K800 Crankshaft Rebuild: Converting to Shell Bearings

I’ve been itching to share this one. The K800’s crankshaft has finally been regenerated and converted for new connecting rods with shell bearings. It’s one of those milestones that quietly decides the future of an engine: either you do it right, or you do it twice. Here’s how I approached it, why I made the choices I did, and what the photos don’t immediately say.

First look after removal

Moments after the crank came out of the cases, I set it down on the bench and just took it in. Honest patina, a film of age, and the sharp reminder that every K800 mile passes right through this piece of steel. It’s the moment where hope and reality meet.

In “as-found” glory with the original rods

With the original connecting rods still in place, you get the full picture of the factory concept. I love moments like this—when a part looks tired but complete. It tells you what you’re working with and what you’ll keep, improve, or retire.

Better than it looks: standard-size journals

Cosmetically, it wasn’t winning any prizes. But the pleasant surprise: this crank had never been rebuilt. The journals measured at standard size, which is gold for a restorer—it means there’s material in reserve for future work and the part isn’t on its last life.

Why move on from needle bearings?

The original rods ran needle bearings in cages. Clever for their era, but here the hardened surface had started to flake—a classic sign that the surface layer is giving up. On a crank this delicate, it’s not a hill I want the engine to die on. Converting to shell bearings spreads the load, builds a stable hydrodynamic oil film, and sets the bike up for long, quiet miles.

Custom rods, classic bearings

The new connecting rods were made to order, with big ends sized for shell bearings from the first Fiat 500—the original city car, not the modern reboot. It’s a smart match of dimensions and availability. I love this kind of cross-pollination: keep the heart of the machine, upgrade the wear parts with proven, serviceable components.

Deep clean: opening and flushing the oilways

Before any metal is removed, the crank’s oil channels need attention. I unplugged the galleries, flushed out the hidden grime, and then let the ultrasonic cleaner do its magic. Only when the inside is spotless does the outside work make sense. With that done, the crank was ready for careful grinding.

Precision grinding with minimal removal

All rod journals and the front main journal were ground for the new shells. Here’s the win: the rod journals only needed 0.007 inch of oversize (about 0.18 mm). That’s barely a whisper of material, which keeps the crank healthy and leaves room for future rebuilds. I breathed a little easier when I saw those numbers.

Final touches: oil gallery plugs

With the machining done, the last job is preparing fresh plugs to seal the oilways. Small parts, big consequences—good sealing means stable oil pressure and happy bearings. I catch myself grinning at this crank every time I walk by it. It’s ready for a long, long service life.

Frequently Asked Questions

• Why switch from needle bearings to shell bearings? Shells distribute load over a broader surface and maintain a hydrodynamic oil film, which is kinder to a delicate crank. They’re also easier to service and source in the future.

• Why choose Fiat 500 bearings? Dimensions and availability. The classic 500 shells are a proven, reliable part that match the new rods well—serviceable now and years from now.

• Does minimal grinding matter? Absolutely. Removing only 0.007 in (≈0.18 mm) preserves strength and leaves headroom for future regrinds if the engine ever needs another refresh.

TL;DR – Quick Recap

• Pulled the K800 crank and assessed a complete, un-rebuilt core with standard journals.
• Retired the needle-bearing setup due to flaking; converted to shell bearings.
• Fitted custom rods sized for classic Fiat 500 shells.
• Opened and cleaned the oil galleries; ultrasonic wash for a spotless interior.
• Ground all rod journals and the front main; only 0.007 in removed on the rods.
• Next up: fresh oilway plugs—then years of smooth, reliable running.