From the experiences with our Chryslers, at about 80,000 miles on a 1960-70s vintage B/RB V-8, you can expect to have a burnt valve. Seems like ours were usually on #5 or #7 cylinders.
First, some background information. When the rocker arm pushes down on the valve stem to open the valve, it exerts a sideways motion into the stem as it moves the valve open and then a similar opposite force as the valve closes. The rocker arm wipes across the top of the valve stem instead of just pushing straight down, so that is where the sideways motion comes from. This causes the stem and guide to wear where they contact each other. There is a clearance spec for the "stem to guide clearance" in the service manual. In extreme cases, this clearance can sound like a flat tappet solid lifter camshaft that need adjusting, from what some manuals say.
Now, as the stem wears against the machined valve guide area, the clearance will widen. When the clearance gets wide enough, it will let the valve wobble in the guide as it move up and down during the close-open-close cycle. This wobble and the sideways force exerted from the rocker arm will not let the valve squarely seat in its valve seat area of the cylinder heat. When it doesn't seat squarely for the combustion cycle, a small amount of hot gas can escape past the area it's not fully seated in. With time, this will erode the valve head and the valve seat and result in the "burnt" valve situation.
When the valve starts wobbling, it will also wear the valve stem seal and possibly result in more oil getting down into the guide area than is necessary, which might figure into the mix too, but usually this might not be a factor as it's usually the exhaust valve that has the issues so the extra oil goes out the exhaust pipe.
The fix? There are several from "just enough" to upgraded valve guides and valves.
The "just enough" would be to knurl the valve guide and put a new valve in the head to replace the burnt one. New OEM spec seals are usually in the cylinder head valve grind gasket set. Many times, only the head with the valve problem is worked on. This route probably costs the least.
From there, you can disassemble both heads, get them surfaced on a lathe for trueness and inspect all of the valves, check the springs for correct tension, check for cracks, and other things that you might desire to look at with everything apart. Then, you recondition the valve guides completely by machining them for bronze helicoil guide inserts. From what I've read, a bronze helicoil guide insert and a chrome stem valve make for a great interface with respect to long life.
In this situation, you put the heads on a drill press and tap the guides to install the bronze helicoil, just like a helicoil repair of a normal bolt hole in aluminum. After you thread the insert into the guide and fully seat it into the guide, then you trim the ends. Then, you insert a new valve and check for freeness in the guide.
Once the guide is reconditioned, then the work on the valve seats can progress. It has to be done in this order too--recondition valve guide and then recondition the seat.
There is also a "knock-in guide" variation too. Basically, the existing cast-in guide is machined to accept an interference fit valve guide insert, much like sleeving a cylinder wall. This would be in the most extreme cases, I suspect, but was more common in prior decades of the 1970s.
Using a knock-in guide tends to defeat one purpose of the integral valve guide. The knock-in guide is reputed to form an extra heat barrier between it and the cylinder head casting, which can delay the heat dissipation from the valve stem into the cooling system. Hence, the valve and guide run hotter.
Whether or not you also add hardened valve seat inserts to your heads when the valve guide operations are done is your judgment call.
When at the Buick Centennial Celebration last July, I attended a seminar by a retired Buick V-8 engine engineer. The question of unleaded fuel came up. He noted that when the GM "word" came down to use unleaded fuel, Buick did not immediately go to the induction hardened valve seats as other GM divisions did. He noted that as they were already using nickel steel valves, the hardened seats were not really necessary (according to their research). In a year or so, they did get on the induction hardened valve seat bandwagon, though.
On that note, the induction hardening is only a surface treatment. My machine shop operative advises that after the first valve job, it's ground away. But without it, Chrysler research indicated that normal seats and unleaded fuel, when used in the "max load" trailering activity, would destroy a cylinder head inside of 12,000 miles of such use. I saw that article in an "Automotive Industries" magazine in the early 1970s. But, we don't drive our older vehicles with massive trailers around the proving grounds continually at WOT, typically.
On valve seals, the OEM spec seals are fine. There is a orange silicone version that will work too (from a big block Chevy 454 application, but I think there might be a similar Chrysler supplied seal too). Key thing is to not get the guide too dry, oil wise. Some oil has to get past the seal for the guide to get sufficient lubrication, just not too much. In other words, NO Perfect Circle style "scraper" seals on a street engine for that very reason.
How much money you spend and how is your determination. Finding a competent machine shop can be a task too! Good guides that are the reference point for a good OEM spec valve job with at least OEM spec quality valves is probably the best defense against future burnt valves. Also figure a quality motor oil into the mix too.
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