The Hard Truth About Engine Run-In: Load It or Lose It.
You’ve stood over the bench. You’ve torqued the final bolt. Your fresh engine is in the frame. Now, you’ve got one critical job left before the open road calls: The Run-In.
This isn't just about putting miles on the clock. It's the final, non-negotiable step that determines whether your engine lives up to its potential or just bogs down as a slow-breathing disappointment.
That window to get the job done is small. If you waste the first few hours with timid, soft riding, you glaze the bores. That kills your seal, kills your compression, and robs your bike of power forever. Don't waste the build.
The technique is simple, but it demands attention to detail. There are a lot of opinions about how to do this, and this is ours. Here's the Purpose Built Moto Blueprint to running your engine in right.
Rule One: Load to Live, Don’t Lug to Kill
Every engine needs load to properly seat the piston rings. That’s what creates the perfect, gas-tight seal against the cylinder walls. Without that pressure, the rings never fully set.
Your mantra for the first few hours is simple: Constantly vary the load, vary the RPM, and avoid sustained, steady speeds.
You need the pressure of acceleration to push the rings out, and the vacuum of strong engine braking to pull oil out of the bore. That combination makes the seal quick and effective.
Never lug the motor. Lugging (low RPM, high gear load) creates destructive heat spikes. That’s worse than running near the redline. Ride it smart.
Mastering the Run-In Process
We break the ride down into three stages: initial heat cycles, aggressive seating, and final bedding. The idea is the same, but how you handle the heat changes drastically depending on your engine.
Phase 1: Thermal Conditioning and Oil Choice
The initial goal is to settle your new gaskets and introduce the parts to heat without warping anything. Oil choice matters here.
- Flat-Tappet V-Twins (Shovelheads, Panheads, Pre-’84): These are fragile. Their flat cams need high ZDDP (Zinc) to prevent failure. You must use conventional mineral break-in oil. Modern, low-zinc synthetics will murder these cams. The Heat-Cycle Ritual: Run for one minute at a fast idle (1,200–1,500 RPM), shut down, and let it cool completely. Repeat this 3-4 times, allowing the gaskets to settle before you put serious heat into the thing.
- Air/Oil Cooled Classics (CB750, GS1000): These aluminum heads hate sitting still without airflow. Use a quality conventional 10W-40 or 20W-50. Cycles are shorter and quicker—get it warm, shut it down, cool, and repeat 3-4 times.
- Modern Water-Cooled: These are tough. They don't need cautious cycling. After you bleed the coolant and check the fan, a single thorough warm-up to operating temp is usually all you need before hitting the road.
Final Check: After the last cool-down, check for any leaks. If you touched the head, always re-torque the cylinder head fasteners. They need to be seated to spec.
Phase 2: Aggressive Seating (0–300 km)
This is the window where you either win or lose compression.
The Technique: Get on the road immediately. Accelerate firmly (60–75% throttle), keeping it below 50–60% of your redline. Immediately chop the throttle and use hard engine braking to slow down. Repeat this pressure-vacuum cycle constantly. No steady speed.
- V-Twins: Keep the RPMs low (below 3,500) but increase the throttle percentage quickly. Don’t hold high gear.
- Air/Oil Cooled: You need airflow. Keep speeds varied but brisk. Because these engines run hotter, keep the individual high-throttle pulls short (3-5 seconds max) to manage heat spikes, then back off immediately for the engine braking/cooling cycle.
- Modern Bikes: You can push the acceleration harder and faster; the liquid cooling will handle the heat.
The First Flush: At 300km, pull the drain plug. Get the oil out + Swap the filter. It may be full of assembly lube and the abrasive particles that just created your seal. Refill with fresh conventional oil. Skipping this step means that trash stays in the engine. Check the oil filter for any large particles or metal flake
Phase 3: Final Bedding and Progression (300–1,000 km)
- 300–800 km: Keep varying the riding, but now you can gradually increase your maximum RPM and throttle use. Introduce a few brief, full-throttle bursts (3–5 seconds max). This ensures the ring is seating across the entire bore.
- 800–1,000 km: The hardest work is done. Just maintain smooth, varied riding to let everything stabilize.
The Final Service (1,000 km): This is the celebration. Drain the oil and replace the filter. You should see minimal particulate. Now, and only now, should you switch to the full synthetic oil your engine will run on for life.
The Reward is more power, for longer.
The run-in isn’t a chore; it’s the final act of craftsmanship. It requires discipline and the confidence to apply the load necessary to create the perfect seal.
You respected the thermal needs of the engine, and you used the pressure-vacuum cycle like a tool. The engine is tight, the compression is perfect, and the final result is that feeling of knowing you've finished the job right.
It’s not just rebuilt—it’s reborn. Go ride it hard.
WE BUILD BIKES WITH PURPOSE.
Purpose Built Moto is home for unique custom motorcycle builds. We offer a unique motorcycle customizing experience in the heart of the Gold Coast.
