Accurate Measurement

When a need arises for an oil seal or a bearing the dial caliper or digital caliper can be used for accurate measurement. The digital caliper has become common and inexpensive and will perform all measurements needed for the  advanced mechanic.

First of all every mechanic has his or her personal way of measurement, this means that the touch or feel of a caliper needs to be soft and gentle and not a forceful pushing or pulling when measuring mechanical shafts, or housings. The situation with force involved in measurement will result in multiple different readings of the dial face when measurement gets into the thousandths of an inch. Seals bearings and shafts are precise and just a few thousandths off in either the bore diameter or shaft diameter will get you the incorrect one at the sales counter. The ability to measure accurately needs experience however if your new at the caliper measurement method the following text should allow you to understand what is needed to do that.


Understanding tolerances and metal parts.


When metal is used to make parts the machine making them uses a measurement that is four decimals out. This means that the machine recognizes one ten thousandth of an inch as a cut number. In reality only NASA machinists can work in that tolerance range and normally for regular parts using oil seals and such work in the three decimal tolerance range.


For example three decimals out would mean we are working with a tolerance factor of a thousandth of an inch .001". A human hair is on average three thousandths of an inch in diameter or .003". So for seals that are used in hydraulic cylinders, engines, and other machinery three decimals out is fine.



There are rods and shafts both look the same to a machine shop worker, the machinist will use a lathe to make a rotating shaft. Use this link to view a shaft being made in a machine shop.





Bearings or bushings are made to slide over shafts, the places they are supposed to fit onto are called journals. Shafts rotate normally and require bearings that are lubricated and oil seals keep the lubrication inside the machine part or housing.


To get a better understanding of a caliper measurement and the measurements you can make with a digital caliper use this link to watch a caliper being used.




Oil Seal or Bearing Bore Diameter


An oil seal that is used to seal in oil that is pressurized or simply oil in containment has an outside diameter (O.D.) and fits into a bore machined into a block of metal called a housing with a bore diameter. A bearing outer race has an O.D. also and fits into a bore or hole. This hole is cut to a dimension of a very specific size, this size can be measured accurately by an advanced mechanic with a digital caliper. Below the three video links will demonstrate the process to bore hole in metal. The important fact here is that metal is not tolerant to deviations from a specific size, all bores need to be very accurate or metal parts like bearings and seals will not fit. These tolerances are in the three decimal out range (.001").








There are three U-tube video links above this text, for some reason I could not change colors on the last two, they are in blue and hard to see. The important point remains the tolerances. In the first video of boring with the mill you can see the tolerances are in inches. On most all American made machinery this is the system that is used to manufacture the inch measurement system. The digital caliper will measure inches or Millimeters for now we will concentrate and write on the American inch system.

In picture 1 shows a common oil seal that holds pressured oil inside a case. The outer metal surface is called the outside dimension or O.D. and the side you are looking at is the oil seal side which has a spring around the inner area of the rubber seal lip. The other side of the seal is called the face and does not have a metal spring on the inside lip and most of the time there are numbers etched or stamped into the metal face.


No matter if this is used simply as a dust seal in a hydraulic cylinder or a high pressure oil seal in a hydraulic pump the side that is made to seal is the cupped side with the metal spring around the inner lip or the inside surface relative a sealed oil system.


In picture 2 shows a variety of oil seals know as a kit. All seals are measured the same way outside dimension O.D. inside dimension I.D. and the width or thickness in inches.



Picture 3 shows measuring a bore with the teeth of the caliper, notice they are on the top side of the caliper and small to measure I.D. or inside diameter in inches. These bores are machined to hold the outside diameter or circumference of the oil seal metal housing solid when pressed together. It is the preferred method to accurately measure the size of oil seal you need. Important to understand is that the bore diameter will be better than measuring the seal outside surface. When these seals are removed they distort the O.D. of the seal and it is difficult when guessing however multiple measurements and an average is usually close however for sure the bore in the metal housing that holds the seal is where the measurement should happen with the teeth of the caliper.


Picture 4 shows measuring the shaft O.D. which rotates inside the seal I.D. the measurement here is taken with the jaws of the caliper and the thickness can be taken in the bore with the depth rod at the end of the caliper or at the seal itself with the jaws of the caliper. A preferred method is to measure the depth of the bore in case the seal thickness you need is not available and you can always substitute depending on the depth of the bore.



When ordering or purchasing a replacement oil seal the bore diameter and the shaft diameter are critical and that's how a mechanic has to order them. The thickness can be variable and depending how deep the bore is.  

These are the dimensions for the common oil seal, all seals are measured the same way.


Outside dimension O.D., Inside dimension I.D. and Height meaning the Thickness.


American made seals measured in inches mostly measure in the 1/16th" range for example the bore in the picture 3 above shows vaguely 1.5" on the slide gauge so the seal being ordered would be 1.5" bore and the shaft in picture 4 shows .375" or 3.8" shaft. Bore depth is 1/8" so the seal number usually reflects these three dimensions.

When ordering the seal in picture 1 from a bearing and seal supplier such as MaGuire Bearing Company Kahului 871-8305 the numbers would be 1.5" x .375" x .125" and on the seal face would appear like 1.500 - 0.375 - 0.125. Most all American made products are manufactured using the 1/16" scale of measurement so oddball reading out of that range can be assumed close to the nearest 1/16th" or .0625".



