Moparts Tech Archive
Motor General
Cam lobe failure Q,
All those posts about wiping lobes make me nervous!!Just wanna ask some common Q´s and Im sure all my doubs will be solved!
1.Its the surface on the lobe thats the protection against wear?
2.Once the wipe out started it will destroy the camshaft in some minutes?
3.You shouldent be able to measure any "normal" wear on a used camshat?
4.Is it possible to tell by the sparkplugs if a lobe is gone?
An exhaust is for sure,, but what about if a intake lobe is gone?
5.Will a hurted lobe end up at the same diam as the basecircle?
Please tell me the true about this frequently happen accident!
Best R Olle B
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some simple answers to the questions to start conversation:1. yes, that and a thin layer of oil. the surface of the cam and lifter are "honed" by each other during break in; once that happens the only other protection is a thin layer of oil
2. no, could be minutes during initial break-in, over over many miles; but the many miles issue is a different failure mode, like a soft cam or a stuck lifter that will not rotate.
3. well, maybe a couple thousandths, but not .050"
4. not necessarily, best way to tell is by watching hte suspect rocker arms, checking for constant need for adjusting, loss of power, rough idle, erratic vacuum signal, etc. Wiped lobe will cause misfires but one that is going may not effect the sparkplug at all.
5. no, but almost.
It just takes time and patience to "help" insure that a cam will break-in and wear properly.
1) The lobes do have some type of coating to help break-in.
Proper cam lube/paste, using racing oil, or I guess diesel oil for break-in also helps.
2) I've never seen a lobe wear out "quickly". But it does wear down over time once it starts and is usually noticable after a short time. Periodically checking the valve adjustment/preload after break-in will catch the problem. Spring pressures and rpm would have an affect on the rate of wear.
3) No, you shouldn't be able to measure normal wear.
4) I'm not sure. A "ticking" or "miss" sound would probably be the best indicator. A bent pushrod will sound similar.
5) I haven't had a lobe wear down anywhere near that much before catching it. But I suppose it's possible.
I always removed inner springs for break in and now use a softer single spring in my high-rate single spring applications. Priming the oil pump with a shaft through the distributer hole helps get the protecting oil circulating faster after start-up.
Worn lifter bores will cause problems as will too much spring pressure after proper break-in, or parts contacting each other that shouldn't! Piston-to-valve, retainer-to-valve guide or coil bind in the springs. This will initally show up as bent pushrods.
As you can see, there are a lot of possibilities besides "soft" cams and lifters. Although that can still be a cause.
In 25+ years, I've wiped 2-3 cams that I remember. But none that were broken-in with the inner springs removed or softer outer springs.
Myron
I think you got some real good info. here and your questions answered. Just a couple things that I have done to prevent these mishaps.
1.) Use the engine assembley moly lube on the lobes of the cam before installation.
2.) After degreeing the cam with the #1 intake lobe, re-lube that lobe as best you can.
3.) With your finger and some motor oil, oil the lifter bores of the block.
4.) put a dab of engine assembley lube on the lifter face ONLY. Dont get any on the sides. Drop the lifters in.
5.) Set the balancer to 10 degrees before TDC. This will establish a good ballpark initial timing position.
6.) Add one bottle of STP motor treatment to your engine oil.
7.) Lastly, and after the upper half is buttoned up, prime the engine and stab the distributor right at #1 cylinder.
8.) Fill the carburetor float bowls with gas and give the dry engine about 5 shots of fuel.
It should fire up instantly. Monitor engine temps. and dont be afraid to do a couple 10 minute break-ins at 2000-2400 rpm's instead of one long one. I like to use a box fan at the front of the car to control under hood temps.
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