4.0 broken crankshaft keyway | Ford Explorer Forums - Serious Explorations

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4.0 broken crankshaft keyway


New Member
May 5, 2015
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Year, Model & Trim Level
2002 Ford Explorer XLT
Well, my wife's 2002 Explorer (4x4) 4.0 experienced timing issues (stopped running suddenly, in traffic). I pulled the engine to replace the cassettes/tensioners. The front had already been replaced, the rear looks okay. When I turned the crank via the flywheel, only the crank turns. The keyway sheared off at the crank to jackshaft gear. Now I know the cams and chains are still timed together, along with the jackshaft gear, (and even the crank gear. Can I get away by lining up the crank to the gear keyway, pull the jackshaft bolt to ease the crank gear forward (after locking down the cams) and replace the halfmoon key? I am replacing the tensioners in the heads too. Should I do a compression check afterwards to see if the valves were bent? Or is this a non-interference motor?

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It's an interference motor. I'd assume you'd need to find top dead center and properly associate the cams at that point. Then test your compression.

Broken Woodruff key

If the Woodruff key broke off it may have been due to pistons striking the valves or a piece of a broken cassette getting jammed against one of the chains. What surprises me is that you posted the cassettes look OK. I wonder if a connecting rod bent causing the Woodruff key to break. Anyway, if the crankshaft rotated one revolution with the jackshaft not rotating it is almost certain that at least one valve is bent.

Did you remove the front cover to determine that the Woodruff key broke off?

The keyway should be above the axis of the crankshaft and aligned with the axis of the crankshaft and jackshaft when cylinder 1 is at TDC.

The camshaft position sensor "nub" should be above the axis of the camshaft when cylinder 1 is on the compression stroke.

The camshaft timing slots should be below the axis of the camshaft and parallel to the head surface that mates with the valve cover.

You can't just lock the camshafts in position and rotate the crankshaft because the pistons will strike the valves.

Is the harmonic balancer in good condition?

Known facts

As it sits, the crank gear, jackshaft and cam gears all are in the position shown on the front view of the engine. The camshaft position sensor nub is up. Someone replaced the left (front) cassette/guides and the jackshaft tensioner with the upgraded tensioner. The cams/jackshaft sit in the position where the failure occurred. Yes, I pulled the engine and the front cover. I can easily turn the crank using the flex plate, but I have NOT gone more than a few degrees back and forth. HOWEVER, my wife said she tried turning the engine over as did the tow truck driver that picked up the vehicle. There is no readily apparent problem with the crank/rods, no clunking or scraping. Using a flashlight, I checked the right (rear) cassette and it is intact although it is obvious that it was not changed out when the other repairs were done. I have not checked the rear jackshaft gear or attempted to move the valvetrain in any way. I was thinking that whoever did the repairs to it the first time may not have installed the key correctly or may have installed a damaged one. The jackshaft bolt also appears to have not been changed out. The balancer is okay as far as I can tell. Are there known issues with the balancer/crank?

upgraded components

The upgraded primary chain tensioner with 6 leaves instead of 3 and the wide base was incorporated in the 2002 and later models as was the improved front cassette. To my knowledge there have been no improvements to the timing related components after 2002. The newest rear cassette guide assembly still has no metal reinforcement on the traction side.

If someone did previously work on the engine they may have installed the crankshaft sprocket backwards which would also impact the alignment of the harmonic balancer relative to the Woodruff key.


Or the Woodruff key may have been installed upside down leaving only a small section to engage the gears.

Harmonic balancer failure

I suggest installing a new Woodruff key and the crankshaft sprocket and then carefully manually rotating the crankshaft checking for proper camshaft operation and interference. However, it may not be possible to remove the crankshaft gear without loosening the jackshaft front sprocket retaining bolt. If that is done then both camshafts will have to be retimed.

Woodruff key tip broke

Okay, only about 1/16 of an inch was making contact with the crank sprocket. As per your picture, the sprocket recess is towards the front of the motor. I'll line up the cams before removing the jackshaft bolt to turn the gear around. Then I'll replace the jackshaft bolt as it looks to be the original anyway. Thanks for your help, I'll keep you posted on my progress.

crankshaft sprocket not jackshaft sprocket

. . . As per your picture, the sprocket recess is towards the front of the motor. I'll line up the cams before removing the jackshaft bolt to turn the gear around. . .

You'll need some way to prevent the cams from rotating when you release and tighten the jackshaft front sprocket retaining bolt.

The photo is of the sprocket that fits on the crankshaft - not the jackshaft. The recess on the sprocket should be visible when installed on the crankshaft.