Crankshaft Flange, Flywheel/Flexplate, Pilot Bearing, and Ring Gear Inspection

The short version — the crank flange and flywheel/flexplate faces must be clean and flat with no cracks or corrosion, the ring gear must have no damaged teeth, and any pilot bearing/bushing gets inspected (and replaced if worn) before the transmission goes back on.

Why this connection matters

Everything here exists to move torque in two directions. The crankshaft rear flange, flywheel/flexplate, and pilot bearing/bushing form the connection that transfers engine torque to the clutch or torque converter, and the ring gear on that same flywheel/flexplate is what the starter drives to crank the engine. So you're really inspecting one assembly that does two jobs: send power out to the transmission, and receive power in from the starter.

The flywheel is used with a manual transmission, and the flexplate is used with an automatic. Both bolt directly to the crankshaft rear flange and must sit flush against it — no gap, no rock, no debris sandwiched in between. This flush fit is not cosmetic. Any burr, corrosion, or trapped debris between the two mating faces tilts the flywheel/flexplate slightly, and that tilt shows up as runout and vibration once the engine is running and the clutch or converter is loaded.

Inspecting the mating faces and ring gear

Before bolting anything up, check both the crank flange face and the flywheel/flexplate face visually and with a straightedge and feeler gauge — you're looking for flatness, scoring, bluing (a sign of hot spots from slipping or overheating), and cracks. Pay particular attention to cracks radiating outward from the bolt holes, since that's where stress concentrates every time the assembly is torqued down and put to work.

Separately, walk the ring gear teeth all the way around. You're checking for chipping, pitting, or missing teeth. A damaged ring gear doesn't always announce itself clearly — it can cause a grinding noise when the starter engages, or worse, an intermittent no-crank/no-start condition if the starter pinion happens to land on the damaged spot. If a customer describes a starting problem that comes and goes, the ring gear should be on your inspection list, not just the starter and battery.

The pilot bearing/bushing — and when it's even there

The pilot bearing or bushing, when the vehicle has one, presses into the center of the crankshaft flange and supports the front tip of the transmission input shaft. It's found on manual-transmission applications, and on some automatics too — but it is not universal, so don't assume every engine has one. If it's worn, chattery, or dry, replace it while the transmission is already out; you don't want a comeback for a two-dollar part because access is hard once everything's bolted back together.

Dual-mass flywheel — what's different about it

A dual-mass flywheel (DMF) is not a solid disc like a conventional flywheel. It has a primary mass bolted to the crank and a secondary mass that's isolated from the primary by an internal spring/damper system, which allows a limited amount of relative rotation between the two halves. That's the whole point of the design: it absorbs torsional vibration from the engine before it reaches the transmission, smoothing out the power pulses instead of letting them slam into the gearbox. When you're inspecting a DMF, keep in mind it's still the same flywheel assembly as far as flange fit, runout, and ring gear inspection go — the spring/damper action is an added system on top of those same basic checks, not a replacement for them.

Easy to mix up

• Flywheel vs. flexplate — flywheel goes with manual transmissions, flexplate goes with automatics. Both mount to the same crank flange and both get inspected the same way for flatness and cracks.
• Pilot bearing vs. pilot bushing — same job (supporting the input shaft tip), different design; neither is guaranteed to be present on a given engine.
• Runout caused by the flywheel itself vs. runout caused by debris/corrosion at the mating face — a straightedge/feeler gauge check on both faces before assembly is what separates the two; don't assume a runout problem is always in the flywheel casting.
• Grinding noise from a bad starter vs. from ring gear tooth damage — both can sound similar during cranking, but only a visual inspection of the ring gear teeth confirms which one it is.

Check yourself

Question: A customer complains of an intermittent no-crank condition — sometimes it starts fine, sometimes the starter just doesn't engage. What flywheel-related component should you inspect, and why does the problem come and go?

Inspect the ring gear teeth for chipping, pitting, or missing teeth. Since the starter pinion engages at a different spot on the ring gear each time depending on where the engine stops, the problem only shows up when the pinion happens to land on the damaged section — which is why it's intermittent.

Question: Technician A says the flywheel/flexplate must sit flush against the crank flange, and that any debris or corrosion between the two surfaces can cause runout and vibration. Technician B says every engine has a pilot bearing or bushing pressed into the crank flange. Who is right?

Technician A is right. Technician B is wrong — the pilot bearing/bushing is used on manual-transmission (and some automatic) applications, but it is not present on every engine.

Question: What makes a dual-mass flywheel different from a conventional flywheel, and what is that difference designed to do?

A dual-mass flywheel has a primary mass bolted to the crank and a secondary mass isolated by an internal spring/damper system, allowing limited relative rotation between the two masses. This design absorbs torsional vibration from the engine before it reaches the transmission.