Motorcycles Surviving Big Bore Surgery
Motorcyclists often find that as their skill and ability to use their bike increases, the mount often suffers as it is ridden closer and closer to it's limits. Whether it be for pure racing modifications, a rebuild after terminal breakdown, the need to give a commuter longer legs, a more competitive ride on club days or just for the hell of it, the Big Bore kits on the market today are often the simplest and most reliable way to get more grunt out of the bike you currently own.
The thinking behind the big-bore is relatively sound - there is no substitute for cubic inches. After-market turbo kits, nitrous systems, over-sized valves and accessory exhausts all make the current bore-and-stroke configuration work harder and produce more horses. The big bore just has more horses to offer. The other way to raise the cubic capacity is to re-stroke the engine by modifying the flywheel and crank to give the engine more stroke. This can be expensive and is a highly specialized procedure.
Benefits of the Big-Bore kit are numerous - improved power to weight ratio, more torque to lift you out of corners or kick you out of harms way, improved fuel economy due to less need for engine revolutions, and of course, a faster more powerful machine. The absolute joy of getting previously only dreamed about performance out of a faithful old favorite can hardly be described. It's like rubbing the genie's lamp and wishing your bike had extra everything in the reactor plant and getting it - for keeps!
There are, as with every modification you make to an engine, cautions and drawbacks to be aware of when considering the big-bore for your bike. First, you've got to find out if there is a kit available for your bike. Some reputable engineers and mechanics, usually part of the motorcycle sales and racing fraternity, will offer in-house big-bore services, often for smaller mini motocrossers or open-class race bikes sponsored by the shop. Sometimes, the selection of available big-bore kits may be limited by brand affiliation or regional demand. It is recommended that before you put the Paso in for a 194cc improvement procedure, you ask previous customers and other mechanics working on the same brand about the quality of these jobs.
It is safer in the long run to work with companies already established and specializing in big-bore work. Wiseco of the USA and Yoshimura from Japan have both had many years in producing oversize kits for bikes, and many different kits are available. Don't rely Wiseco, however, for assistance with jetting information, fitting recommendations or advice on suitability. They are parts manufacturers, and not motorcycle enthusiasts, their range covering automotive usage as well. Yoshimura are specialist motorcycle parts manufacturers and their after-sales service and advice is excellent. Both companies have excellent big-bore kits, and are known for reliability and performance.
In some cases, your model bike will just not have been considered for big-bore development. Your 1977 CB400N for instance, may not be in the running. In this case, either buying a bike worth big-boring, or getting a specialist engineer to fiddle with the dimensions may be the best bet. As a guide, the following points should help to assess your bikes suitability and a offer relatively pain-free procedure to follow:
1. Do you need a big-bore conversion? Riding the bike to it's capacity, blowing up the top end and needing a re-bore, coming consistently third in Sunday's club-man class, or wanting more performance and less strain on the engine for long commutes may all be factors in deciding this.
2. Find out if there is a kit available for my bike, or is there a specialist in my area doing reputable big-bore work?
3. Assess my bike, note frame strength: Does it flex through corners with the existing power output? Frame strengthening is relatively cheap and ultimately a good investment for higher power outputs.
4. Suspension: This will get an extra workout through corners and braking. Think progressive fork springs, higher rebound and dampening settings and aftermarket springs.
5. Brakes: Most Important! If your brakes are garbage now, they'll be far less than suitable when you've got thirty more horses stuffed under the seat and overcook a corner. Get better ones...NOW! Think bigger master cylinder, bigger callipers, better pad compounds and new lines and cables.
6. Exhaust: Do you already have a bigger diameter aftermarket item or power-pipe? If so, contact the manufacturer to assess its suitability for use with a larger capacity motor. If you have a standard system, it may suit the big-bore by providing extra back pressure. Alternately with a standard system, big-bore conversions may suffer from inadequate exhaust removal, and strangle under load. It is also important to consider the condition of your exhaust system, as higher capacity often means higher compression ratios. This can lead to a sharper sonic "strike" against the system as the "bang" occurs, putting pressure on seals, gaskets and weld seams in the exhaust. A rusty old aftermarket four-into-one system may well self-destruct.
