Introduction: The 'correct' Way to Measure a Cylinder Bore and Measure Cylinder Using Piston Diameter

Hi All,

Purpose of this Instructable is to hopefully teach how to measure a cylinder and piston, the correct place to measure a piston and why you find the largest possible diameter.

From my activities in many on-line motorcycle groups I've seen a number of people posting things are 'worn out' or wrong parts fitted because they can rock or move top of piston in cylinder bore after cylinder head has been removed

The piston and cylinder being measured are from a 1968 Suzuki T305

Suzuki also made a T250 which looked pretty much identical and a year later a T350 so make sure you have correct specifications

Before making any decisions on parts, you will need the stock
specifications, it's hard to judge clearances between parts when you don't know what exactly they should be so get service manual or technical data manual

In this case, the standard piston size is 59.955mm~59.940" but it's also given as an inch size of 2.360"~2.359"

During operation (ie, when engine is running) the top of piston is exposed to full heat of combustion which is normally around 7~800f or higher. Aluminium has a very high expansion when heated so at running temperature the piston top expands to 'fill' the cylinder bore (top of piston is probably averaging over 300f ?) This is a two stroke (two cycle) motor, conduction removes heat from top to underside of piston and fresh charge helps cool it and prevent a 'melt down' (mostly)

Pistons are machined so the largest mass of metal has room to expand and thinner sections (which don't expand as much) are different diameters. The sides of pistons where gudgeon (piston) pin fit, having more material then the thinner sections of skirt are also smaller than the 'nominal' diameter. It may be easier to picture a piston as being barrel shaped top to bottom and kind of 'pear' shaped looking down from top. The reasons are also linked to the way piston is 'pressed' into front or back of cylinder due to the connecting rod angle and direction of rotation of crankshaft

Step 1: Measure the Piston

As previously stated, 'we' are looking for the largest diameter of the piston so in this case it is inverted and measuring point (usually 5~10mm from base) this particular bike uses 26mm from base of piston. Very surprisingly (in view of age and mileage of bike) the actual size of piston is in specification, 2.359 and a few ten-thousandths of an inch ( I no longer have micrometer reading to 1/10,000" but it isn't really needed - most of the time)

Step 2: Gather Material and Find a Flat Surface to Work On........

My wife borrowed fold up workbench to paint house, although you may think she only painted work surface.

You will need a pad and something to write with as your going to make multiple measurements at various points of cylinder. A couple of blocks of wood to hold cylinder off bench are also handy (in my opinion) Personally I prefer to measure cylinders from the top down but some people will invert them and measure from the bottom up (as cylinder is inverted your still going from top to bottom)

The wood is needed because the cylinder spigot protrudes through cylinder and makes it unstable on a 'small base', much easier to use the flat gasket face plus you can measure close to bottom of cylinder without gauge contacting bench (and messing up readings)

Step 3: Some Measuring Equipment and Set Up Bore Gauge

To accurately make measurements you will need some specialty measuring equipment, in this case, 'cheap' micrometer and bore gauge (I've had and used expensive ones, these do the same job)

To set up bore gauge, you need to know size of bore or size of piston. (2.360") In this case I had piston which was still in specification so didn't re-set micrometer plus it will give a direct reading of the actual clearance between cylinder and piston. (actual piston size closer to 2.3595")

The bore gauge has a range of only 0.050" measuring in ten-thousandths of inch. The contact end of gauge is about 2" long so a contact tip is needed to reach 2.360", the tip is for 2.400" bore so gauge will be compressed at least 0.040" (nicely within range)

The pictures show the disassembled head with contact tip and nut then assembled in between micrometer anvils

The top cover of bore gauge box has soft foam lining, from experience I've found the easiest most convenient way to set gauge or 'hold' micrometer is just sit it on the foam.

The flare from flash pretty much obscures the secondary dial (slightly above and to right of '4') It is important as it tells how many full revolutions the primary dial (with long pointer) has made. Picture of gauge set at 'zero' wasn't too difficult to take as I didn't have to worry about sliding out of micrometer anvils (I know, it's 'off by 1/10,000", actual setting is correct.

The majority of video's or tutorials will tell you to use a micrometer stand but I've found it really difficult to keep gauge contact points between micrometer anvils, the contacts are rounded and about 3/32", the micrometer anvils, 1/4" diameter. The hardened/carbide faces don't want to stay in place (it is possible but why make life deliberately difficult?)

Step 4: Taking Measurements.................

The bore gauge doesn't take direct measurements, that is to say, it doesn't tell you the actual bore size, you need a little bit of math later on.

What it does do is compare the size it was set at to the hole size.

We know gauge is set to piston size and will be taking measurements at top of cylinder 'side to side' and 'front to back' getting a reading of the actual piston clearance. Max allowable is around 0.006", after that things start breaking up very quickly

Just so you know where you are, it's common practice to measure as X and Y, the X is side to side, the Y is front to back. Your also measuring from the top to the bottom, ABC, Top Mid Bot, etc (whatever floats your boat)

From the numbers, it's easy to see that the top of cylinder is close to danger zone, the middle is well into catastrophic failure region and the bottom has 'strange' numbers which are actually easily explained.

Going back to piston measurement, the sides of piston are removed for gas to flow through transfer ports (remember, it's a two stroke -picture of side of piston) There isn't anything to wear the cylinder so it's most likely the 0.0024" was the standard clearance when bike was made..................... but.............. the pistons show no wear so they have probably been swapped for new ones and bores may possibly have been honed for new rings will 'bed in' meaning original clearance was less than 0.0024" ???

If anyone wants to do the math, just add the clearances to the piston size of 2.359"(+3 or 4 or 5 ten thousandths of inch, take your pick from micrometer picture - LOL) to get actual bore size

Step 5: And So.............................. Setting Up for the Next Section - Honing Cylinders 0.50mm Oversize

With the numbers in and from many years experience, the motor would have had a catastrophic failure first time it was run hard, piston rattling around would have broken off the bottom part and possible damaged other components

Made a DIY honing tank from old storage tub and scrap I had laying around.