Randall Young wrote:
>
> BTW, I agree. Bob's post is entirely on target.
> Don't tear down a perfectly good engine just to put hardened exhaust valve
>seats in.
This has been pretty much the general assessment throughout this
occasional thread over the last couple of years. If there's no sign of
valve damage, there's no point in getting worried. Most engines are due
for a valve job before the bottom end gives up, anyway, so it's
something to be considered at the time a valve job is required.
However, there are a couple of things to be considered at the time of a
valve job. One, which has not been mentioned in this context, is valve
guides. I would generally recommend replacing the cast-iron guides with
silicon-bronze-aluminum guides, and installing valve stem seals. The
guides are not expensive ($3 each, or so), stem seals about a buck
apiece, and the labor to install the guides is quite small. The bronze
guides hold oil well and allow for installation of stem seals. Anyone
running the engine regularly at over 4000 rpm (as I do on longer trips)
will find that oil consumption will go down, noticeably.
I have been making regular bi-weekly 800-mile round trips to Texas
lately, and have found that the leg out, done during the day (at the
daytime speed limit in Texas of 70 mph) is about 4200-4300 rpm in my GT6
with the 3.89:1 rear and no overdrive. Coming back Sunday nights, at 65
mph speed limits, the rpm is generally around 3800-3900. Oil consumption
on the way to Texas at the higher speeds is about 2-1/2 pints. Oil
consumption on the way back is less than 1/2 pint. It's my belief that
over 4000 rpm, the rate of oil pumped up to the top end begins to exceed
the rate of drainage back to the sump, and the oil level in the rocker
cover begins to rise, increasing the pumping rate down the guides.
This is probably necessary with the cast-iron guide (and probably the
reason why the engine was never fitted with stem seals from the
factory). However, it seems to me that with the right guide and stem
seal, oil consumption would go down significantly without harming the
valve stems. Bronze guides have the additional benefit of being capable
of being knurled and reamed to fit at the next valve job if they show
any wear--something which can't be done with the cast-iron guide, since
it is not ductile enough.
It's my contention that the valve seats on engines with ordinary demands
placed on them don't suffer too much erosion or precession under leaded
fuel use because of one principle. When tetraethyl lead is burned, it
forms lead oxide and some free lead particles. A lot of research has
shown that the valve benefits derive from these lead products depositing
on the seats, creating a cushion for the valve face.
In a typical Triumph engine running unleaded fuel, there's a lot of oil
unnecessarily passing the valve guides, and that oil becomes carbon,
which is deposited on the valve heads, faces, and seats. Therefore,
carbon, to some smaller degree, acts as the missing cushion formerly
afforded by lead combustion products.
This may seem like a good thing, but anyone who has disassembled an
engine with worn guides knows why head gasket kits are referred to as
"decoking" kits in England. In fact, decoking is a recommended practice
about halfway between valve jobs, probably because the drop in
volumetric efficiency gets quite significant because of accumulated
carbon on the valves. Moreover, carbon buildup from oil ingestion is a
not a good thing from the standpoint of combustion chamber
efficiency--more hot spots to cause detonation on hard acceleration.
As some have suggested, overheating may reduce valve life on unleaded
fuel. This stands to reason, since overheating will produce higher
temperatures at the valve seat and head, possibly high enough to burn
off accumulated carbon. So--oil passage through the guides and deposited
onto the valve seats is not good insurance against valve damage,
especially if the car runs hot, or hot and hard for any noticeably
period of time.
The real solution, to my mind, is to use the materials available to make
the engine more efficient and survivable. When it does come time for a
valve job, it makes good sense to adopt newer technology--install
good-quality bronze guides, stem seals and at least hardened seats on
the exhausts. The money saved in oil usage alone will pay for a good
part of the next valve job if the car is run often, or hard.
It's worth noting, too, that the cast-iron heads on Triumphs are not
heat-treated to any appreciable degree. Almost by definition, the valve
seats ground directly into the head are not going to be as hard as an
aftermarket seat insert, so, providing good quality valves are used,
time between valve jobs will be longer with inserts. The stock
arrangement may wear reasonably well, but there's nothing wrong with a
valve job which enables the head to go further between valve jobs.
Cheers.
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