Recommended Additional Upgrades
01
Uma 63mm Ceramic Cylinder Block
Get The Best Cylinder Block for your Big Bore upgrade.
03
Uma Laser Iridium Spark Plug
Improve ignition with Uma Racing Laser Iridium Spark Plug.
The UMA Racing Bore Up Connecting Rod (99mm) is a heavy-duty engine component designed for riders pushing the limits of the Yamaha 155cc VVA engine. When you increase displacement with a 63mm or 65mm bore, the stock connecting rod becomes the “weakest link” under the increased combustion pressure and higher RPM limits.
The 99mm length is the standard OEM length, meaning this is a high-strength replacement rather than a “long-rod” conversion, making it perfect for high-compression builds that retain the stock crankshaft stroke.
Key Features
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Forged Chromoly Steel: Unlike the stock rod, which is cast, the UMA rod is forged from high-tensile Chromoly steel. This process aligns the grain structure of the metal, making it significantly more resistant to snapping or “bowing” under high boost or compression.
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Precision Balancing: Each rod is precision-balanced to reduce engine vibration at high RPMs. This is crucial for maintaining the longevity of your crankshaft bearings when revving past 12,000 RPM.
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Hardened Big-End & Small-End: The contact surfaces for the crank pin and piston pin are hardened to resist “ovaling” (where the holes become egg-shaped over time), ensuring your engine timing and compression remain consistent.
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Enhanced Oil Flow: Designed with optimized oiling holes to ensure the big-end bearing stays lubricated even under extreme centrifugal forces.
Technical Specifications
| Feature | UMA Connecting Rod Details |
| Rod Length | 99mm (Center-to-Center) |
| Material | Forged Chromoly Alloy Steel |
| Piston Pin Dia. | 14mm (Standard R15 V3/V4 size) |
| Compatibility | R15 V3/V4/M, MT-15, XSR 155, WR155R |
| Ideal For | 62mm, 63mm, 65mm Bore Up Kits |
Why Upgrade to a Forged Rod?
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Prevent Engine Failure: On high-compression builds (e.g., 12.5:1 or higher), the stock rod can literally snap or bend. If a rod snaps, it usually destroys the cylinder block, piston, and crankcases instantly.
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High RPM Reliability: If you have adjusted your ECU (M5/M9) to increase the RPM limiter, the inertial forces on the rod increase exponentially. The UMA rod is designed to handle these “redline” stresses.
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Turbo/Nitrous Ready: If you are planning more extreme modifications like a turbocharger or nitrous oxide, a forged connecting rod is a mandatory safety requirement.
Installation Note: “The Big-End Bearing”
When installing a new connecting rod, it is highly recommended to replace the Big-End Bearing and the Crank Pin at the same time. Since the crankshaft must be “split” to install this rod, it is the perfect time to ensure your entire bottom end is refreshed.
Expert Guide Note: This installation requires a Hydraulic Press and professional “truing” of the crankshaft. This is not a DIY job; it should be handled by a specialized engine machine shop to ensure the crank is perfectly centered.
FAQs
Why upgrade connecting rod?
To improve engine durability when using big bore kits or higher RPM performance setups.
Is 99MM correct for R15 V3/V4 platform?
UMA official listings show a 99MM option for the Y16ZR / R15-V3 platform.
Can I install this on stock engine?
Yes, during rebuilds, but it’s most beneficial for tuned engines.
