Drilled them with a 5/8 bit!!! Wow.
The bronze doesn't have a memory--at least not any better than mine, but
it does get permanently distorted because the heating is so varied along
it's length.
There's a good chance your guides were too tight to begin with--it's
worthwhile to run the correct size ream through them before assembly (not
a 5/8 drill bit). It's amazing how few shops do this--they bang them in
and call it good. Overheating can tweak the guides too, of course. We
could go on and on about guides--it's amazing that they work at all. I've
tried a zillion things, particularly with a Matchless G80CS that I own
that loves to stick the exhaust guide. The most recent iteration is steel,
with a bronze insert that looks like a helicoil. I think Kas recommends
these. I got mine three years ago from a 90 year old machinist who kicked
the bucket before I could find out where he got them. So far so good. I'd
like to install these in Triumph motors, but can't find any.
-----Original Message-----
From: Richard Taylor [mailto:tarch@bellsouth.net]
Sent: Monday, September 15, 2003 7:05 AM
To: fot@autox.team.net
Subject: Barking Heads
Group,
Here's the next wrinkle in the old "barking carburetor" syndrome. The
original problem turned out to be that three exhaust push rods had off-set
their respective rocker arm sockets. I figured that I had over-revved the
motor, put them back in place and entered the HSR Historic Race at Road
Atlanta last weekend. During practice Saturday, my motor did the same
thing, but this time at a carefully monitored 5500 rpms.
My friend, Neil Estes, and I pulled off the rocker arm rack and found that
all four of the exhaust valves were sticking convincingly. We pulled the
head, hand-drilled out the valve guides with a 5/8th drill bit,
reinstalled the head, it ran fine and took second place in the race Sunday
(as the lone TR-4 in a class of MGB's).
At the Saturday night refreshment session, the collective wisdom was that
the initial displacement of pushrods was caused by over-heating, not
over-revving; that the bronze guides had expanded within a steel head and
deformed inwardly against the valve stems and that this deformity remains
after the head has cooled.
The motor had 17 races on it when it first had this malady so too-tight
fitting valve guides is probably not an issue and, yes, I was having
cooling
(over-heating) issues when the problem first occurred. I never associated
that with sticking valves.
What is the collective experience of the group with heat-expanded valve
guides which lose their memory when cooled. Quite frankly I am quite
comfortable trusting my Saturday night drinking buddies' thesis, but a
second opinion might help us all.
Thanks,
Richard Taylor
TR-4
Atlanta
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