7. Engine: First, does your cylinder head have enough gap and removable metal between the cylinders also allowing for cooling, be it liquid or air-fins? If the bores are too close on a multi-cylinder, or the walls are too thin on a bike already factory designed to be close to it's capacity extremes will be unsuitable for a big-bore conversion. Self destruction through metal fatigue and collapse or overheating through lack of cooling capacity vs. bore size can spell disaster for engines just not designed for oversizing. The engine of choice for big-borers is usually the single-cylinder dirt bike, road single, or multi-cylinder with a bulletproof reputation. A model proved in the past to be successfully oversized as featured in motorcycle media is also backed by a choice of aftermarket parts to suit, experienced engineers familiar with the task and ready answers should difficulties arise. As a rule, the easiest and most reliable big-bore kits to install will be the most available on the market. Obscure kits mean problems.
8. Cylinder head: As the capacity and compression are to increase, it is necessary to assess whether the valves, camshafts, cam profiles and inlet/outlet dimensions are suitable for the big-bore kit. Some engines are just suited to the process, having large valve diameters, strong cranks and conrods, sporty cam lobes and generous exhaust and carburettor ports. Others may need help in breathing in order not to choke them. Ask the mechanic who completes the conversion (you're not doing this yourself, are you?!)
9. Gearbox and Clutch: The Crank, conrod and gearbox must be strong and in good condition or risk serious bending at high revs. Consider higher ratings on main bearings, a tougher conrod, and heavier weight gearbox oil for higher temperature running. Your clutch may need heavier springs or higher friction plates to cope with the extra load. Radical workmanship may require a heavier chain and sprocket arrangement, but this is rare. You will have a far better range of sprocket gearing to play with thanks to the extra horsepower.
10. Injection,Carburettors and Jetting: Probably the more tedious task of the process, setting up the fuel delivery is time consuming and expensive. If your bike is injected, check to see if the system will cope, can be modified to cope, or if it is a complete waste of time to attempt. Carburettors should be of a more popular type, like Mikuni, Kehin, MIC, S&S, Screamin' Eagle, etc. These manufacturers offer jetting ranges to suit varying conditions and states of tune, as well as covering a diverse manufacturer client base with their products. Orphan carburettors made by the bike's manufacturer in small batches, or dedicated carburettors made for certain bikes may be limited in jet replacement ranges. Replacement carburettors may be needed to allow more jetting experimentation, but throwing the bike over a rolling road dyno at the local mechanic's is a good way to diagnose carb problems. Usually, you will have to go up on the main jet, richen the needle and pilot jets and replace the slide-spring with a stiffer model. The balancing hole in the slide may have to be slightly bored out to increase rate of movement. If you can find a suitable jetting kit for your model from Dynojet, your problems will disappear as quickly as you can install it.
11. Peripherals: You may need to increase the breather hole in the fuel tank to allow for extra flow, and look at the fuel lines and petcock bores as well to ensure efficient fuel flow. Upgrading your tires as well as your brakes is a good idea. Your airbox may need some snorkel trimming and restrictor removal, but this may be illegal, and you should get an expert to do it, as to not to make the mixture too lean, and burn your new piston.
The process for completing the big-bore operation is straightforward. The barrel is assessed for strength and cooling and oil journal clearance. The existing cylinder is machined out and a new bore is pressed in. If it is a two-stroke application, the ports are cut in, otherwise the cylinders are honed and the crankcase is machined to accept the bigger cylinder. New sized gaskets are required.
Note that complete engine disassembly, gearbox removal, and the successful re-assemblage of same are required to complete the metamorphosis. DO NOT ATTEMPT THIS YOURSELF, unless you are a competent engineer or motorcycle mechanic. Always choose a qualified mechanic with previous experience in big-bore work. Due to the sometimes fickle nature of big-bore outcomes, it is prudent to have the backing of a workshop which will guarantee its work in relation to this particular job in writing. Although the above-mentioned points will encourage caution in getting the big-bore kit installed in your machine, it is quite achievable and the outcome and outlay is often the best and most economical of performance engine modifications.
