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Big Bore Kits: What’s Really Involved?

Motorcycle Mods December 18, 2016

Big Bore KitIt seems like big bore kits are all the rage lately, and for good reason. A big bore kit is the single easiest way to add power to your bike and the old adage still holds true, “There is no replacement for displacement.” For those who don’t know, a big bore kit replaces, or over bores, your jugs which increases the displacement of the engine. Think of an engine as a big air pump. The more air it can flow, the more power it is going to produce. Following this train of thought you can see why increasing your displacement from, say 883cc on a Harley Iron to 1250cc will give you a significant power boost, but it is not as simple as just installing the kit. In this article I am going to cover the other steps you will need to take to get the most out of your kit.

Breathe Easy

The first thing you will need is an upgraded air intake. While this is not 100% necessary you are going to be artificially limiting your bike without it since the bigger engine is now going to struggle to draw in enough air through a restrictive stock intake. Most stock intakes are going to be an enclosed system similar to what you would find on a car. While this is fine for stock applications it is usually already restrictive for emissions purposes. To get around this you will need to either purchase a system specifically for your bike, such as the Screamin’ Eagle kit for the Iron, or, if you have a carbureted bike especially, you can use pod filters. The advantage of pod filters is that they are fairly cheap and offer plenty of flow. The downside is that they are universal, so some fabrication or rigging may be needed. They also don’t protect from the elements like a stock filter will, so they can cause issues in the rain.

The second thing you will need is a better flowing exhaust. Most riders will want to replace this anyway, but once you start getting into big bore territory it is no longer optional. It doesn’t matter how much your bike can breathe in if it can’t exhale fast enough. Remember, this is a big air pump we are working with and all of the parts need to work together for maximum performance. Stock exhaust systems will generally be restrictive for noise control reasons, but this extra baffling also causes bottlenecks in the system. Unlike air filters, however, there isn’t really a universal exhaust solution unless you are planning to cut and weld it together yourself or have a shop make one for you. The cheapest, and usually best, option is to buy one from one of the big manufacturers.

That’s my Tune

The last thing you will need, other than your kit itself, is a tuner. This only applies to fuel injected bikes, but most newer bikes are fuel injected, so it is worth mentioning. There are several tuners out there for different model bikes. The one I use is the Vance and Hines Fuelpak, but this tuner is only available on a limited number of bikes. Some others are Harley’s own street and pro tuners, but I don’t recommend these as the interface and support are lacking at best. This is one area where the aftermarket definitely outshines the OEMs. There are dozens of tuners out there and it is not possible to cover them all. Be sure to crawl the forums and find the best tuner for your bike.

If you have a carbureted bike then you won’t need a tuner, but you will need a bigger carburetor. Depending on the increase in displacement you may be able to get away with rejetting, but it is usually going to be better to replace the carburetor. If you are doing a big bore upgrade with a carbureted bike be sure to ask the manufacturer what they recommend. If they aren’t willing to help you pick out a carburetor when you are buying a $1000+ kit from them then they don’t deserve your business and you should find another company to work with.

Size Does Matter

OK, we are finally at the meat of the article, what big bore kit should you look for? There are a couple of different options here. First, most kits are exactly that, a kit that comes with everything you need. This means you get new jugs, new pistons, and new pins, everything you need to do the swap all at once without having to ship any parts. If you are looking for a project you can finish in a weekend and a six pack this is the way to go.

On the other hand, having your current jugs bored out can save you a bit of money, but will require you to ship your jugs to a machine shop which could have a turnaround time of over a month depending on their workload. This option is also limited by the sleeve thickness of stock jugs since they can only be bored so far before you risk destroying the jug. As a rule, I would advise against going this route if you can. The cost savings just doesn’t justify the extra time investment and lower displacement in my opinion. On top of that it can also decrease the life of the engine. Since the sleeve is being bored into it can wear out much faster than a stock bike or new jugs and, if the engine needs to be overhauled later the jugs will have to be replaced since they can’t be over bored anymore and wouldn’t be able to be trued up.


As you can see a big bore kit is not a small investment, but it is also going to give you more power for your money than any other upgrade. At the same time, the other upgrades can be done prior to installing the kit, so there is no reason you can’t upgrade your intake and exhaust as the money is available, then get a big bore kit.

How much power can you gain? Let me give you an example, my Iron 883, stock, had about 45 horsepower. It was pretty quick because it is a light bike, but now with all of the upgrades I have closer to 80 horsepower. No, it is not all from the kit, but it definitely provided the lion’s share of that increase.

Lastly, don’t forget to keep your tire pressure up, your motorcycle oil clean, and your chain in top-notch condition. It’s the little things that are often forgotten that are both cheap, and make quite a difference together.

Author Bio: Aaron is one of the writers for www.TheRevver.com, a website dedicated to providing all sorts of motorcycle related information. He has owned more vehicles than most, and loves getting his hands dirty. Aaron is our number one gearhead, and all techy questions get answered by him.

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