THE CORRECT USE OF MEASURING EQUIPMENT

What follows may be second nature to many of our karters who have some form of an engineering background, but many of the Competitors or Entrants these days don’t fall into that category.  How to correctly measure a parameter laid down in the Technical Regulations takes on whole new meaning when one realises that if non-conformance is found by the Scrutineers it inevitably results in a disqualification, and that’s something to make you day at the track memorable for all the wrong reasons.

The cost of measuring equipment depends largely on the brand of the given item and one should obviously purchase the best available tools within your budget constraints.  When checking or setting a particular size, reference should be made to the latest version of the technical specifications applicable to the kart class rather than relying on your memory of what you think the specification says.

Tape measure

If you thought that all tape measures were the same you’d be wrong.  There are a number of different ‘classes’ of tape measure available and the class determines its accuracy.  There are four generally accepted levels of tape measure accuracy viz. Class 1 (denoted by the Roman numeral 'I') is the most accurate, then Class 2 (denoted by 'II' in the picture) the second most accurate with Class 3 (III) logically being the third.  Such marking is found within the first 100mm of the tape, and an unclassified tape i.e. unknown accuracy will not feature any marking on its blade.

  • Class I is accurate to ±1.1mm over a 10m length

  • Class II is accurate to ±2.3mm over a 10m length

  • Class III is accurate to ±4.6mm over a 10m length.


Tape measures manufactured under the brand name Stanley or Ross are usually Class II and these are good for our purposes (accurate to better than ±0.2mm over the length of a kart’s axle).  There is a ‘claw’ fixed to the end of the blade and this allows for internal or external measurements to be made.  Consequently any damage to the claw that causes it to bend or elongate the holes allowing it to move slightly on the blade, will affect the accuracy of the measurement.  Therefore any rough usage, bending on the claw, dropping the tape, or even letting it retract violently back into its housing can seriously impact its accuracy.

This instrument is largely used for measuring the length of a kart axle, or the rear track width over the tyres or rims whichever is the larger.  Consequently, you should ensure that when purchasing one that the width of the blade is such that it can easily fit inside the bore of your kart’s axle.  In order to save the cost of expensive templates, jigs and the like, the most cost effective way of measuring the rear track is to slip the tape measure through the axle, let it butt up against a flat object such as a clipboard pressed against the far-side tyre, hold a steel rule against the near-side tyre and observe the reading on the tape where it crosses the steel rule.

Vernier caliper

There are a number of different types of caliper (all usually referred to as a vernier) available viz. a digital caliper, a dial caliper, and what is simply known as a vernier caliper, all of which are shown in the picture.  All are capable of measuring an outside size (using the lower jaws), an inside size (using the upper jaws), or a depth (using the depth tang extending from the RH end of the instrument).  The easiest to use is obviously the digital type, followed by the dial version, and lastly the vernier type.  All are capable of providing a reading to the second decimal place of a millimetre with greater or lesser precision depending on the type of instrument.

If available, always use a digital vernier as almost no skill is required to interpret the readout displayed.  For a dial caliper, one needs to add the reading on the main scale to that displayed on the dial which usually has 100 divisions.  There are two variations on the vernier caliper viz. one which has a vernier scale with 20 divisions (each is 0.05mm) and one with 50 divisions (each is 0.02mm).  In both cases the actual reading is that displayed on the main scale plus what one sees on the vernier scale.  From the illustrations provided, the vernier provided with 50 divisions has a greater precision and is the preferred one to use.

When conducting a measurement with any of the above instruments, there are a few things to be borne in mind.

  • Open the caliper and clean off the jaws (use your thumb or a cloth) before proceeding.

  • For a digital instrument, switch the instrument on and check the display with the jaws closed.  If it is not reading zero, depress the ‘zero’ button and check again.  For a dial Vernier, close the jaws and check the pointer is showing zero.  If not, rotate the bezel on the dial until it lines up.

  • When measuring always apply light and constant pressure via the thumbwheel of the caliper i.e. don’t overly squeeze the component being measured.

  • Ensure that the jaws and/or the depth tang are as perpendicular as possible to the direction of the measurement required.  Failure to do so will result in a reading that is either oversized (external jaws or depth tang), or undersized (internal jaws).

  • The jaws of all of these instruments may be locked in place to preserve a set opening by using the small thumbscrew located on the top of the sliding jaw.  Don’t overtighten this.

Spring caliper

There are two different types viz. one for internal use, and one for external use.  The internal type (refer picture) is useful for checking hard to reach bores such as a carburettor throat where one cannot gain access with a vernier.  The easiest way to conduct multiple checks of whether a dimension is within specification, is to use the instrument as a comparator i.e. don’t use the instrument to measure the actual dimension each time, but rather to check if it exceeds a set value.  As an example, the inlet venturi diameter of a Bambino carburettor is specified as 12.1mm maximum.  Firstly set a vernier to this size, and lock the jaws.  Lay the vernier on a flat surface and adjust the jaws of the spring caliper against the vernier’s jaws until a slight resistance is felt when rocking the caliper over centre (similar to that described under the telescopic gauge).  The spring caliper now has the correct dimension set, so it can be used to check (say) a number of carburettor bores one after the other, providing no alteration to the setting is made.  If less resistance (this only applies to an internal measurement) is encountered when rocking the caliper inside the bore is found, then the bore is more than likely oversize.

