General Replacement Engine Tips
The most important thing is to compare the main specifications and or features from your existing engine and find the most suitable replacement engine that is as close as possible in specifications and features.
- Brand / Model Number
Engine model numbers can be located in a number of different places depending not he manufacturer and is normally located somewhere on the outer portion of the engine, it will be either stamped, engraved or printed on the engine. For Honda models its usually clearly printed on the from of the engine. All genuine engines have a model and spec on the engine from when they were built. Some times they might be hard to find, but they are definitely there.
As manufactures introduce new models, you might not always find the replacement engine for your application will always be the exact same horsepower as your previous engine. In this case look at the nearest model either smaller or preferably larger in horsepower with the same specs and features as your old model engine. For example if your replacing a 11.5Hp Briggs & Stratton lawnmower engine, and in the latest range from Briggs & Stratton a 12.5hp with the same specs and features is available, then this will be your replacement engine option.
- Crankshaft Position (Vertical or Horizontal)
Vertical shaft engines are most commonly found on lawnmowers, vertical shaft means the crankshaft is pointing towards the ground.
Horizontal shaft engines are most commonly found on equipment such as water pumps, generators, dingo machines etc. Horizontal shaft means the shaft is parallel with the ground
- Number of Cylinders
It is important to replace your old engine with a engine that has the same amount of cylinders. For example a large ride on mower with a v-twin engine fitted will use its engine full torque and power from the engine so the equipment can be used to its full capacity. A single cylinder engine would not be able to produce the same specs as a v-twin engine.
- Cylinder Head Style (Flat Head or Over Head Valve)
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.
- Crankshaft Diameter / Crankshaft Length / Crankshaft Height
For horizontal shaft engines, 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 as this distance will vary depending on engine model and brand. Depending on the application, this can be a critical measurement!
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.
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. 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 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.
- Charging System
(Some manufacturers use a bigger charging system than standard)
Obviously the first thing you must consider is whether your existing engine or application has or requires any type of charging system output. At first thought, you might think that a "recoil starter only" type of engine would have no need for electrical output and while that is largely true, there are some applications that do still have a small charging coil incorporated into the design. One example might be on an engine designed for a go kart application that has headlights and/or tail-lights. In this case, even though it's equipped with only a recoil starter and no battery, there would be a small charging coil needed to drive the lighting system or some other accessory.
Another more common example would be on a commercial walk-behind mower that is of the "recoil only" variety, but needs a small charging coil to provide power to an electric PTO clutch. Speaking of PTO clutches, be aware that these units require a minimal charging system output of at least 9 amps.
The first and foremost concern is that the older "Flat Head" style engines have been mostly out of production for several years now and unfortunately, there are literally hundreds of thousands of these engines (most major brands) that are just now wearing out. This can be crucial situation for a couple of reasons. Some of the newer replacement OHV style engines have the exhaust ports located on the opposite side of the engine from the flat head counterpart, which means:
Your existing OEM supplied exhaust system or muffler will not fit and cannot be re-used and a "bolt on" replacement may not exist.
Even if the new engine comes equipped with a muffler or exhaust system, it may be located in a position that will not work for the application.
The good news is that many of the smaller engines powering rotary lawnmowers, pumps and such, this is often not an issue. On the other hand, if you are repowering a riding mower or commercial product, you would do well to get as much information together as you can.
The second part of the equation has to do with the fact that some of the larger engines are often sold without an exhaust system provided by the engine manufacturer. In these cases you are generally expected to re-use your old system or purchase a new one from the equipment manufacturer. This can be a tricky and difficult situation if you are repowering with a different brand engine or one of a different size range.
The bottom line in regard to exhaust systems and mufflers is that even if you select a replacement engine model that has been specifically recommended by the engine manufacturer, it may be a perfect match for all other factors, but there isn't any specific guarantee that it will be suitable for your particular application. If you are repowering with a different engine brand, be sure to do you homework and be aware that there may be some fabrication required.
As mentioned elsewhere, the best bet is that when possible, select a replacement engine of the same brand, size range and family. Otherwise, do your research, ask questions and/or contact us if you need additional advice.
- Fuel Tank or Fuel Pump
The fuel system requirements for most small engines are pretty simple, which means there is not too much to be concerned about when repowering. If fact, the primary thing you need to know is whether your old system has a fuel pump or not and then match the new one up accordingly. Simple, huh?
Actually, the bigger problem is in cases where you do not have an old engine to start with or if you are building that "special project" from scratch. In those situations, here are the primary things to consider:
Where is the fuel tank mounted in relation to the carburetor?
If the fuel tank is mounted directly to the engine, chances are good that a fuel pump is not required. If the fuel tank is located remotely, but within a foot or so of the engine and is mounted up higher than the engine, chances are good that a fuel pump will not be required.
For all situations where the tank is mounted remotely and has a distance to the engine greater than about 18 inches, a fuel pump will likely be required. For situations where the fuel tank is mounted level with or lower than the carburetor on the engine, a fuel pump will likely be required.
Assuming that you should find an engine that has a pump where none is needed, it's usually a non-factor since you can just bypass (not use) the fuel pump altogether.
Should you find an engine that meets all other criteria with the exception of needing a fuel pump, you may be able to install a low pressure (typically 3 PSI or less and preferably adjustable) electric pump.
When considering switching between different brands of engines, the engine overall dimensions and exhaust location are the two main points to consider first.