Indeed your mechanic is correct, valve seals are of benefit in a stock
Your "bronze" guides are more tolerant of tight clearances...read any info
on the subject and they suggest the lubricative properties of the silicone
impregnation (?!?) in the guides allows lubrication and therefore resistance
to valve sticking !!
It is very important, though, when fitting these guides that your valve
stems are in PERFECT condition. Wear on the stems, usually in ovality of
.001-.002 will be unacceptable for these tighter tolerances.
So, given you are going to use new guides and new valves, the question
becomes "how often do I want to recondition the head?". One thing I've
learned about the TR 6 cyl heads is the lack of lubrication caused in part
the valve guide wear. This wear in turn causes the all to familiar smoking
motor. Starting with a fresh top end AND an external oiler line (top end
must be "perfect" to avoid oil fouling plugs).
I have used the above guidelines on my own cars since 1989 without incident
with one exception. Bronze guides WILL stick if you overheat the motor
during the running in period. New valves and bronze guides did not avoid a
stuck valve on my race GT6 motor last summer.
From: Brian Sanborn <firstname.lastname@example.org>
To: Triumphs List <email@example.com>
Date: January 5, 1999 6:50 PM
Subject: TR4 Engine - Valve Seals Question???
>Another engine rebuild question.....
>The machine shop owner asked me where the valve seal parts were. I told
>that they were not used in the original engine design. He recommended
>he modify the standard bronze valve inserts to accept a modern valve seal.