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| Batavus Laura Head
Shaving |
Page 2
| Click on photos to enlarge. WARNING: These
modifications are experimental. I take no responsibility in the accuracy
or damage this information or modification may cause to you Bat. |
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Updated: 9/22/01
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PART 2
Initial Measurements
Initial measurements must be taken to determine how much material can
safely be removed from the head to prevent the piston from hitting the
spark plug or the head. It cannot be assumed that every engine is exactly
a like and in fact I have found some differences in measurements with just
the few engine samples I have. With that in mind here are some of the measuring
tool you will need.
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dial calipers or depth micrometer
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feeler gauge set
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parallel (any piece of material known to be flat)
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Display of various tools for
reference purposes. Not all tools pictured are needed.
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Tools in picture
Across top: 6" digital calipers
Left to Right: depth micrometer, 4" dial calipers, straight edge (pictured
5/16" lathe tool bit), feeler gauge set.
Piston Height
The engine does not have to be removed from the moped to perform any of
these operations. The first thing is to remove the head. Although not pictured
below it is recommended that you refasten the cylinder and torque the nuts
to allow the most accurate measurement of the top travel of the piston.
This can be achieved by placing larger nuts over the cylinder studs and
retightening the cylinder head nuts.
Side Note: The cylinder to piston relationship is not necessarily
a constant. Example: If you remove the cylinder from the engine and it
had the original gasket between the cylinder and engine, but when you put
the engine back together without the gasket and use a form a gasket material,
you have just unknowingly increased the piston height out of the cylinder
by the thickness of the original gasket.
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| Use of straight edge and dial calipers. |
Use of straight edge and feeler gauge set. |
I have shown above two ways of measuring the piston height. The first
way is with a straight edge and dial calipers and the second way is with
a straight edge and a feeler gauge set. Both measurements are taken with
the piston on TDC (top dead center).
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Using dial calipers: Place the straight edge across the machined
gasket surface of the cylinder and use dial calipers against the straight
edge to measure the height of the piston at it's highest point. The straight
edge in this example measures .308". The dial caliper reading is .286".
Subtract the straight edge thickness from the dial caliper measurement
and you have the piston height. (.286 - .308 = -.022) The negative measurement
means that the piston protrudes out of the cylinder.
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Using feeler gauge set: If the piston protrudes out of the cylinder
use two neighboring blades of a feeler gauge set and place them on the
gasket surface of the cylinder head. Put the straight edge on the
feeler gauge blades and slide the straight edge over the top of the piston.
Keep increasing or decreasing the feeler gauge blades until the straight
edge just brushes the top of the piston. In this example a .021 and .022
feeler blade were used. For recessed pistons lay the straight edge across
the cylinder and use the feeler gauge to check the clearance between the
piston and straight edge.
Spark Plug Height
With the spark plug tightened into the head, place the straight edge across
the machined gasket surface of the head. The dial caliper reading in this
example is .432". Subtract the straight edge thickness from the dial caliper
measurement and you have the spark plug recess. (.432 - .308 = .124)
Now figure out the existing piston to spark plug clearance. Since our
piston in this example protrudes out of the cylinder, the piston height
must be subtracted from the spark plug height. (.124 - .022 = .102) There
must be some clearance between the spark plug and piston when the head
shaving is done so I chose about .020" clearance. Subtract the final clearance
from the above existing clearance and you have the safe amount to shave
off the cylinder head. (.102 - .020 = .082) or .080".
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Measuring the spark plug to head relationship.
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Page 2
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