Made to Measure - Micro Measurement
Just because they are relatively cheap, don't rush out and buy a set of feeler gauges until you have looked in the handbook to check whether the settings on your motorcycle are in inches or millimetresYou can, of course, convert inches to metres and vice-versa, but you wont always have the exact feeler blade to match the answer, So get Imperial or metric feelers as applicable. Let's see how they are used:
Plug Gaps.  Measure the gap between the centre electrode and the side electrode, if necessary laminating two feeler blades together to get the correct thickness. If the gap needs adjusting, bend the side electrode only, using a plug gapping tool to open or close the gap until the gauge is a firm sliding fit. Worn plugs will give a false reading if the side electrode has been etched away by the spark (1).
With these, bend the side electrode vertical (it won't fall off) and file its innersurface flat. You should also file the centre electrode so that it is flat alsso at the end (don't do this on precious metal plugs - leave the centre electrode alone on these) so that when the side electrode is re-positioned you have a parallel gap (2).
A Set of Feeler Guages
Contact Braker Points  The points gap is responsible for triggering the spark at the plug, and if it's wrong the ignition timing will be wrong too. As it is an electro-mechanical gadget, the contact-breaker wears in two areas. Mechanical wear takes place on the plastic or fibre rubbing block that the cam uses to lift up the moving contact. Electrical wear comes about each time the contacts separate when a spark transfers a tiny spot of molten metal from one contact face to the other. 
To correct mechanical wear, check the gap between the contacts when the moving contact is at its maximum lift on the cam. The gap can be altered by loosening a lock screw and moving the fixed contact. The feeler blade should be just a sliding fit between the points (3).  After setting, try to re-insert the feeler gauge - it should go back in without having to prise up the moving contact.
Electrical wear results in a small 'pile' developing on one contact, with a corresponding crater in the other. This interlocking of the contact face has little effect on performance, providing the gap is correct, but it can give a false reading when you use a feeler gauge to check the gap (4). The ideal remedy for pitting and piling is a new contact set, but a lot of manufacturers hide their contact-breakers behind the generator flywheel which needs a special tool to get it off. On these, you may be able to remove the pile (don't worry about the crater, the spark will find the closest surface to work from) using a thin file or a small slip-stone. If you can't, setting the gap at the edge of the contacts will give the most accurate result.
Valves  The majority of four-strokes have an overhead camshaft layout where the cam lobe operates the valve through a short rocker. From your handbook you need to know whether the valve gap is set at the camshaft end or the valve end of the rocker, and whether the engine should be hot or cold. Once this is determined, check the gap with the appropriate feeler blade - it should be a firm sliding fit (5).
On motorcycles with push-rod operated valves, it's possible for wear on the rocker pads to give a false reading (6). If the valve gear seems noisy after repeated adjustment, examine the rocker pads for wear. Any indentations can be corrected by stoning away the unworn portion until the pad surface is flat— don't use the oil stone to go below the level of the worn section or you'll damage the case-hardening. This work should not be carried out on a revolving grindstone by the inexperienced.
Rings,bores and clutches You can measure the thickness of something using a feeler gauge providing you can somehow transform the thickness into a gap. The best example is when checking the thickness of piston rings. First, clean and measure the ring gap in the piston. 
Suppose our feeler gauge shows this to be 0.060 in. Now fit the ring and measure the clearance between the ring and the gap (7). Let's say this is 0.005 in. This means the ring is 0.005 in thinner than the gap, in fact it is 0.055 ins. thick. Using a piston ring you can also get a very good idea of how much the bore is worn, using just a feeler gauge.
This is what you do. Clean the top of the bore, the bit the piston ring doesn't touch and fit the ring into it so it is parallel with the top of the bore (use the piston to push it up) and measure the ring gap (8). Now push the ring down until it is midway down the bore and measure the gap again. If the engine has been used, this measurement will be bigger than the first one. Lastly, measure the ring gap again at the bottom.
Suppose our end gap on the top (unworn) section of the bore was 0.005 in. and half-way down it was 0.020 in. and at the bottom it was 0.017 in. Take away the number we first thought of the 0.005 gap for an unworn bore and we know the circumference is worn by 0.02 —0.005 =0.015 in. in the middle and 0.017-0.005 = 0.012 in. at the bottom.

Bore wear is normally quoted across the diameter and to get this we divide our wear measurements by 3.142, or just by three if you're in a hurry, though remember that this will then be only an approximate measurement and this gives across-the-bore wear of 0.005 in. in the centre and 0.004 in. at the bottom. Comparing these with the data figures in the workshop manual will indicate whether the cylinder needs re-boring.

