Also one other thing to check:
The surface of the rockers where they contact the valves. If they have become
concave over the years then when you adjust the rockers you will be getting a
false
reading from your feeler guage to the tune of .002/.003 inches. Enough to cause
the sound your referring to.
Carl G
Barney Gaylord wrote:
> At 11:31 AM 6/24/2001 +0200, DenverD wrote:
> >.... adjusted the valves (cold engine, .015)--imagine my disappointment
> when it sounded just about the same when it was started.
> >
> >my friend theorizes that the noise is coming from a worn cam lobe. he
> said he thought the area just after the valve full open peak has been worn
> down and is allowing the lifter to "slam" down onto the cam, rather than
> "ride" in contact with the cam..
> >
> >so, we've made plans to pull the engine and replace the cam ....
>
> Changing a cam can be a particularly expensive and aggrevating chore,
> especially if you still have the noise afterwards. Do some diagnostic work
> to try to isolate the true problem before getting into expensive surgury.
> But go ahead and check the cam first, because it's fairly easy to do.
>
> You may be planning a lot of work for nothing. When a cam lobe wears down
> it generally wears down the peak of the apex, and maybe a bit more on the
> leading side of the peak. I have never heard of one wearing on the far
> side of the apex without first wearing down the top. You can tell without
> major disassembly if the cam is worn. Just remove the valve cover and
> mount a long travel dial indicator to measure the travel of the valves,
> each in turn. First screw all the valve adjusters down to zero clearance
> (tight) to remove all clearance, then turn the engine through one or two
> turns and observe the reading on the indicator to get the valve travel.
> Repeat for each valve. The lift for each valve should be the same within a
> few thousandths of an inch. If it is, stop worrying about the cam. If you
> find a lift variation of more than .005", then you need a new cam.
>
> There are other possible sources of tapping noises. A worn connecting rod
> bearing can sound about the same. A rod knock will generally be more
> prevalent under light load and accelleration at about 2500 rpm (just blip
> the throttle when running and parked), and may tap only slightly at idle.
> Remove one spark plug wire at a time and run the engine. If the noise
> subsides considerably when one wire is removed, it's a good probability
> that it's a loose rod bearing. Slightly worn rod bearings can be changed
> by removing the oil pan without pulling the engine. Worn bearing journals
> on the crankshaft can only be repaired by removing and disassembling the
> engine to remove the crankshaft for regrinding.
>
> I have also had a freshly rebuilt engine tap in about the same manner,
> tapping consistently at idle, sounding like a loose valve adjuster, and not
> getting noticeably louder with variation of engine speed. This is usually
> cam related, but not necessarily a bad cam. It could just be a tappet
> sticking in its bore and dropping late to tap on the cam. This might be
> cured with a little solvent around the tappets.
>
> The end float of the cam is controlled by the front retaining plate which
> is screwed to the face of the block and is trapped between the front face
> of the front male journal and the back of the cam sprocket. This is a
> steel plate with a soft bearing surface on the back. With much use the
> front cam journal wears slightly into the soft back surface of the plate,
> thinning it out slightly and allowing a little excess end float for the
> cam. Many engine rebuilds are done with reusing the original cam retainer
> plate, with the end result of an audible tapping noise at idle as the cam
> and sprocket oscellate endwise and slap at the front retaining plate. This
> tapping noise is nothing to worry about, just annoying.
>
> Occasionally a cam can warp slightly, more often if the car has been stored
> for a few years or more without running. A cam that is not perfectly
> straight can slap audibly in one of the journal bearings, especially if the
> engine has a lot of miles on it. This tapping noise is generally not
> anything to worry about unless it is particularly LOUD.
>
> Check the end float in the distributor shaft. If the dizzy shaft has
> excessive end float it can oscellate by floating endwise and tap audibly
> when running.
>
> Use a stroboscopic timing light on the crankshaft pulley to observe the
> timing mark when running. If the timing mark appears to jump around more
> than one or two degrees while running it could be caused by end float of
> the dizzy shaft, or by a worn shaft bearing in the distributor, or by a
> loose timing chain, or by end float in the camshaft, any of which can cause
> a tapping noise. The loose timing chain can often be cured by replacing
> the tensioner. The sloppy dizzy can be repaired by replacing the shaft
> bearing or adjusting a shim just above the drive gear.
>
> For an engine with a lot of miles (maybe 150,000+) on the original block
> the cylinder bores can be worn to the extent that the piston will slap
> against the side of the bore causing a rapping noise that sounds a lot like
> a bearing knock. Also don't over look some obvious little external thing,
> like a vibrating radiator shroud or exhaust pipe, or fan blade, or worn
> water pump bearing.
>
> I recently acquired a nearly perfect engine for a very small price.
> Internally it was in almost new condition, and the only obvious problem was
> that the water pump pulley hub was loose on the shaft where it should be a
> tight press fit. This would probably cause a nasty knocking noise and may
> have been the reason this engine was taken out of service (and ultimatley
> sold cheap).
>
> Food for thought, and things to check.
>
> Barney Gaylord
> 1958 MGA with an attitude
> http://www.ntsource.com/~barneymg
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