Damon
04-03-2004, 11:58 AM
Well, after 20 years of futzing with engines I have jsut killed my first cam at startup. Wiped about half the lobes off the cam- mostly exhaust lobes. And I can't figure out why.... so I'm down to weird stuff.
It's a Comp Cams XE282 solid flat tappet cam, straight off the shelf. I have been through the valvetrain over and over and it's not the problem. Valve spring pressure on the seat and open are all just a smidge less than spec for the cam (I'm talking about only 10lbs- and it's LESS than spec, not more, so I doubt that's what killed the cam). Valve spring installed height is dead-on. I'm not coil-binding the springs at max lift (plenty of room before coil bind). Retainer-to-guide seal clearance is WAY more than cam lift. Pushrods are stock length, they aren't bent or worn and rocker geomentry is dead-on. Rockers are just standard 1.5 aluminum Harland Sharp rollers. Valve lash was corrrectly set. Disassembly reveled no bent parts or signs of contact between anything- in the valvetrain or valve-to-piston. Like I said, the valve train is NOT the problem.
THis is an in-car cam swap. The old cam we pulled out (also a solid flat tappet but of unknown origin) was in the motor and running without problems for 2 years. This is not a "new engine" scenario. We just replaced the cam and lifters with new (Comp cam and Comp lifters- not a frankenstein) and screwed it back together, everything else is the same. Something I've done MANY MANY times.
Proper assembly lube was used on the cam lobes and faces of the lifters, engine fired quickly and was brought up to 2000-2500 RPMs for 10 minutes immediately after startup, oil pressure was good from the instant after startup, oil level in the engine was good (even overfilled it slightly to make sure plenty got thrown onto the cam by the crank). In short, it was broken-in correctly, just like I've done MANY times before. And yet the cam was dead in 20 minutes.
I'm down to weird stuff.
I checked, on a suggestion by a friend, the location of the cam lobes vs. the centers of the lifter bores. Sure enough, the cam lobes are slightly to the rear of the block relative to the lifter bore centerline. Further checking reveled that the "cheap old" top timing chain gear only has about .155" offset vs. a new GMPP timing chain that has .120" offset. That's at least part of the reason the cam lobes are slightly to the rear vs. the lifter bores. BUT COULD THAT HAVE KILLED THE CAM?? SHOULD THE CAM LOBES EVEN BE CENTERED EXACTLY ON THE LIFTERS??
I have also double-verified this slight offset situation by looking at the wear pattern on the worst wiped lifter. It had worn down and stopped spinning so the lobe wore a nice stright path across it's face. That path is definitely slightly offset. Enough you can tell just looking at it by eye.
I'm stumped on this one, guys. Am I on the right path or is there something else I may be missing? I am VERY reluctant to stab in a new cam with all the same parts just to chew it up 20 minutes later.
It's a Comp Cams XE282 solid flat tappet cam, straight off the shelf. I have been through the valvetrain over and over and it's not the problem. Valve spring pressure on the seat and open are all just a smidge less than spec for the cam (I'm talking about only 10lbs- and it's LESS than spec, not more, so I doubt that's what killed the cam). Valve spring installed height is dead-on. I'm not coil-binding the springs at max lift (plenty of room before coil bind). Retainer-to-guide seal clearance is WAY more than cam lift. Pushrods are stock length, they aren't bent or worn and rocker geomentry is dead-on. Rockers are just standard 1.5 aluminum Harland Sharp rollers. Valve lash was corrrectly set. Disassembly reveled no bent parts or signs of contact between anything- in the valvetrain or valve-to-piston. Like I said, the valve train is NOT the problem.
THis is an in-car cam swap. The old cam we pulled out (also a solid flat tappet but of unknown origin) was in the motor and running without problems for 2 years. This is not a "new engine" scenario. We just replaced the cam and lifters with new (Comp cam and Comp lifters- not a frankenstein) and screwed it back together, everything else is the same. Something I've done MANY MANY times.
Proper assembly lube was used on the cam lobes and faces of the lifters, engine fired quickly and was brought up to 2000-2500 RPMs for 10 minutes immediately after startup, oil pressure was good from the instant after startup, oil level in the engine was good (even overfilled it slightly to make sure plenty got thrown onto the cam by the crank). In short, it was broken-in correctly, just like I've done MANY times before. And yet the cam was dead in 20 minutes.
I'm down to weird stuff.
I checked, on a suggestion by a friend, the location of the cam lobes vs. the centers of the lifter bores. Sure enough, the cam lobes are slightly to the rear of the block relative to the lifter bore centerline. Further checking reveled that the "cheap old" top timing chain gear only has about .155" offset vs. a new GMPP timing chain that has .120" offset. That's at least part of the reason the cam lobes are slightly to the rear vs. the lifter bores. BUT COULD THAT HAVE KILLED THE CAM?? SHOULD THE CAM LOBES EVEN BE CENTERED EXACTLY ON THE LIFTERS??
I have also double-verified this slight offset situation by looking at the wear pattern on the worst wiped lifter. It had worn down and stopped spinning so the lobe wore a nice stright path across it's face. That path is definitely slightly offset. Enough you can tell just looking at it by eye.
I'm stumped on this one, guys. Am I on the right path or is there something else I may be missing? I am VERY reluctant to stab in a new cam with all the same parts just to chew it up 20 minutes later.