[zotoa]

Does anyone know what the cause of a tap-tap noise which increase with speed in my 1972 Dodge Dart slant 6 engine could be?

 

[ImperialCrown]

Welcome to Allpar. The valve clearance is adjustable (.010" intake; .020" exhaust). Get a service manual and new valve cover gasket and start here if the noise is at the top of the engine.
http://www.allpar.com/mopar/valve-adjustment.html

 

[chuzz]

If it's not the valves, check for a cracked exhaust manifold. Some of the older ones were bad about cracking.

 

[Bob Lincoln]

VERY common for a slant-6 to have lifter noise. They are mechanical lifters up to about 1981 or so, and always have some noise. A valve clearance adjustment will reduce this, and it must be done with engine hot and running, so be careful of hot oil drops splashing you, and get the measurement as close as possible to spec.

 

[Volunteer]

If these earlier L.T's.O.P. were set up right, there should be very little discernible sounds from under the valve cover. Agree that a possibility of crack exists - as does leak at the flange itself, but, such an anomaly should produce noise at all engine speeds and loads. Another tidbit about the valve-train is that the (OEM) springs were very 'weak' - ie. low seat pressure with valves closed. With .375" valve lift, the pressure was only rated about 144 lbs. - compared to 318's rating of 177, (2Bbl) 360 of 208, and 340/360 HP at near 240 lbs. ALL OF THESE springs are interchangeable. My own stock springs, with 45 years of use on them were between 125 and 135 lbs. so I installed a matched set of 318 springs. Even with low-revvers, (like most stock sixes), valves not closing 'promptly' is something to be aware of - which can result in burned valves. Now, this leads us into one more top-end noise possible cause and that could even be a leaking intake valve. Although rare, it would produce more of a subdued 'popping' sound in the intake manifold - as load increases. We have no idea how many miles on O.P's engine so all I can suggest is to continue trouble-shooting and eliminate causes, one by one. Oh, and a compression test would also be a good idea. Good luck.

 

[Bob Lincoln]

The slant-6 had flat-bottomed lifters, and as they wore against the cam lobes, they became slightly concave, which results in some of the tapping noise. That's normal wear. These engines were nearly as quiet as those with hydraulic lifters for the first 50k miles or so, but above 100K often had a tapping sound.

 

[Volunteer]

The slant-6 had flat-bottomed lifters, and as they wore against the cam lobes, they became slightly concave, which results in some of the tapping noise. That's normal wear. These engines were nearly as quiet as those with hydraulic lifters for the first 50k miles or so, but above 100K often had a tapping sound.

This would be likely with many other solid-liftered engines and also factor in the lifter to bore clearance and you have potential for 'rocking' or slapping in each bore - which also makes it more difficult to measure and adjust ( hot and running) clearances. My own Signet had virtually no wear on bottoms or sides of lifters so if not for the broken compression rings I wouldn't have had to disturb it.

 

[saltydog]

My Valiant has some tapping noise once warm, but not bad. Ensure that valves are adjusted and that made the biggest difference for me.

My slant has almost 300k, I am very interested in a rebuild, but there is very little oil usage and still has great drive-ability. However I cant wait to see what it looks like inside after all these years.

 

[valiant67]

Your Valiant would have solid lifters which require a periodic lash adjustment - unless the head on that slant 6 is much newer than the rest of the car.

 

[saltydog]

Your Valiant would have solid lifters which require a periodic lash adjustment - unless the head on that slant 6 is much newer than the rest of the car.

Its the original head, so is there a valve and lash adjustments? I know my dad adjusted the valves in April.

 

[Bob Lincoln]

Post #2 tells the specification.

It's vital that the adjustment is done with the engine hot and running (so, ignore those directions in the link, which describe doing it with engine off. It MUST be done while idling). If it was not done this way, it will be off and can result in burned valves. It should be checked every 12 months or 12,0000 miles, according to the book; but in reality can go 50K mile intervals.

 

[68RT]

If you engine has been run for quite a while with the valve settings too loose, you may have created worn spots on the rockers that will not allow correct adjustment by feeler gauges and will require re-facing of the rocker arm tips. it is not that hard to do but requires correct equipment.

 

[A-Body]

All else set aside, tapping on a hot engine could also be the oil breaking(maybe use a higher rating) bearing clearances are wearing and the oil pressure is dropping or even possible the oil pump needs replacing for the same reason.

 

[wheelsport]

Here is a snazzy trick to determine the source of engine noise.

Diagnose Noises with a timing light? Valve train noises occur at half of crankshaft speed so even if your ear can't tell whether the noise is happening at 700 rpm (raps per minute) or only 350 rpm, your eyes can. Hook the timing light to any one cylinder and watch the flash illuminate the timing mark. Stare at it for a while and see if the flash jives with the knock. If it does, then it is more likely to be rocker arms, pushrods, lifters, camshaft, cam bearings, timing chain and gears. If the noise seems twice as fast it is probably in the crank, mains, rods, rod bearings, wristpins and pistons.

 

[Budd Cochran]

Your Valiant would have solid lifters which require a periodic lash adjustment - unless the head on that slant 6 is much newer than the rest of the car.

The heads are interchangeable between solid and hydraulic lifter engines. the difference is in the pushrods (hollow for lubing the lifters from the rocker arms ... yeah, backwards but it works!), the lifters themselves and the camshaft.