Swapping Motorcycle Engines? Consider These Essential Elements

Two english women on Excelsior motorcycles in England, 1928
Two english women on Excelsior motorcycles in England, 1928.

 Apic/RETIRED /  Getty Images

Over the years, some of the greatest bikes have been made, or at least assembled, by private individuals. Probably the best example is the Triton. The exceptional handling qualities of the Norton Featherbed, equipped with a Triumph Bonneville engine and gearbox, made one of the best café racers of all time.

But engine changing, or swapping, has not been limited to café racers. Many motorcycle owners have created their own versions of the ideal motorcycle by replacing the stock power unit—some out of necessity, some by choice. Occasionally a manufacturer will use the same frame for two different engine capacities. A good example of which is the Triumph Tiger 90 and Tiger 100 range as, for the most part, these two models were identical except for their engines.

During the 60s, it was common to see owners attempt to be different by using a different manufacturer’s engine in their frame. However, although it sounds simple to do, fitting an engine into another manufacture’s frame is not easy and there are many safety implications to consider first. For example, fitting an engine with a larger capacity, and therefore typically with more power, may result in a motorcycle with inadequate brakes.

The following list represents the essential elements to consider and research before fitting a different engine. Although the list is not exhaustive, it will give the potential motorcycle builder directions to research before committing.

  • Physical sizes
  • Engine mounting locations
  • Chain alignment
  • Chain and sprocket sizes (gearing)
  • Instrumentation and drive ratios
  • Cables
  • Electrical system
  • Exhaust pipe routing
  • Vibration frequencies
  • Legal and insurance implications
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Physical sizes

The first limitation, when successfully fitting another engine to a frame, is the physical size. Needless to say, if the engine is considerably bigger than the original, there may be interference issues such as the header pipes may hit a down tube, or the rocker box may rub against a top frame rail.

In extreme cases, a mechanic may decide that modifying a frame by welding in different tubes (for example) is worth the effort to get an engine to fit with sufficient clearances.

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Engine mounting locations

If the new engine has a similar mounting configuration as the old one, such as plates from a down tube to the front of the engine, it may simply be a case of making new plates with holes in the appropriate place. However, major problems will be encountered where the original engine/gearbox assembly was mounted in a stressed configuration, or if the original engine was hung on mounting from a top rail and these types of mounts will not be used in the new frame. Although possible, this type of engine fitting will require the input of a qualified engineer who will almost certainly say it is not worth the expense and trouble. Note: See also vibration frequencies below.

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Chain alignment Chain alignment Chain alignment

Another element of engine changing that can cause major problems is the position of the final drive chain. Besides the obvious problem of the final drive being on the opposite side on some bikes, the sprockets may not line up even though the engine is mounted on the centerline of the frame/wheels.

Occasionally it is possible to machine or shim the sprockets to get the required alignment. However, this again requires the input of a qualified engineer for obvious reasons.

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It is very unlikely that the gearing on two motorcycles of different engine capacities will have the same gearing. Therefore, the mechanic must calculate the gearing he or she will require when changing engines.

In addition, the final drive chain/sprockets may be of a different size/pitch. If this happens the rear sprocket must be changed to match the front (it is far easier to change the rear sprocket than the front).

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Instrumentation and drive ratios

If the speedometer drive is taken from either the front or rear wheels, changing the engine will not make any difference to the accuracy of the meter. However, if the drive is from the engine the ratios must be checked. Alternatively, an electronic unit could be fitted that takes pulses from an HT lead.

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Control cables must be routed properly. When changing engines the mechanic must ensure the cables will not get damaged in use from heat (exhausts) or caught in steering stops, etc.

Needless to say, the mechanic must check that the handlebars will turn from side to side without adversely affecting the throttle position (generally caused by a short throttle cable).

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Electrical system

Unless the engine and frame are from the same manufacturer and from a similar model, the chances of the electrical system being compatible are slim. However, the older bikes had relatively simple electrical systems and rewiring should not be a problem to a knowledgeable mechanic.

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Exhaust pipe routing

If the engine change is a simple twin cylinder for a twin cylinder of a different capacity, the exhaust system for the engine must be used and should offer few problems. However, if a multi-cylinder engine is replacing a twin or a single, the exhaust system can present all sorts of problems, especially issues of clearance and heat transfer. Again, this is a consideration the mechanic must allow for when researching the possibility of changing engines.

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Vibration frequencies

It is often a surprise, and not a good one, to find that having changed an engine the bike is very uncomfortable to ride due to vibrations. Throughout the history of twin-cylinder motorcycles, for example, the vibration was a problem theme running throughout the years of production. As the Triumph or Norton twins got bigger, so too did the problems associated with vibrations. (Anyone who has experienced carpal tunnel problems through riding will know those vibration problems can result in the need to stop riding altogether.)

In light of this known problem, the mechanic should try wherever possible to use the same type of engine mountings as the original motorcycle of the donor engine.

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Legal and insurance implications

In many countries it is not legal to change the engine in a motorcycle for one of a different capacity--generally, this relates to a maximum capacity limit. However, older bikes may be exempt from any such laws. But again, the mechanic must conduct research before starting a project like this.

The same consideration and research must be given to getting insurance for the finished bike. As all riders are aware, most insurance applications have a question relating to modifications to the motorcycle. The insurance companies ask this as they must know what they are letting themselves in for! (Finding out that your insurance is invalid after an accident is an expensive mistake.)