Clutch plates can be checked for warpage with a feeler gauge. Put them on a flat surface (a piece of thick plate glass is ideal) and try to fit a feeler gauge between the plate and the glass. The manufacturer will quote a maximum warpage figure. On the Suzuki GT750, for instance, it's (0.3 mm) and this is the feeler gauge to use. If it won't fit under the plate, it's not warped. If it does,scrap it.
 Warp: The simple straight-edge has its uses still. Suppose you're blowing head gaskets with monotonous regularity and you're pretty sure you have tightened all the head nuts to the correct torque. Could the head be warped? You can often tell by holding the head face up to the light, and puttinga straight-edge across it length ways, crossways, and diagonally. If you can see light between the head face and the straight-edge - or get a feeler gauge under it - the head's warped and this is why the gaskets are blowing. Have it surface-ground to correct the problem.

Timing: Ignition timing is one of the more important engine adjustments, because it triggers the spark at the plug. If it's wrong, the engine produces less power or overheats. What you are measuring is the position of the piston at the instant the contact-breakerpoints open; on a two-stroke this is often done using some sort of depth gauge through the plug hole to measure piston position, while a small bulb or a meter indicates when the contacts open. On a four-stroke you can use stroboscopic timing light.

Thickness: Although feeler gauges will measure small gaps, if you need to check the skirt diameter of a piston or the exact dimensions of a cam lobe, you need something more sophisticated, and you have the choice of three methods - a vernier, which is a sophisticated caliper capable of measuring to 0.001 in. or 0.02mm; a micrometer, which has less range than a vernier, but is even more accurate; or a clock gauge (which can be set up to measure practically anything, so long as you have suitable attachments) showing its results to the nearest 0.001 in. or 0.01 mm on a clock face.

‘So much for theory’. The only flaw so far is that while most measuring instruments are delightful pieces of equipment, built to clinical standards, the things they have to measure are often filthy, worn, or seem to be deliberately positioned so you can't get at them! What's more,without a few minutes' tuition nobody can read a vernier scale or a micrometer at all.

So, in an effort to add a little practical knowledge to the theory, we've put some of the more popular measuring tools in their environment to show how to use them properly and how to avoid the main pitfalls that lead to inaccuracies,
PYRAMID builders, I imagine, measured their work to the nearest two or three inches; but times have changed since then! Where motorcycles (or any other mechanical contrivances) are concerned such tolerances are just not good enough. Imagine a wheel hub two inches off-centre! In a straight line the motorcycle would buck like a bronco, while round corners. . . well, I shudder to think.

In engineering terms even half an inch is too big. So is a quarter of an inch, or a sixteenth of an inch. It's not until we get down to a thousandth of an inch or a fiftieth of a millimetre, that we begin dealing in the sort of measurements that engineers use when they build up and adjust the engine, gearbox or any other moving part on your motorcycle. For the record, a cigarette paper (Rizla green) is one thousandth of an inch thick.

You might wonder how anybody can measure anything to an accuracy of one 'thou' (written 0.001 in.), but in fact this can be done using one of the cheapest and readily available of measuring tools - a set of feeler gauges. Of the four adjustments which are essential to keep four-stroke engine running sweetly, valve gaps, points gap, plug gap, and ignition timing, three are measured using feeler gauges. The fourth needs a timing bulb or a stroboscope but more about this later.
Not all motorcycle measurements are of gaps. Here are some of the others you need to check from time to time:—

Unless you are in the business of modifying engines, it is unlikely you will need to use a depth gauge to measure the travel of the piston, but as long as you use public roads,you will need to know how much tread you have on your tyres. The legal minimum is 1 mm and the best way to measure it is using a small depth gauge, cheaply available from accessory shops.

You’ve guessed it, tyres again. On the whole, garage pressure gauges give a general idea of tyre pressure, but because all and sundry use them they get rough treatment; so,don't be surprised if they are a pound or two out on occasion. The best bet isto get yourself a pocket pressure gauge and use it to check the tyre pressures on a regular basis. Most manufacturers suggest checking the tyres when they arecold, incidentally.

