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Engine Style & Design

While there are some very obvious differences in both engine styles and design, this section focuses on how the differences should be considered when repowering with a new engine. Here are a few of the more common options:

Vertical Shaft

Single Cylinder


Flat or "L" Head

Vertical Shaft Engine

Single Cylinder Model

V-Twin Engine Design

L-Head or Flat Head Design

Horizontal Shaft

Twin Cylinder

Opposed Twin


Horizontal Shaft Engine

Twin Cylinder Model

Opposed Twin Engine Design

OHV Design

While it should be apparent that you cannot substitute a horizontal shaft engine in place of a vertical shaft version, here are a few of the "easier" considerations that must be made when trying to change from one engine brand or style to a different one:

Single - vs- Twin Cylinder
Typically, as long as the overall dimensions and other required specifications are comparable, there should not be a problem with substituting one style for the other and the twin cylinder variety will almost always have a lower vibration level than the single. You must consider the exhaust system requirements, since they will always be incompatible.
Opposed Twin - vs- V-Twin

One of the major differences between the opposed twin in comparison to the V-Twin will be in the overall size of each. Regardless of whether the engine is a vertical shaft or horizontal design, often there can be serious differences in overall size and mounting considerations and you must consider the exhaust system requirements between the two since they will not be the least bit compatible. This is probably not a swap that should be considered by the inexperienced unless you have a lot of patience and the capabilities to do some custom fabrication.

"L" or Flat Head - vs- OHV

For all but a very few engine models, the older flat head style engines are now mostly out of production and are getting scarce to come by. As such, most of our focus here is pointed toward the newer OHV engines and issues that you may likely encounter with them.

The OHV models are typically longer (vertical shaft models) or taller (horizontal shaft models) which may present a problem in some cases, but if all other factors are comparable, this is a very "do-able" swap. The OHV version is also usually more efficient and powerful than it's flat head counterpart which is a definite bonus. However, you must again consider the exhaust system requirements, since they will most likely be incompatible between the two engine styles.

Note - The OHV version may have the exhaust system mounted on the opposite side of the engine from the flat head model (B&S models in particular) which can play havoc with any installation.


A Common Theme

By now you may have noticed that when switching between engine styles and/or brands, there are at least two common guidelines (dimensions and exhaust system) that must be considered right up front. For comparison purposes, we try to keep a current list of line drawings, complete with dimensional information at our main site and you will find more details about exhaust system components later on in this guide.


In our next topic, we will start delving into the "nitty-gritty" details of crankshafts, PTO's and gear reduction units. This is where the differences can start to become enormous...


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Crankshaft & PTO



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