For example if a seal is distorted after removal and the O.D. measures 1.504" that is closest to the 1.5" 16th and so on so shafts come in as raw rods that are turned to size using a lathe as seen in the first U-tube video on this page.

Measurement of Bolts


When a bolt is needed there are several measurements, length, diameter and thread pitch. Using the American system in inches bolts are also made in the 1/16th" range normally speaking. A bolt has the threaded portion and a hex head or head of some type. A bolt measurement is the threaded end and does not include the head. On an American bolt the outside diameter of the threaded portion is the diameter of the bolt, the head is a different measurement.



Using the Jaws ( lower section digital caliper) measure the bolt outside diameter. If a caliper is not available there are other options to measure the American bolt. As seen below in the pictures a wrench can be used for the diameter of both the bolt and the hex head.

Bolt Length & Diameter -        Measurements using Wrenches


When in the field a bolt diameter and length can be measured using just regular open ended wrenches. If a tape measure or ruler is not available as seen in the middle picture the open end wrenches are really close to the size stamped on them. The diameter of a bolt can be measured accurately using an open end wrench on the threaded part. When the 5/16" wrench just fits around the threads on the bolt it is a 5/16" diameter bolt and the other sizes of wrenches can be used to measure most any round shaft or bolt.


Important to know is the length of the bolt when you are buying a new one does not include the length of the head. As seen in the far left picture a bolt length is measured from the head face to the end of the threads. When working with American bolts there is a thread and grove, each thread is counted per inch. A bolt with a diameter of 1/2" will have a specific number of threads per inch (TPI) and a 5/16" diameter bolt will have it's own specific number of threads per inch.


Essentially in American measurement is was always course of fine thread but in reality you should always count the threads per inch of any bolt you may need to replace. For example a wrench fitting over a bolt to be ordered is 3/4" diameter and the TPI is 8, the length is measured @ 5" so when the order for the new bolt goes in you would say it's a 3/4" X 8 TPI x 5". The only additional information needed is the strength of the bolt which can be viewed in the pictures below.

American Bolts have these marks -   Metric Bolts have Numbers


In simple the cheapest American bolt is the one with no marks and I would use them if I was working with wood or a very light application but normally an American bolt with three marks as seen in the middle picture is common and a good quality bolt as far as strength is concerned. The American bolt with five marks is better quality and strength than the bolt with three marks and these are used in heavy equipment applications, general purpose. The American bolt with six marks is top quality and hardness and is generally used inside engines and differentials where a broken bolt could cost a person thousands of dollars in damages.


The picture on the far right is a Metric bolt and those have numbers on them as seen, click on the picture to enlarge so you can read those. These are the same as the marks on the American bolt Hex Heads.


For most applications simply the diameter and course or fine thread is sufficient but that relies on someone very familiar with bolts. The diameter is consistent to the TPI and there are only a couple variations from that in equipment. Most manufacturers do not make their own bolts and nuts so the sizes are very generic and common.

It is necessary to be able to measure accurately when working with metal. A differential of .001" will not allow a bearing to slip freely onto a shaft, a human hair is averaged @ .003" we measured quite a lot of them when practicing for accuracy using a caliper. So split the human hair diameter into three equal parts and if your off by one hair piece or .001" you may have to use a torch. Bottom line is any mechanic wanting to advance has to learn how to use a digital or dial caliper efficiently and accurately. Use the U-Tube link below before you go purchase a cheap caliper. Generally speaking a $20.00 caliper is sufficient for general applications like the oil seals and the bolts.

Choosing the digital caliper


Mechanics Method NOTED:


As an upcoming advanced mechanic I had to adopt a method of consistency to really get the picture on numbers and measurements. How I did it was every time I went over to pick up a wrench I broke every fractional number into 16ths. For example wrenches are stamped in fractions 1/4" -5/16" -3/8" -7/16" -1/2" -9/16" -5/8" and so on so what I did as a young mechanic is to change the in my mind all to the same fractional number which was 16ths or .0625". Every time I needed a wrench I got used to saying 16ths so it ended up consistent and in order in my mind and went like this.


4/16 -5/16 -6/16 -7/16 -8/16 and so on until I got it clear on the exact sizes quickly. This saves time in several situations like getting the correct wrench and understanding decimals easily and quickly.


Once all those sizes became an automatic response a system to memorize decimals in inches which is the half system. A quarter is 25 cents, a quarter inch is .250, a 50 cent coin is 50 cents and one inch divided into two is .500" and so on, I reduced all fractions to .0625" or 1/16". By using this method daily it allows a general mechanic to be able to recite decimal numbers on the fly without looking them up.


When this method is working for a  mechanic he knows half and inch is

500 thousandths of one inch, and that half of that is 1/4" or .250", and that half of that is 1/8" or .125", and 1/16" is .0625", and 1/32" is .03125" and so on. The half system works well so for anyone wanting to quickly recite a decimal equivalent to a fraction the wrench system and the half system needs to become automatic in response instead of looking up a number. 

Numbers helpful for common every day measurements.


1 inch is equal to 25.4 millimeters.


1 millimeter is .03937 inches.



1 liter or 1000cc volume is equal to 61 cubic inches