If you've ever flicked through the engine re-assembly section in a workshop manual you will know that virtually all nuts and bolts must be done up so tight, and no more. To prevent stripping threads or breaking studs you use a torque wrench, which indicates when the correct tightness has been achieved. Torque is measured in lb/ftlb/in, or kg/m.
Some Other Tools Covered Later
If you wanted to be crude, you would describe the micrometer as a very accurate G-clamp. The most usual sort is an external micrometer which measures from zero to 1 in. or 0-25mm. To use it, you place the item to be measured inside the G' and turn the thimble control until the object is gripped.  To prevent over-straining the thimble usually has as pring-loaded ratchet on the end, which should be used for the final contact (9).
For a better view of a Micrometer this picture show the component parts and uses the common names.  Some lockng systems are either a knurled collar on the sleeve or a small turnscrew on the frame.
Once you reach this stage, all you need do is read off the measurement on the thimble and sleeve, but to do this you need to know a little about how it works. This is what happens on a metric micrometer reading to a hundredth of a millimetre. The screw on the micrometer has a pitch of 0.5 mm, so that two revolutions of the thimble will move it down the thread 1 mm. On the sleeve (10) is a datum line graduated in millimetres (below the line) and half millimetres (above the line). The thimble is divided into 50 equal divisions, so that each division represent some fiftieth of 0.5mm, which equals a hundredth of a millimetre.
To read this micrometer you first note the number of millimetre divisions (major divisions) on the sleeve, then note whether there is a half millimetre visible (minor divisions). You then read off the thimble division that aligns with the datum line. In drawing No. 10 the reading is, 

Major divisions     lOx1 mm =10.00mm 
Minor divisions     lxO.5mm= 0.50mm 
Thimble divisions  16x001 mm =0.16mm
Reading =            10.66 mm
On the Imperial micrometer, the sleeve has major divisions representing 1/10th of an inch (0.1 in.) each and each major division is divided into four minor divisions representing 0.025 in. each. One complete revolution of the thimble advances it 0.025 in. and it is divided into 25 graduations so one division on the thimble equals 0.001 in. In drawing (11) the reading is:—

Major divisions    = 2x0. 100 = 0.200 in
Minor divisions    = 3x0.025 = 0.075 in
Thimble divisions =  llxO.001 =  0.011 in
Reading =              0.286 in.
There are internal micrometers for doing such jobs as measuring the inside diameter of cylinder bores—these are usually mounted on an extension (12) so they can be lowered and held horizontally with some accuracy while the measurement is taken. The method of taking readings is exactly the same as already detailed.
The Vernier Caliper
The vernier caliper produces results in the same way as a slide rule. It normally has two main scales one in millimetres and one in inches (13) and a vernier scale on each. On the metric side, the vernier scale is divided into fifty divisions over a distance of 49mm, so each division equals 49/50ths of a millimetre (0.98mm) and the difference between a division on the main scale (11 mm) and a division on the vernier scale (0.98mm) is 0.02mm
To take a reading (14) note the position of the zero line of the vernier scale in relation to the main scale (in our example 53mm). To this, add the number of divisions from zero on the vernier scale to the point where one of the vernier graduations aligns exactly with a line on the main scale in our example 42 divisions or 42/50ths of a millimetre (0.84mm).
The reading is therefore:—
Main scale        53.00mm
+ Vernier              0.84mm
Reading      =    53.84mm
On the Imperial side, the main scale is graduated in twentieths of an inch (0.050 in.) and the vernier is divided into 50 divisions over a distance of 2.450 in., each division equalling 0.049 in. The difference between a division on the main scale and the vernier scale is 0.001 in.
In Fig. 15 the zero on the vernier just clears 1.450in. Add to this the divisions of the vernier from zero the the point where it aligns with the main scale (0.031 in.) to give a reading of 1.450 + 0.031in.  =1.481 in.

Besides measuring such things as clutch plate thickness and clutch spring free lengths, verniers can be used to measure internal diameters (16), and most have a depth gauge as well.
Torque Wrenches 
There are two types:
The cheaper one has a long pointer attached to the business-end and the pointer aligns with a scale calibrated in lb.ft or kg.m on the bar of the wrench. As you tighten a nut or bolt, the wrench bar bends but the pointer stays still - and indicates on the scale the turning torque you are putting on the nut or bolt.
More expensive torque wrenches are spring-loaded, and the shaft goes over-centre and 'breaks' with a distinct click when the correct torque is reached.  These are pre-set by winding a thimble on the handle.
The tightening torques given by the manufacturer are calculated on the assumption that the nut or bolt has free-running oiled threads, so make sure it tightens down easily before using a torque wrench.

