Cam Lobe Wear Cause Question Please

[ahh911]

Hello,

Had top end rebuilt at 85k miles.

Camshaft intake lobes on cyl 4,5,6 had wear on the same side, the surface facing the front of the engine.

Camshaft and rocker arm reworked, new rocker shafts during topend.

Now 12k miles after top end, same wear pattern developing. No wear on exhaust lobes. The only conclusion I can come up with is that the bore for the intakes was slightly off angle to the camshaft bore.

Experts and others, what's the cause? And the origin. Factory or other, workable solution next time around and possible measurements to take to solve this. Street car. Can the rocker arm pads be worked at an angle to compensate? Experienced guys will have seen this and know the answer, please share in general so these cars can go on.

Regards,
Phil

Generic picture from ebay modified.

[Classic 911]

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[Walt Fricke]

Maybe find a tube or rod which is about 0.707" in diameter, or a bit smaller, and longer than the cam carrier casting. Maybe get two, and run both through a bare (assumes engine apart) carrier, and see if the ends are the same distance apart?

Interesting the wear is on the lobes and not the rockers.

[ahh911]

Hi Walt,
Wear on the rocker shaft bushings as well I believe, these three were loose compared to the others, as far as the pad goes, didn't check.

That's a test for sure I'll try next time it's apart. What else might cause these wear patterns to appear across the intake lobes/rockers on the same angles? The original lobes were pitted in the wear area as well.

[targa72e]

worn rocker shaft bushings can cause angular wear. If the clearance is to large between the rockers and shafts the rockers will run at a angle.

john

[ahh911]

Thanks John,

The rocker shafts are new and the bushings/pads have all been replaced/redone. I could not feel play. To be honest, cyl 5I is showing up first in the same area, that was the worst with the original equipment, I just suspect that intake 4 and 6 are not far behind. I was looking for loose/leaking rockers (none), cam lobe pitting (none, but a wear ridge on cyl5) and then found the exhaust headstuds loose, so didn't study the lobe wear too closely, yet. Car is away for winter I suspect, last night was the final drive after a compression test

Phil

[racing97]

Sometimes you can have one lobe go bad do to contamination in the spray tube itself and most of the time it comes from the chain guide rails.

regards