79 f150 slight clatter at certain rpms

It has to be the lifter.

If the valve was bent, then it would stick open. The lifter fits between the cam lobe and the pushrod. If the valve or pushrod was bent, they would be loose and you would see the problem. If the lifter wasn't working right, you would see the rocker move less than the desired amount, and depending on intake or exhaust, I suppose you should have driveability problems too. Maybe not. Since only one cylinder would be affected, you may not notice the performance loss.

It is more likely the cam lobe is worn down if the valve only moves half as far since that is what determines valve lift.

Lugnut

Yup, my vote's on the cam too. Lifter may cause a ticking noise, but you won't be able to pick up the loss of travel with your eye.

Careful hand cranking and a dial indicator will tell the OP whether it is the lobe gone, or the lifter. OK, just to be difficult, I'm going with lifter--unusual (in my experience) to have just one lobe worn down that much (but it can happen!)

The OP does not say which engine, but many of those had the

351M engine which was a 335 series like the 351C and the 400. All shared an oiling problem to the rear of the engine which frequently led to premature bearing and cam wear. It is very unsual that only one cam lobe would wear. It is probable that most, if not all of the others, are also worn. When you see one that is obviously not going the distance, it is likely that that lobe was simply the first to wear through the hardened surface of the lobe into the relatively soft cam core where it will wear rapidly. If this has happened, the cam and crank bearings are probably pretty close to being toast because of the wear particles in the system.

Lugnut

In any case, it would seem that the repair activity is going to be very deep and should very closely approach an Engine Rebuild, if not actually demand that much work.

The lifters will not fit through the holes that the pushrods fit through, so the head(s) must be removed. Maybe on a V8, the cam can be exposed by removing a plate under the intake manifold, if this is true then the heads will not have to come off to get to the lifters and expose the cam for inspection. The Inline 6 might have a large plate on the side of the motor to expose the cam, saving the need to pull the head.

But in any event, a worn cam or collapsed lifters is a significant repair. If the cam is worn down, then it would be a very good idea to replace the bearings while the engine is torn apart.

If it is high mileage, that may be the best thing to do. A good used engine may also be available since there were son many of all of them built.

The lifters can be removed by removing the intake manifold. The cam wear can be measured for failure by removing the rocker and measuring P/R travel. The cam is covered from the top side and would only be visible through the lifter bores. The oil pan would have to be removed for a look from below. Again, a lot of trouble on a high miler.

Keep in mind that these engines were also rough on cam bearings which results in greatly reduced oil pressure and flow to the crank. One of the oiling problems was loss of a lifter or excessive cam bearing clearance resulted in dropped oil flow to the rear crank bearings.

Lugnut

Heh, I almost 100 percent guaranty that if there's a flat cam lobe, the lifter probably won't come out thru the top. I just changed out a cam and lifters in a 351 Cleveland which was newly overhauled with about 50 miles on it. Ended up pulling the motor and going thru the whole thing to clean it out. Used to do quite a few SBC flat cams back when I worked at the dealerships.

You are correct. At least 90% of the time, the lifters are not going to be easy to get along with if the lobes are worn much. If that is the case, you just have to pull everything off the top and turn the engine upsidedown and drive the lifters as far back as you can with a ladys foot, remove the cam and then drive the lifters back into the cam bore.

One thing that many engine builders or installers neglect is to setup the engine as nearly as possible to operating specs. Prime the oil system with a drill and drive shaft in the distributor bore before cranking. Have it filled with a good on the low viscosity end of the scale. When it starts, take it above 2K RPM as soone as it has good oil pressure and keep it there for the first 15 to 20 minutes to braek in the cam and lifter surfaces. The reason for this (before someone ask) is that the cam lobes are lubricated by oil throwoff from the crank. New crank bearings will not have enough throwoff with cold oil at low RPM to lube the cam. The life of the cam is pretty much sealed during the first 20 minutes or so of fresh engine operation.

Lugnut

All of that was performed as you state and in addition, a zinc compound cam break-in additive was used. That particlar car has just been more trouble than it's worth. It was bought off of the 'net as a rolling project. Personally, I believe some guy had a yard full of Mustangs and built this one to sell out of a bunch of old, mismatched, worn out crap he had laying around.

I think I may haver owned that sucker a few years back. Traded it for a 6 cyl '82 Capri that kicked around trouble free for 50K miles before I refurbed mechanically, had it painted and gave it to my nephew as his first car. Three months later, he had pretty much destroyed it and back to walking. At lease he learned what happens when you don't care for one.

Lugnut