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Crankshaft, PTO and Gear Reduction

Although the subject matter is pretty straightforward, in this section we will try to help you understand the "how to's" of locating an engine with the correct crankshaft and/or auxiliary PTO shaft to meet your requirements. We will also give a brief description of gear reduction units in general. This section of the guide will be a fairly lengthy and important, so please stay with us...

Measuring Your Old Crankshaft

While not at all difficult, learning to take the proper measurements from the crankshaft on your old engine will take you a long way towards selecting a suitable replacement engine of any brand or type. The procedures for horizontal and vertical shaft engines differ, so let's take a look at the proper method for each.

Using the diagram, you will notice that there are three measurements that you will need to take for horizontal shaft engines, which are:

  • A.) Diameter
  • B.) Length
  • C.) Crankshaft Height

While the diameter measurement is obvious, be aware that you should take the length measurement from the oil seal out to the end of the crankshaft. Also, don't overlook the importance of the crankshaft height (measurement C) as this distance will vary depending on engine model and brand. Depending on the application, this can be a critical measurement!

Horizontal Shaft

Guide For Horizontal Shaft Measurement

Note - The crankshaft height measurements are not included in the database so be sure to consult our line drawing section and make comparisons with your old engine.

Vertical Shaft

For vertical shaft engines, there are two primary measurements that need to be considered, along with the type and quantity of key-ways required by your application.

  • A.) Diameter
  • B.) Length
  • C.) Key-way (Size & Quantity)

As with the horizontal shaft engines, the diameter is an obvious measurement, so for now, we will focus on length. Notice in the diagram that we show two separate measurements for length. Unlike the horizontal shaft engine, you must measure from the mounting flange of the engine to the end of the crankshaft B instead of from the oil seal as indicated by the red coloring.

Guide For Vertical Shaft Measurement

One easy way of taking the length measurement is to hold a ruler or long flat block against the bottom of the crankshaft and then measure from the flange (bolt hole) down to the top of the block.

In addition to diameter and length, it is also important that you be aware of the size of the key-way (figure C) and number of woodruff keys (if any) required for your application. Woodruff keys are the "half moon" shaped keys that may be required to attach pulleys or other accessories to the crankshaft. While these details are not included in the database, they are generally shown in the line drawings available at our main site.

Number Conversions

As you will notice, depending on the manufacturer and situation, measurements are provided in fractional, decimal and metric formats and depending on the tool you use, you may need to do some conversions. Here's a little math refresher to help in that regard:

Fraction to Decimal

Decimal to Metric

To convert a fraction to a decimal, you need to divide the top (small) number by the bottom or larger number. Example:

To convert a decimal to the metric equivalent, you need to multiply the decimal number by 25.4 rounding the number accordingly Example:

7/8 will convert to 0.875 in decimal format

0.875 converts to 22.225 mm which would typically be rounded down to 22.0 mm

Click here to view our quick reference chart


Auxiliary PTO Shafts

Auxiliary PTO shafts can be found on both horizontal and vertical shaft engine models and are used to drive accessories on various types of equipment.

Image showing an auxiliary PTO shaft

For example, the shaft shown in the image is used on a tiller application for the purpose of driving the reverse portion of the drive system. Depending on the application requirements, these shafts can protrude from the engine case in either a horizontal or vertical arrangement.

You should also be aware that they depending on the application, they will be configured to operate in either clockwise (CW) or counter-clockwise (CCW) rotation and cannot be interchanged. Shaft diameters and rotation info is included in the database where applicable.


Gear Reduction

As the name implies, gear reduction systems are used to reduce the output speed at the PTO while allowing the engine to maintain a sufficient speed that will deliver the power required for the application.

Image showing a common gear reduction unit

For example, an 8 H.P. engine used on a cement mixer would likely require a constant speed of around 3600 RPM to deliver peak power output. Obviously, the mixer itself cannot run at the same speed to be effective, so a gear reduction system is used to reduce the RPM at the PTO shaft. In this case, a 6:1 reduction system would output around 600 RPM (1/6th the engine speed) which would be in the correct range for the application. The internal gearing of the reduction unit also provides a massive amount of torque (power) that would otherwise be unavailable to the application from the engine alone.


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