Choosing a 98 big bore kit is probably one of the most satisfying upgrades you can do if you're riding an older Twin Cam 88. It's that perfect middle ground where you get a massive boost in personality and power without having to tear the entire engine down to the crankcase. If you've been feeling like your 88-inch motor is struggling a bit on the highway or just lacks that "get up and go" when you twist the throttle, bumping it up to a 98 is usually the answer most riders are looking for.
Why the Jump to 98 Makes So Much Sense
For a long time, the standard move was to go from an 88 to a 95. It was the safe, catalog-recommended path. But as machining got better and piston technology evolved, we realized there was a bit more meat left on those cylinder bones. A 98 big bore kit takes things just a step further by using a slightly larger bore size—3.938 inches, to be exact—while still using your stock engine cases.
The reason this matters is simple: displacement is king, but convenience is queen. To go any bigger than 98 inches on an early Twin Cam, you usually have to pull the motor, split the cases, and send them off to a machine shop to be bored out. That's a huge jump in both cost and downtime. The 98 kit is the absolute limit of what you can "bolt on" to that stock bottom end. You get the maximum possible displacement with the least amount of mechanical headaches.
What Actually Comes in the Kit?
When you finally pull the trigger and order a 98 big bore kit, you aren't just getting a couple of heavy boxes. You're getting a transformed top end. Most kits out there are going to include a set of new cylinders (or your old ones bored out and replated), high-quality pistons, rings, wrist pins, and a full top-end gasket set.
Some guys prefer the kits that come with brand-new cylinders, often made from aluminum with a Nikasil coating. These are great because they shed heat way better than the old cast iron stuff. If you're stuck in traffic on a hot July afternoon, you'll be glad you have that extra cooling capacity. Others prefer boring out their stock jugs because it keeps the original look of the bike perfectly intact. Either way, the magic is in the pistons. They're usually forged and designed to bump your compression up just enough to make the bike feel snappy without making it impossible to start on a cold morning.
The Performance Reality: What You'll Feel
Let's be real—nobody spends the weekend covered in oil just for a few extra numbers on a dyno sheet. You do it for the "seat of the pants" feel. When you install a 98 big bore kit, the first thing you'll notice isn't necessarily the top-end speed; it's the torque.
The bike just feels "fuller." When you're cruising at 65 mph and you need to pass a semi-truck, you don't have to downshift twice and pray. You just roll the throttle on, and the bike moves. It's a smooth, linear pull that makes the motorcycle feel lighter than it actually is. You're looking at a jump from maybe 65-70 horsepower up into the 90s, and torque often climbs into the triple digits if you've picked the right supporting parts. It turns a sluggish bagger into a bike that actually wants to play.
Don't Forget the Supporting Cast
Here is the thing a lot of people miss: you can't just throw a 98 big bore kit on an otherwise stock bike and expect it to be a world-beater. Think of the engine as a giant air pump. If you make the pump bigger but keep the tiny straws for breathing, you're bottlenecking your investment.
The Camshaft Factor
If you're doing the cylinders, you really should be doing the cams at the same time. A 98-inch motor loves a cam that can take advantage of that extra volume. Something like an S&S 510 or a Woods 6 is a classic choice. You want a cam that moves the power down into the range where you actually ride, not something designed for a drag strip.
Fuel and Air
Your stock carburetor or EFI system is going to need some love. If you're running the old CV carb, you'll need to re-jet it to make sure you aren't running lean and melting those expensive new pistons. If you're fuel-injected, a proper tuner and a session on a dyno are non-negotiable. More air and more displacement require more fuel. It's a simple equation, but skipping it is the fastest way to ruin a brand-new build.
Exhaust Flow
Those stock mufflers with the heavy baffling? They've gotta go. To get the most out of a 98 big bore kit, you need a high-flow exhaust. A good 2-into-1 system is usually the gold standard for torque, though a nice set of true duals can work too if you prefer that classic look and sound.
Is it a Garage Project or a Shop Job?
Whether you should tackle a 98 big bore kit yourself depends on how much you trust your torque wrench and your patience. If you've changed your own oil, swapped out bars, and maybe done a few other bolt-ons, you can probably handle this. You don't need a degree in mechanical engineering, but you do need a factory service manual and a very clean workspace.
The biggest hurdle for most DIYers is the ring filing and making sure the piston pin clips are seated correctly. If you mess that up, the engine will eat itself in about ten minutes. But, if you take your time, keep everything surgical-room clean, and double-check your specs, it's a very rewarding weekend project. There's nothing quite like the feeling of hitting the starter button on an engine you just built yourself.
Reliability and Longevity
One concern people always have is whether a 98 big bore kit will kill their engine's lifespan. The short answer is: not if it's built right.
The 98-inch setup is actually very reliable because it's not "straining" the motor. You aren't revving it to 7,000 RPM or running nitro. You're just making the engine more efficient at moving air. Because the cylinders are often better quality than stock (especially the aftermarket aluminum ones), the engine actually runs cooler in many cases. The main thing is to make sure your oiling system is up to par. A lot of guys will swap in a high-flow oil pump while they have the cam chest open, just for peace of mind.
Final Thoughts on the 98 Upgrade
At the end of the day, the 98 big bore kit is about making your bike what it should have been from the factory. It fills in the gaps in the powerband and gives the Twin Cam 88 the lungs it always needed. It's an investment, sure, but compared to the cost of buying a brand-new bike, it's a bargain.
You get to keep the bike you already love, the one that fits you perfectly, but you give it a whole new lease on life. It's the difference between a bike that just gets you there and a bike that makes you want to take the long way home every single time. If you're on the fence, just look at the torque curves—the numbers don't lie, and the smile on your face when you first crack that throttle won't either.