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     Scott Crank and Rod Assembly

 

Text 3076  26-02-05  Re Scott crank / rod assy.                                                      

 

First let us dispel a common myth.  The crank does not flex in use to any degree that is significant.

Crank flexure is not the cause of blue rollers and dished side plates.

So what does cause this?  Let us start with the relationship of the crankpin and crankpin bush.

We are assuming that the crankpin is round and parallel.  Where crankpin bushes have rotated in the past, the crankpin is often found to be both oval and tapered.  In this case you cannot use these until they have been reground to restore roundness and parallelism.  Compare the wall thickness of the crankpin and the crankpin bush.  You will see that the crankpin bush is thicker than the wall of the crankpin.  In order that the crankpin bush remains on the crankpin, there must be a condition called “Interference”, that is, where the bore of the crankpin bush is smaller than the diameter of the crankpin.  So where does this extra metal go?   If we call this extra metal 100%, then if we start to push the crankpin bush on the crankpin at its outer end, approx 60% will crush the crankpin and 30% will expand the crankpin bush.

This is because the different thickness of the two components, give each of them a different strength to resist deformation.  In this case the crankpin bush wins!

When the crankpin bush is pushed completely on the crankpin, then the strength to resist crushing of the crankpin near the point where the crankpin meets the flange of the crank, is dramatically different.  The strength of the flange aids the strength of the crankpin, so that it exceeds the strength of the crankpin bush. The crankpin wins!  In this area, the bush will expand 60% and the crankpin will crush 30%.  The remaining 10% is absorbed internally in the components.

The result will be a fitted crankpin bush that is bigger near the flange than at the outer end, unless compensation steps are taken to avoid this.  Like a flat belt that climbs to the crest of a crowned pulley. The rollers will run towards the inner side.  If there is correct running clearance for oil in the assembly, this need not be a considerable problem.  The inner roller plate will withstand some side thrust, although at the expense of a little power loss.  A more serious problem will occur if the sizes of the crankpin bush and the con rod big end inner ring are so arranged that the rollers are a tight fit.  In this case the tendency for the crank to run inwards will be more pronounced and with greater force.

I prefer to so arrange the sizes so that the rollers can be extracted using a small magnet.  If this will not extract them, they are too tight!

The Scott big end assembly is narrow so as to keep the crankcase volume as small as possible and optimise pumping pressures.  A consequence of this is that the big end assembly has little inherent ability to control itself in an upright stance.  The crankpin bush is 0.4425” wide and the con rod with inner roller plate is 0.4375” wide, so the side clearance of the assembly is 0.005” as new, but there are considerable variations found from engine to engine..  The real control of the vertical stance of the rod is by the gudgeon pin in the piston.

The secret is the relationship between the diameter and the length.

If we consider the big end, the, the internal diameter of the rod is much bigger than its width.

The diameter of the gudgeon pin, by comparison to the width of the rod little end is entirely opposite, the diameter being much smaller than the width.  

The durability of the Scott engine relies on the alignment accuracy of its assembly and the little end is the most crucial item in this equation.

If you wish to test the integrity of an engine when you are building it, assemble without rings and big ends without roller plates.  Measure the distance of the rod to the crank cheek each side and record it.

Rotate the engine 20 revolutions forward, then stop and record the distance of the respective rods to the crank cheeks.

Rotate the engine 20 revolutions backwards and repeat the measurement.

The rod big end should not have travelled sideways by any noticeable amount.

If your engine is assembled, Take off doors and remove crankpin screw and outer roller plate.

Remove spark plugs, then check that rods are against the inner roller plate and rotate forwards as before.

Are the rods against the inner roller plate and if so, is the roller plate entirely free to rotate, or has the rod “wound over” and trapped the roller plate making it stiff to turn?

Have the rods “wound out” so that there is now a gap between the inner roller plate and the rod?

Rotate the opposite direction and recheck.

The rod should not track across the bearing to any great degree.

Problems associated with inaccuracies in these areas will cause the following:-

Outer roller plate blue and dished outwards

Blue rollers

Inner roller plate with wear on side faces

Very rough engine if little end out of alignment causes rapid rod wag sideways.

Rapid big end wear

Rapid little end wear

Rapid wear of bores for gudgeon pin in pistons

Loss of power.

Because of the accuracy required to get an acceptable result from a design that was of necessity a compromise between mechanical integrity and gas flow efficiency, even in the early lower powered slower revving engines which are more true to Alfred’s original design, the company took as its logo, the “Made to limit gauge”

Personally I prefer to control the stance and position of the little end by fitting control washers either side of the rod little end within the piston.  This may not have been economical for the Scott Company to consider, as, after all, they were a commercial concern trying to survive in a difficult commercial world. Anything that increased production costs and thus retail costs would have been difficult to justify.

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email roger@mossengineering.co.uk or richard@mossengineering.co.uk