quantity to cope with the leakage, it doesn't of itself provide load bearing capacity as such.

The more load you apply to a hydrodynamic plain bearing the greater the eccentricity becomes (and so the steeper the wedge angle) up to the point where the surfaces get closer together than the size of contaminants or the surface roughness itself, and wear takes place. The bearing (i.e. the shell) surface is usually a soft material (aluminium-tin with a bronze underlay typically) which is designed to allow hard contaminants to embed into the surface which minimises damage.

There are very often grooves around the bearing, or sometimes just half the bearing (to minimise load capacity reduction), or across the width, to ensure the oil gets supplied to the appropriate areas.

What can also occur particularly in big-ends is that under rapid reversal of load (typically TDC exhaust) the bearing can move across the clearance rapidly and the sudden reduction in local oil film pressure can result in cavitation which can fatigue the surface of the soft bearing surface and pitting can result.

Crank thrust bearings also work hydrodynamically up to a point, the oil is supplied usually from the centre and feeds out through the grooves, and the rotation entrains it into the gap between the faces, but of course the wedge only operates for a short distance around the circumference and the specific load capacity isn't all that high. For very high load thrust bearings, for example in ship propeller shafts, the thrust bearing is made up of tilting pads ("Michell") which automatically optimise the oil film wedge action and provide very high capacity in a small size.

I've not seen an SV in pieces, but from the parts diagrams I assume the crank endfloat is between the thrust washer inboard of the right side main bearing panel and the camchain sprocket flange outboard, the left side simply runs with clearance. Those with experience of the SV in bits will correct me if wrong.

There isn't any real crank end-thrust in the SV engine, what there is tends to come from deflection of the crank under firing and inertia forces. Car engines usually have the thrust bearings either side of a centre main panel (e.g. 5 main bearings in most 4 cyl engines, thrust on centre main). They are either separate semicircular bearings set into the main bearing panel, quite commonly these days they are integral in the centre main bearing in a flanged arrangement. They of course take the thrust from the clutch release, which can be quite significant. Engines used only with automatic transmissions often have only a half thrust bearing rather than the full circumference, usually the case half, since thrust is less and it reduces cost and drag.

HTH

[yorkie_chris]

I've not seen an SV in pieces, but from the parts diagrams I assume the crank endfloat is between the thrust washer inboard of the right side main bearing panel and the camchain sprocket flange outboard, the left side simply runs with clearance. Those with experience of the SV in bits will correct me if wrong.

Yup, without the primary drive sprocket and rear camchain sprocket on there and torqued down the crank can move left to right couple of mm.

[squirrel_hunter]

Embee, thank you very much for that explanation. The way these bearings work now makes sense.

However bad news. The engine received a little damage in transit. Anyone ever replaced the main crankshaft bearings (item 3 in your diagrams) before? Squirrel is not a happy person.

[yorkie_chris]

How did that happen?

I think you need a press.

[squirrel_hunter]

The cases were not bolted together, the crate gave way, the cam chain gear cogs on the crank have left a lovely dent in the bearing. Sh!t happens, but I'm close to giving up.