Another pitfall to watch for is the way the manufacturer quotes his torque settings. Yamaha, for instance, quote torque figures in relation to stud diameter, not the hexagon across the flat size. If you inadvertently torque a 10mm nut to the figure they quote for a 10mm stud, you'll break something. We know - we've done it!
Compression Testers
A compression tester works like an oversize tyre pressure gauge and measures the pressure generated above the piston when the engine is kicked over. To use it, remove the spark plug, screw in the tester, open the throttle fully, and kick over the engine.
In the technical data published by the motorcycle manufacturer you'll find the recommended compression figure for your bike. On a two-stroke, if you're more than 15lb/ below this, the rings or bore are worn. Strip off the heads and barrels, and check further with a piston ring and feeler gauges, or an internal micrometer.
On a four-stroke, a low reading can be due to borewear or a leaking valve. You only need a teaspoonful of engine oil to tell which it is.

This is what you do: Take the compression pressure at full throttle, with the engine cold. Say it's 100 lb/sq. in,, and it should be 140 lb/ Somewhere we've lost 40 lb/ Is it getting past the piston, or is has a leaking valve? Take out the tester from the plug hole and pour in a teaspoonful of engine oil. Refit the tester, and kick the engine over again. If the reading is the same, the leakage is definitely past a valve, and you need to have the head off and do a grind-in job. If the reading improves, the leakage is past the piston - the addition of the oil has improved the seal and given the improved reading and it could be that your motor is due for a re-bore.
The most spectacular instrument of them all, the stroboscope is used to measure the ignition timing on a four-stroke engine while it is running connected in to the high-tension feed to the spark plug, it produces a flash of light the instant the plug sparks. Shine it at the timing marks, and if the ignition timing is correct the marks will 'freeze' and appear to be aligned (19).
There are two types of stroboscope cheap ones, and good ones! The cheap ones have a neon light which has an orange hue to it and the unit is simply connected in series with the HT cable to the spark plug. The light output is feeble but useable, providing you operate it in a darkened garage and have picked out the timing marks with white paint beforehand.

The good strobes cost more and are recognisable by having three wires: One goes to the spark plug, one to earth, and one to the battery positive terminal. The Xenon tube behind the lens gives a brilliant white light that enables you to see the timing marks in daylight
Vacuum Gauges
The opposite to a pressure gauge, the vacuum gauge measures depression — or 'suck' and is principally used on motorcycles when balancing multiple carburettor layouts. Honda use a 'nest' of four gauges for setting up the carburettors on the CR750/550/500/400 F ranges, for instance.
The Bing constant-vacuum carburettors on our BMW R90/6 each have a pick-up pipe for a vacuum gauge and this is used to check piston-lift at part throttle (20). Ideally, you need two identical vacuum gauges which are mounted on the handlebars. Run the engine at 2,000 rpm and both gauges should register the same.
This tells you the state of charge of the battery. What you do is use the flexible bulb to draw off a sample of battery electrolyte (21) and then read off the specific gravity of the electrolyte using a calibrated float inside. Readings should be taken at room temperature (60°F) (19° - 21°c).
If the float is well out of the electrolyte and gives a reading of 1.270 to 1.290 then the battery is fully charged. A half-charged battery will give a reading of 1.190 to 1.210, whereas a flat battery will cause the float to sink lower and give a reading of 1.120 or less. If the battery needs topping-up with distilled water when you come to take a hydrometer reading, run the machine for at least half an hour or leave over-night before checking the electrolyte.
Dial Gauges
Dial gauges are extremely accurate, as easy to read as a clock, and are used to measure wanted or unwanted movement.

Ideally you would use a dial gauge to check if a brake disc had too much run-out. In this instance all you do is clamp the dial to the front forks so it cannot move, set the probe so it touches the disc and the dial indicates, say 5mm, and then rotate the wheel slowly. If the disc is perfectly true (which it won't be) the needle on the dial gauge will stay still. However much it wavers on each side of the 5mm reading indicates therun-Out of the disc, which should be within the tolerance allowed by the manufacturer.
Dial gauges can be adapted to measure practically anything. The reason you don't see them in amateur hands very often is the clamping problem. The gauge on its own isn't enough: you must also have a means of anchoring it securely if it is to give meaningful readings, and different clamps are needed for different applications. In the circumstances it's usually cheaper to go to a specialist and let him measure the disc brake run-out, or the exact moment when the piston gets to top dead centre, than buy a dial gauge and all the bits and pieces.  However using one can be an exciting experience..

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