Micrometer

Micrometers are available in three different styles that are used to measure outside sizes, inside sizes, or the depth of a component.  As the outside micrometer shown in the picture is the type used most often, it is discussed in further detail but they all operate on similar principles.  Once again, the measuring faces need to be clean, and it is imperative that one checks that the reading on the scale displays zero when the spindle is rotated and closed against the anvil.  Always use the ratchet when operating the instrument as this ensures a repeatable pressure on the component being measured (even when checking the instruments zero).  If the closed reading is not zero, the discrepancy needs to be accounted for in subsequent readings, but the best solution is to have the instrument zeroed by someone who knows how to do this correctly.

Reading a micrometer is fairly similar to that of a vernier.  The reading displayed on the main scale is added to that displayed on the thimble to provide the final dimension.  In the picture, this is 7mm on the main scale + 0.14mm on the thimble = 7.14mm.  If the instrument is used to set a size on a spring caliper, the thimble may be prevented from rotating once the correct size has been dialled in by using the locking lever on the frame.  Remember to unlock the instrument prior to using it for subsequent measurements.


Telescopic gauge

This is an instrument that is seldom used, but can result in incorrect measurements if used incorrectly.  Choose the proper size gauge for the hole to be measured and proceed as follows.

  • Push the plungers in, and lock lightly (never tighten more than necessary to hold the plungers in place).

  • Place into the hole, tilt slightly off centre, release the lock allowing plungers to contact sides of the hole and retighten, but do not over-tighten.

  • Rock the gauge over the centre as shown, remove and measure with a caliper (or micrometer) using a light touch as too much pressure will destroy the accuracy of the reading.

  • Do not repeatedly go over-centre as the tool is designed to be self-centering and excessive rocking could result in a reading that is smaller than the bore being measured.

  • Always take a few readings to ensure repeatability and that the measurement is accurate.

If the gauge is used incorrectly e.g. it is not rocked, or locked into position when the handle is not 100% horizontal, and then extracted from the hole, it is possible to get a reading that is over-sized.  Typical sizes that will result from a hole that is exactly 12.00mm diameter are as follows:


Checking squish

The combustion chamber shape on a kart engine is usually some form of a dome that assists in ‘squishing’ the trapped fuel/air mixture towards the spark plug as the piston rises thus promoting better combustion.   There is a minimum value that this squish thickness is allowed to be and it varies between classes.  It is measured by inserting a length of solder wire (of the correct diameter) through the spark plug hole, manually turning the engine over past top dead centre (TDC), removing the wire and measuring it with a vernier.

When conducting this test there a number of important points to keep in mind.  Always ensure that the end of the correctly sized solder wire is trimmed off square using a utility knife blade rather than side cutters as the latter tends to distort the end of the wire.  The smallest value of squish is closest to the cylinder wall, so it is imperative that the bent end of the solder wire is long enough to reach this.  Squish is never measured on the Bambino class, so let’s set that one aside.

The cylinder bore on the Kid Rok and Mini Rok is near enough 42mm, whilst for all other engines (Rotax or Rok) it is close to 54mm, so when bending a length of solder wire to fit as shown in the previous picture, ensure that the bent portion is just a bit longer than half of the bore diameter of the engine.  If the wire needs to be straightened out for any reason, try not to stretch it as that tends to thin the wire.  The cylinder ports on a kart engine usually face fore/aft and squish is measured at 90deg to this i.e. in line with the gudgeon pin of the piston.  Insert the bent solder wire through the spark plug hole, ensure the orientation is correct, get the end of the wire to touch the cylinder wall, and slowly rotate the engine by hand.  This can be done using a spanner on the pinion nut of the clutch, or using one’s hand on the teeth of the starter ring.  A DD2 engine requires it to be ‘turned over’ using a 8mm bolt screwed into the end of the crankshaft after removing the plug on the side of the crankcase with a coin (item 2 in the picture).  In all cases some resistance will be felt as the solder wire is compressed when the piston passes TDC.

Carefully remove the solder wire and check the measurement approximately 1mm from the end of the wire using a vernier.  Apply only sufficient pressure on the jaws to get the correct reading without squashing the wire itself.  This measurement is not necessarily the same on either side of the piston due to manufacturing tolerances, carbon build-up, etc.  The removal of carbon from the cylinder head or piston to get the reading to comply with the minimum value is not allowed.

 

Feeler gauge

This is an instrument that is essentially only used when conducting two checks viz. spark plug gap, or when confirming the port timing on the Rok and Comer engines.  If you are going to check the port timing, then have a read of the Technical Regulations for that class because the feeler gauge invariably needs to have the correct blade width and also needs to be inserted into the port at a specific angle.  Be careful when purchasing a set of feeler gauges as cheap ones from the ‘corner café’ won’t necessarily be very accurate.  Using these and getting the correct ‘feel’ of a gap, can be likened to a ‘Goldilocks moment’ – one feels too loose, one too tight, and one is just right.

To achieve this feel, use the following guideline.  If the maximum gap allowed in the regulation is say 1.0mm then set your vernier to that size and lock its jaws in place.  Choose the correct feeler gauge blade (or combination of blades) and slide it into the jaw opening because the resistance you are looking for when checking the spark plug gap is identical to that.  If you require to use more than one blade to achieve a certain size (the blades usually go up to 1.00mm in 0.05mm increments) then use two blades that are located close to each other in the pack.  As an example, the 0.60mm + 0.70mm blades to get 1.30mm is a better choice than 1.00mm + 0.30mm.


Emile McGregor - MSA Technical Consultant