On Thu, 16 Oct 1997, ROBERT G. HOWARD wrote:
> A few months ago, Chip Old posted some wisdom on this list about those
> shims under the rocker posts. He said, essentially, to remove them since
> they were put in on the recommendation of the factory, but the factory
> was wrong.
> [snip]
Here's the original, typos and all. It stemmed from a discussion about
problems some were having with bronze valve guides. Rocker geometry was
sort of a side issue.
Date: Sun, 20 Jul 1997 12:40:52 -0400 (EDT)
From: Chip Old <fold@mail.bcpl.lib.md.us>
To: mgs@autox.team.net, british-cars-pre-war@autox.team.net
Subject: Re: TF mystery
On Sat, 19 Jul 1997, Leckstein wrote:
> At 07:36 PM 7/19/97 -0400, Chip Old wrote:
> >On Sat, 19 Jul 1997, Leckstein wrote:
> >
> >> As to the seats, he feels that the modern engines with castings made for
> >> the new seats are better prepared for them , taking a 50 year old head and
> >> installing them, no matter how good the machine shop , will always be a
> risk.
> >
> >Mike, that's nonsense. The installation of hardened seat inserts is a
> >repair technique that dates from way before our T-Types were built. The
> >age of the head has nothing to do with it. Some heads are tricky (or even
> >impossible) to install inserts in due to the way their valve pockets are
> >designed, but that's not an issue with the XPAG/XPEG head. If an insert
> >falls out of an XPAG/XPEG head in a car that gets the kind of use you
> >described for yours, then it wasn't installed correctly.
>
> Chip, I was repeating what I was told.
Yes, that was clear from your post.
> Its too late for me, but what is your opinion on Bronze guides and the
> whole matter of danger to the valve train using unleaded?
Bronze guides and precautions against unleaded fuel are two different
issues, so lety's look at them sperately.
Rapid valve guide wear has always been a problem in the XPAG/XPEG series
of engines. Lack of lubricant isn't the problem. There are no valve stem
seals in the modern sense of the word, so more than enough oil works its
way down into the guides. Lack of proper retention of lubrican, however,
is a problem. The real problem is incorrect geometry between the rocker
arm and the valve stem. This puts a lot more sideways thrust on the valve
stems than there should be, which accelerates wear on the guide bores and
on the valve stems.
Ideally when the valve is at half lift, the rocker arm and the valve stem
should form a 90 degree angle. That is seldom the case in any production
line engine, but even with all new parts the geometry on an XPAG/XPEG is
almost always way out of whack. This has to be corrected by either
machining the bottoms of the rocker shaft stands or by inserting shims
under them, depending on which direction the geometry is off. On an old
head that has slightly sunken valve seats due to many rebuilds, the latter
is more common.
On a side note, the practice once advocated of shimming up the rocker
shaft stands to compensate for a radically milled head is dead wrong.
This does give you back a reasonable range of valve lash adjustment, but
it throws the rocker arm geometry way off. The better approach is to
shorten the pushrods.
Getting the geometry right does improve valve guide life, but it's still
far from ideal. The friction surface between rocker tip and valve stem
top is pretty small, and the rocker tips aren't very hard (especially if
they have been reground to compensate for wear). Once the rocker tip
wears, it imposes more sideways thrust on the valve stem even if the
geometry is right. What we really need is thicker valve stems or a thrust
cap to install on top of the stem. Even better, how about roller-tipped
rockers!
The rocker bushings and rocker shaft also wear rapidly, and the resulting
slop imposes even more sideways thrust on the valve stems. Hard chroming
the shaft helps somewhat in this case, but you still need to replace the
bushings more often than on most other engines.
Phosphor bronze guides help to reduce wear because they retain lubricant
better, and because the material has a lower coeficient of friction. They
do expand more when they heat up, so the cold clearance between guide bore
and valve stem has to be slightly greater (as you discovered), otherwise
when it gets good and hot the guide will clamp onto the valve stem. When
this happens, the moving valve tries to take the guide along with it.
Rapid valve seat deterioration due to use of unleade4d fuel is a different
issue. Even with leaded fuel, XPAG/XPEG valves and seats go bad pretty
quickly. The 30 degree seat angle is partly to blame. It doesn't provide
as good a "wedging" seal as a 45 degree seat, so sealing is not as good as
it could be and there is more chance of hot exhaust gas leaking past the
seats. This causes the seats to erode more quickly. Recutting the seats
to 45 degrees helps considerably.
The materials used also have a great deal to do with it. All cast iron is
not alike, and I suspect that the alloy used for XPAG/XPAG heads is a bit
soft. The original MOWOG valves were of fairly high quality, but some of
the replacements sold over the years have been pretty shoddy.
Once the valve seats recede too far below the surface of the combustion
chamber (either from too many regrinds or from long-neglected wear), the
only choice is to install inserts. These days it is also common practice
for older engines to install inserts even if the seats are not receded, as
a preventive measure with unleaded fuel. The installation of inserts is a
common machine shop job, but like any other machinework its success
depends on how much skill the machinist has and how carefully he does the
job. The valve seats are bored out to a slightly smaller diameter than
the outside diameter of the inserts, giving an interference fit on the
inserts. The bore is made very slightly deeper than the depts of the
insert. The inserts are then pressed into place. If the interference fit
is too great, the inserts or the head may crack. If the fit is too loose,
the inserts may fall out. The inserts must be pressed fully into their
bores. If they don't hit bottom, they will be subject to hot exhaust gas
from underneath, which can overheat them. Once pressed in, the insert
should be very slightly below the combustion chamber surface. This allows
the machinist to peen the cast iron over the insert, helping to keep it in
place.
I've never used them, but as a further hedge against unleaded fuel some
swear by stellite-faced stainless steel valves.
Whether or not all this is really necessary for any given engine depends
on how that engine will be used. If the car is rarely driven, ar is
driven like an antique (heaven forbid the rev counter should ever go over
3000 RPM!) then why bother. It won't rack up enough mileage in the
owner's lifetime to wear anthing out. If it racks up a lot of miles,
especially at modern highway speeds, then I think bronze guides,
attention to rocker arm geometry, hardened valve seat inserts, and
high-quality valves are well worth the effort and expense.
Basing opinions of the worth of all this for an XPAG/XPEG engine on the
experience of MGB and Spridget owners with their BMC engines, as some have
done here, is senseless. They are very different designs, with very
different problems.
-----------------------------------------------------------------------
Chip Old 1948 M.G. TC TC6710 NEMGTR #2271
Cub Hill, Maryland 1962 Triumph TR4 CT3154LO (daily driver)
fold@mail.bcpl.lib.md.us
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