I get asked repeatedly by owners of the old engine type if they can buy our BB kits. and of course they cant. but even the native upgrade is not an easy switch:
Technically, you can do it, but it requires "surgery" rather than just a simple replacement. Here is why it’s a complicated and often discouraged project:
1. Crankcase Bore Size
The 500cc cylinder has a much wider "skirt" (the part that sits inside the engine block). To make it fit, you would have to:
The 350cc and 500cc UCE engines share the same stroke (90mm), but the crankshaft balancing is entirely different.
If you manage to get the cylinder on, the 350cc support systems won't keep up:
Technically, you can do it, but it requires "surgery" rather than just a simple replacement. Here is why it’s a complicated and often discouraged project:
1. Crankcase Bore Size
The 500cc cylinder has a much wider "skirt" (the part that sits inside the engine block). To make it fit, you would have to:
- Dismantle the entire engine.
- Take the crankcases to a machine shop to bore out the mouth of the engine block.
- Relocate the cylinder head studs, as the spacing on the 500cc block is wider than on the 350cc.
The 350cc and 500cc UCE engines share the same stroke (90mm), but the crankshaft balancing is entirely different.
- Vibrations: The 500cc piston is significantly heavier. If you put it on a 350cc crankshaft without rebalancing the crank, the vibrations will be bone-jarring and could eventually lead to main bearing or connecting rod failure.
- Connecting Rod: The small-end of the 350cc connecting rod is narrower than the 500cc version. While you can force a fit with spacers/shims, it creates a weak point in the engine.
If you manage to get the cylinder on, the 350cc support systems won't keep up:
- The Head: You would need the 500cc cylinder head as well, because the 350cc valves are too small to breathe for a 500cc displacement.
- Fueling: If your 350 is a carburetor model, you’ll need a larger carb (like a VM32 or higher). If it's EFI, the 350cc ECU mapping will be completely wrong, leading to a lean mix and a melted piston.