From: Dave Baker
Newsgroups: rec.autos.tech
Date: 17 Sep 2001 14:42:38 GMT
Subject: Re: Wear on cams

>Subject: Re: Wear on cams
>From: epbrown01@att.net  (Emanuel Brown)
>Date: 17/09/01 06:50 GMT Daylight Time
>Message-id: <3bcbaa5b.392083546@netnews.worldnet.att.net>
>
>On Sun, 16 Sep 2001 21:07:20 -0700, "Blue" <wingsofwax_1999@yahoo.com>
>wrote:
>>My valve gaps are set by selecting shims of different stock sizes.  The
>>shims are miked and the gaps above the shims are checked with feeler guages.
>>The gap is set to specification by (usually) selecting a larger shim.  The
>>assumption is that the shim has the major wear, causing a larger gap.  The
>>valve seat can also wear, causing a smaller gap.  My question is the wear on
>>the lobe of the cam.  That one is ignored and difficult to assess. Just
>>wondering if anyone knows how significant it is compared to the other two
>>wear points. If I were to hazard a guess I would say that it was not
>>significant until the engine had well over 200,000 miles on it.  Would
>>appreciate the opinion of a professional mechanic  with experience specifit
>>to the question.

Valve clearances are set with the cam lobes on their base circles and these
don't usually wear at all because they do no work. Even with a hydraulic lifter
engine they merely brush the follower lightly. On a solid lifter engine they
aren't touching the lifter at all when the valve is closed. Wear on the lifting
part of the lobe isn't relevant to this question.

Valve clearances stay in adjustment for very long periods of time on modern OHC
engines with hardened buckets or shims. It's pushrod engines that require the
most frequent adjustment and most of the wear is usually on the rocker tip and
rocker shaft bushings with some on the valve stem tip and maybe the follower
too when these are cast iron.

Valve seats and valve head seat faces generally wear very little unless there
is recession due to unleaded fuel and this only really affects cast iron heads
without hardened inserts.


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