Serious overheating will cause valves to stick in bronze valve guides.
At 10:04 AM 9/15/03 -0400, Richard Taylor wrote:
>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
uncle jack
|