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Fixing the sheared key of a crank timing sprocket

posted by  Kuba on  Tue Aug 12 03:43 CST 2003
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Hi, if any of you has sheared the pulley key on the crank timing sprocket (the small one), here's what I did to get it fixed (with some pictures).

0. The key got sheared off by the crank pulley. The pulley started rotating against the sprocket because the crank bolt came loose. The bolt came loose because some #$%@!& mechanic didn't bother torquing it to specs.

1. File the old key away. Make sure to leave a couple of thousands of an inch worth of material still protruding, so that you can easily see the boundaries of the old key.

2. With a caliper, mark the center of the old key. You do it by measuring with and length of the old key, and marking in the half.

3. Use a center punch. Don't miss this step. If you don't have a center punch, get one. It is used to make a guiding "pit" so that the drill won't slip when you start drilling.

4. Drill a 1/16" hole in the spot you've marked. Make it about 1/4" deep, no more.

5. Re-drill the hole to 2/16" and then to 3/16".

6. Get a hammer, super-glue, and a 3/16" drill bit you want to make a tad shorter.

7. With the caliper, measure how deep is your hole (dimension A), how deep is the groove in the crank pulley (B).

8. Add A+B=C, cut off C length off the shank end of the 3/16" drill bit.

7. Put super glue into the hole and on the C-long shank of the 3/16" drill bit, hammer the shank into the hole as far as it will go.

8. Leave the glue to set (15 minutes).

9. File the shank (your new key) with a fine file so that it will fit into the groove in the crank pulley. You need to shave off just a couple of thousandths of an inch (a hundredth of a millimeter or so). Make it a loose interference fit, if you will.

When your key gets sheared, your front timing belt locator plate will be damaged, just get a new one. It costs about $5 at the dealer, IIRC. The front locator plate is a "washer" that goes between the timing belt sprocket and the crank pulley.

I hope the pictures below explain what needs to be done. Unfortunately, I forgot to take the picture of the sheared key before I filed it.





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