We installed a Big Bore cylinder kit: size matters...

Direct-fit Big Bore kits for increasing displacement are becoming increasingly popular when it comes to improving the performance of motorcycle engines. The main reasons are ease of installation, the torque/cost-effectiveness ratio, and reliability, as we're not "squeezing" the engine, but rather replacing it with a larger one.
Another advantage is that this modification is not visible externally, since the components are aesthetically identical to the originals.

The Screamin' Eagle Kit
For this occasion we are going to proceed with the assembly of a kit from the high-performance components brand of the Motor Company, specifically the 103” (1700cc) Big Bore Stage II Kit.

This kit consists of pistons and cylinders, livelier camshafts, a high-performance air filter kit capable of handling the increased demands due to the increased displacement, and a reinforced clutch spring to handle the increased torque. We've also added an oil cooler kit to maintain a stable temperature under maximum effort or in high ambient temperatures, and a manifold without a catalytic converter or active exhaust valve to allow for greater exhaust gas flow.

Disassembly
Our test bike is going to be a 2010 Electra Glide, which we'll start by removing the side cover and disconnecting the maxi-fuse to de-energize the system (safety first).
We then proceed to remove the original exhaust and manifold.

If we look at the back of the manifold we can see the active exhaust valve and the catalytic converter.

We continue disassembling the upper part of the engine until we reach the pistons, which we will remove using the specific tool for this task.

We can now compare the original cylinders and pistons with those in the kit, appreciating the difference in size that will translate into much higher engine torque.

With the "Top End" or upper part removed, we continue with the replacement of the camshafts, which we will perform strictly following all the steps and procedures in the workshop manual.

Assembly and adjustment
With the lower part of the engine finished, we continued installing the new pistons and cylinders using a piston ring compressor.

We'll continue with the original cylinder heads, as this kit has been designed to eliminate the need for any modifications to any of the original engine components, making installation simple.

One of the recurring problems with high-performance Twin Cam engines is excess oil escaping as vapor through the engine breathers, which condenses into droplets and exits through the air filter. The Big Bore kit includes new breathers designed by HD to minimize this problem.

Once the cylinder heads have been tightened (remember, following the manual...), we will install the rocker boxes and the new breathers.

The next step is to reinstall the intake body in its location,

followed by the new higher flow air filter system.

At this point we can reinstall the exhaust, starting with the new unrestricted manifold (catalytic converter).

With the exhaust system complete, all that's left to do is open the primary chain cover to replace the clutch spring with the one included in the kit, which is slightly stronger and better suited to the new features and performance.

The oil radiator
Once the Big Bore kit is installed, we'll continue with the radiator assembly, removing the oil filter and installing the provided adapter.

Next, we will place the radiator at the bottom of the chassis and connect the supply and return tubes.

Final operations
As always, before testing the engine, we must reprogram the ECU to provide it with the new parameters and volumetric efficiency data. We'll perform several runs, recording data and updating the tables. While we're at it, we can adjust the electronic throttle response to achieve more immediate response when turning the throttle grip.

As a final touch, we'll add the air filter trim, which warns the unwary that this bike no longer belongs to the herd and has become a predator of the asphalt.

Frank Burguera

#harleymechanics