It is important to keep your beloved motorcycle in good condition for a long time. Many of the 2 stroke engine sports models have been designed with large intake and exhaust ports to achieve high power. However, there are many cases where the durability of the engine is sacrificed. Here we report on a specific example of the ICBM® and "pillared intake/exhaust ports" that were used to keep the Yamaha SDR running for many years.
What we got out of this modification was just huge! Of course, not only the SDR, but all 2-stroke models seem to have a lot of potential in the future.
A pillar was added to the center of the large exhaust port. What does it mean?
In the case of a 2-stroke engine, the timing of the intake and exhaust ports can change the feeling of power and engine characteristics. Originally, the Yamaha SDR's cylinders were made from cast iron, but the ICBM® conversion is a method of changing the cylinder to a machined aluminum sleeve with a special plating treatment. This time, a pillar was placed in the center of the intake and exhaust ports (exhaust port in photo) to prevent the piston rings from expanding and the piston head from pivoting. 
Since the Yamaha SDR is equipped with an exhaust device as standard, "pillar compatible processing" around the exhaust device is also essential to add a pillar to the exhaust port. On this SDR, the exhaust device was machined to operate the servo motor with standard open/close timing to control the YPVS valve. Of course, the Yamaha SDR is compatible with the exhaust devices of the TZR250R and NSR250R as well, and we have obtained many results.
"Left before vs. right after" the difference is obvious.
The change from a stock Yamaha cylinder with a cast iron sleeve (left) to an ICBM® special plated cylinder with an aluminum sleeve (right) has resulted in an overwhelming weight reduction and improved thermal conductivity. The numerous vertical scratches around the exhaust ports of the stock cylinder were caused by the piston ring edges and piston skirt during the reciprocating motion of the piston. 
The YPVS exhaust device, which is the hallmark of the Yamaha 2-stroke sport, has been retained, but the added pillar has been recessed. The two-piece exhaust system has been modified with a relocation of the left and right locating pins.
SDR for reassembling cylinders without taking down the engine
This is a 2 stroke engine that can be replaced without removing the engine for ease of maintenance. Since it is a water cooled engine, it is necessary to remove the cooling water and radiator related parts. This cylinder and piston swap was also done with the engine installed.
- Point 1 - Improved wear resistance and heat dissipation can be reliably obtained by using ICBM®, a special plated cylinder with aluminum sleeve.
- Point 2 - The "pillared intake and exhaust ports" greatly reduce the scratching noise of the piston rings and the slap noise of the piston (pivoting strike noise). This effect makes it difficult for abnormal wear to occur on both the cylinder and piston.
- Point 3 - The exhaust port pillar is prone to distortion due to exhaust heat concentration. However, with the special processing to release the pillar, there is no concern about the piston seizing or sticking.
The ICBM® cylinder is the saviour of the 2-stroke engine enthusiast: I/Inouye Boring, C/Cylinder, B/Bore, M/Mezotte. ICBM® technology is becoming synonymous with "non-decreasing cylinders" regardless of engine type.
Among the large 2-stroke models are the Kawasaki Mach 500/H1 and 750/H2, which are known for their legendary performance, and these models in particular boast an overwhelmingly high level of popularity due to their numerous achievements.
The number of orders for the 4-stroke Kawasaki Z2 and Z1 engines is also increasing. Of course, not only these popular models, but any 2-stroke or 4-stroke engine can be ordered with ICBM® cylinders as long as the bore size is within the range of 52-91mm. Here is an actual example of a Yamaha SDR cylinder modified to the "ICBM® with pillars" specification.
It was with this Yamaha SDR that I started to notice mechanical noise in the engine. So we looked for and bought oversize pistons. Even if the engine was finished with normal boring and honing, the noise characteristic of a 2-stroke engine would appear after a relatively short distance. This is a characteristic of the 2-stroke sports engine. If you enjoy riding at full throttle (e.g. on a track), you will notice the noise of piston ring strikes and piston skirt slaps even after very short distances. A high-powered two-stroke engine with 'big intake, big exhaust' port timing means that mechanical noise and durability are difficult to maintain, and the same negatives apply to the 200cc SDR.
ICBM® is an internal-combustion-machining technology developed by Inoue Boling Co., Ltd. to replace cast-iron sleeves with special-plated aluminum sleeves, which do not wear out. Many modern engines, even 4-stroke ones, use special plated cylinders. The reason for this is the overwhelming improvement in wear resistance, which is also the result of aiming for high durability. The "ICBM® with pillar", which was put into practice for the SDR, is a combination of ICBM® and the technology of adding a "pillar" in the middle of the intake and exhaust ports of a 2-stroke cylinder. The piston moves back and forth in the cylinder and passes through the large intake and exhaust ports during its journey. This causes the piston rings and skirt to catch on the edges of the intake and exhaust ports on the cylinder wall. This is the cause of mechanical noise and the main cause of wear to the cylinder and piston. As the piston rings pass through the exhaust port area, the physical tension of the rings causes them to push out towards the larger exhaust port. On the intake port side, the piston pivots, causing the skirt to catch on the edge of the port.
The added pillar solves the problem of mechanical noise. The pillars prevent the piston rings from sticking out and also prevent the piston from swiveling during reciprocating motion. In other words, the intake and exhaust port pillars act as guides for the piston as it moves back and forth. Adding a pillar to the exhaust port, where the hot exhaust gases flow, can easily deform the pillar due to heat distortion. However, Inoue Boling, which offers this internal combustion-machining technology, has a special processing technology that allows such exhaust pillars to escape thermal distortion and is highly regarded by motorcycle manufacturers.
Yamaha SDR with Maniac Details.
The Yamaha SDR, with its wide front and rear aluminum rims, has a very light and nimble design. This model is known for its strong fan base, but at the time of the motorcycle boom, it was not a model that did well in business. It was also a model that was introduced too early.
The Yamaha SDR (with a 200cc liquid-cooled crankcase reed valve engine) was born in 1986 as an ultra-lightweight sprinter. Reflecting the popularity of racing at the time, this machine was also a favorite in one-make races. The ultra-lightweight chassis was highly acclaimed for its excellent handling, and this model had an impact on the motorcycle movement of the 1980s. At a time when cast aluminum wheels were the norm, the combination of spoked wheels and wide aluminum rims drew attention in the custom scene. In the case of Yamaha motorcycles, the spoked SRX series and the spoked SDR drew a lot of attention.
For this SDR, 17-inch front and rear Excell aluminum wide rims were chosen. The front and rear spokes are Dirt Freak Dachi brand stainless steel spokes with black anodized aluminum nipples. The front 2 piston calipers and rear calipers are both SDR stock parts. While the calipers are opposed 2-piston calipers, the spoke tension on the caliper side is all "internal" to prevent interference with the spokes. The rear caliper bracket was fabricated and welded using the stock bracket as the base, and the offset was changed to match the dimensions. As for the front and rear hubs, the same disc rotor was used for the front of the Virago 250, and the rear hub was adapted from the rear disc hub of the early Serow series.
Opposed 2-piston calipers have brake pistons on the spoke wheel side as well, so it is physically very difficult to release the spokes. In this SDR, the use of an offset disc rotor and the "all-internal" design of the spokes on the caliper side has succeeded in releasing the brake pistons.
Rear brake hub is for early series of serow. The early model had drum brakes, but the 2nd model had rear disc brakes. I used the one for that model. The offset of the caliper was adjusted by modifying the caliper support (welded off).
Photography cooperation: Inoue Boring Co.
