broken cam key

[John Mason]

Here's a puzzling problem. I have a N/A 264" Stage II odd-fire with all the best pieces. Molodex 3.4 stroke crank, BME coated pistons 15 to 1 comp, MGP aluminum rods, LSM roller cam, Jesel 1.7 rockers, .795 lift, 268/282 duration @.050, Chapman ported Stage II aluminum heads, Hogan sheet metal manifold and a single Holley 780 Carb, all ready to run NHRA SS/DS. All the best pieces, no expense spared. Only problem is it runs for about 10 seconds and shears the cam key!!! It's happened 3 times now. I've checked everything I can think of, measured everything. I had a similar 265" even fire and never a problem.
The only thing I can come up with is odd-fire harmonics, but the engine never has been over 2000 RPM.
Any ideas??

 

[KendallF]

Check the stackup of components on the front of the cam nose..make sure that the retaining bolt has some "crush" on the assembly. If the gears/spacer/etc. aren't as long as the nose of the cam, the bolt won't tighten against this stuff, and the key will be holding all of the load.

 

[turbodave231]

If your key is sloppy in the keyway of the cam or the sprocket that could cause the sprocket to shear the key. Is the cam too tight in the cam bearings?

Dave

 

[EightSecV6]

Check for coil bind or piston to valve......might be ugly!

 

[John Mason]

I appreciate all the input. I found the problem thanks to a call from a tech at Milodon. The gear drive idler plate had elongated the lower bolt holes holding it to the block. Nothing showed up at assembly, but under load the plate would move enough to retard the cam, allow a lobe to hit a crank counterweight and split the key. Replaced the gear drive and everything's fine.