H2-ICBM™ Development Story - Episode 2

H2-ICBM™ Development Story – Episode 2

What the Kawasaki H2 Taught Us About “Bridged Ports”

In the first episode, I talked about why I decided to create a billet cylinder for the Kawasaki H2 Mach IV –
not just to make something rare and flashy, but to give this legendary 2-stroke a real future.

In this second episode, I’d like to go a little deeper inside the engine and talk about one of the long-standing “chronic problems” of Kawasaki triples:

the bridged port (center bridge port).

This is not only a story about the H2 itself,
but also about a basic weakness shared by many high-performance 2-stroke engines –
and how we are trying to address it with ICBM™.

The Hidden Weakness of Kawasaki Triples

When the Center Bridge Disappeared

Kawasaki triples have a very interesting history.

On the very early H1 500,
Kawasaki actually used proper bridged ports in the cylinder design.

But as mass production went on,
that center bridge was dropped because of casting and manufacturing constraints.

To increase power, the intake and exhaust ports were opened up large.
However, there was no bridge where there really should have been one.

What happens when you do that?

• The piston ring and skirt can “fall into” the large port window

• The top and bottom edges of the port wear out much faster than the rest of the bore

• In the worst case, it can lead to a seizure

Behind the romantic image of “fast but wild and fragile” Kawasaki triples,
there is this very real mechanical issue.

This is one of the major reasons why so many triple cylinders are now badly worn, damaged at the port edges,
or already at their limit for safe re-use.

Lessons from Honda Race Cylinders

How to Make a Bridged Port Survive Heat and RPM

One of the reasons we at Inoue Boring (iB) felt we had to tackle this problem
is our long experience with Honda race cylinders.

When Honda was mass-producing RS125 and RS250 cylinders,
iB was involved as a partner supplier.
In that work, we did a lot of machining around the bridge area on 2-stroke cylinders.

A bridged port is not just “put a bridge in the middle and call it done.”

• The bridge area is where heat tends to concentrate the most

• It is also the harshest zone for the piston ring to pass over

If you simply add a bridge with no further thought:

• The bridge expands more than the surrounding area as it heats up

• That expansion makes the bridge “stick out” toward the piston and ring

• The result can be local scuffing and, ultimately, seizure

So, on serious 2-stroke cylinders, another step is essential:

• We remove a tiny amount of material from the bridge in advance

• In other words, we “relieve” the bridge so that even when it expands with heat,
  it does not protrude into the ring’s path

• Then we shape and polish the area so that the ring can pass smoothly across the port

All of this has to be done in microns.
Too much relief, and you lose support or blow-by increases.
Too little, and you risk scuffing.

This “put the bridge in, then carefully relieve it for thermal expansion” know-how
is absolutely critical for making a bridged port survive real-world use.

When I realized that this exact approach could be applied directly to the huge ports on the Kawasaki triples,
I honestly thought:

“OK, this is going to be fun.”

Because it meant we didn’t just have an idea –
we already had the machining experience to back it up.

Designing the H2-ICBM™ Billet Cylinder Around Bridged Ports

Billet Structure, Aluminium Sleeves and NiSiC Plating

Our new billet H2 cylinders were designed from the very beginning
with bridged ports as a core concept.

Using a billet structure gives us several important advantages:

• We can give the bridge area the wall thickness it actually needs

• We can design the surrounding structure to minimize distortion under clamping loads

• We can place material exactly where the gas flow and temperature gradients demand it

Inside that billet cylinder,
we install an ICBM™ aluminium sleeve and then apply NiSiC plating to the bore.

This combination offers:

• Excellent heat transfer from the bore into the cylinder body

• A very hard, wear-resistant running surface for the piston and rings

• Predictable dimensional behavior as temperature changes

On top of that, we apply our “relief” know-how from RS-series race cylinders:

• The bridge is shaped and relieved so that thermal expansion is accounted for

• The edges the ring has to cross are gently blended and polished

• The entire port region is treated as a system: flow, heat and ring motion together

I can’t share every single dimension or process detail –
some things must remain our trade secrets.
But the direction is very simple:

Not “an engine that is fast for one lap at full throttle,”
but “an engine you can live with for 10 or 20 years,
even if the setting isn’t perfect.”

Beyond the Kawasaki H2

Why Bridged Ports Matter to All 2-Stroke ICBM™ Cylinders

You might be thinking:

“Is this only important for the H2?”

In reality, this bridged-port way of thinking applies to almost all high-performance 2-stroke engines.

Any 2-stroke with large intake or exhaust ports runs into the same basic dilemma:

• We all want more power → the ports get bigger

• But as ports get bigger, the mechanical conditions for the ring and piston become more severe

The Kawasaki H1 / H2 triples are simply
the bikes where this problem appeared in one of the most extreme and dramatic forms:

• Very large port windows for power

• No center bridge in places where the ring still needs support

• Many engines now showing damage around the port edges

That’s why, for me, the H2 is not only a “dream bike,” but also:

“the best possible test case, with the worst possible conditions,
for tackling the core problems of 2-stroke engines.”

By taking this extreme case seriously and developing a solution for it,
we can pass the benefits on to other engines.

The “bridged port” techniques we have refined on the H2-ICBM™ billet project
are now being fed back into:

• Other Kawasaki triples

• 2-stroke cylinders that originally had no bridge at all,
  by adding a properly designed bridge and treating it with the same relief know-how

In other words,
the work we do on one very special H2
is helping us build a stronger “backbone” for 2-stroke ICBM™ cylinders in general.

Looking Inside Your 2-Stroke Cylinder

A Question for Triple and 2-Stroke Riders

What kind of gas flow is happening inside your 2-stroke cylinder right now?

When the piston goes up and down,
and the ring crosses each port window at high rpm,
under what conditions are you asking those parts to survive?

With the H2-ICBM™ billet cylinder,
we are trying to shape those conditions in a way that is:

• Faster and cleaner in gas flow

• Kinder to the ring and piston

• More stable over long-term use

In the next episode,
I’d like to talk more about how this billet H2 cylinder and ICBM™
actually change the character of the engine on the road and on the track –
especially in terms of confidence, flexibility and the feeling at the throttle.

(…to be continued.)

FAQ – Bridged Ports, H2-ICBM™ and 2-Stroke Durability