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MG MGA - sticky valve
|Whilst running the engine in my garage setting the carbs I heard a funny noise and stopped the engine immediately. When I opened the rocker cover I found the last valve stuck and after tapping it a little we lifted it with a screwdriver. After that I started the engine again and worked well. Although I have run my engine for a couple of hours since putting back on the chassis it had remained idle for the past two years.|
Is there any cause for concern?
|i had 250 miles on a newly rebuilt stock 69 B when i stuck the exhaust valve on the rear cylinder. The head was rebuilt by a highly regarded machine shop in my area, their first response was how old was the fuel as old fuel turns to a varnish type substance which could stick a valve and was the gas tank boiled out prior to breaking in. It turned out that the shop miscalculated the tolerance and repaired the mistake although i'm the one who had to reassemble the motor. Also, the valve returned to its closed position when the motor cooled down.|
|Charles, this is all news to me. Would you give me a ring tomorrow so we can discuss the matter, and at the same time do an inspection.|
|Valve stems are very close fit in the guides, originally in the range of 0.0015-0.0025 inch (0.0381-0.0635 mm). There is some thermal expansion when the parts get hot, valve stem gets slightly larger, and the guide gets slightly smaller inside (because it is constrained on the OD by the head casting). Too little clearance and too much expansion equals seized valve when it gets hot. This commonly happens under full throttle operation, like winding it up tight for competition, or driving up a long hill (heavy engine load).|
Bronze valve guides are sometimes a problem due to greater thermal expansion of the bronze, so bronze guides need about 0.0010 in more clearance (larger ID).
When you have a sticky valve there is a simple home brewed way to fix it. Remove valve from head. Get an old valve and cut the head off of the valve. Chuck the stem up in an electric drill, put a touch of valve lapping compound on the stem, and run it into the guide at full drill speed, with slow forward progress so not to jam and stall the drill). Keep honing in this manner until the tool will go all the way through the guide with lapping compound on it and free spinning. Then wash it all out clean with solvent, and the good valve should drop right through the guide by gravity alone with just the right amount of running clearance.
|As already indicated above I have started the engine and the valve seems to be working well. However I noticed that the retaining clip for the collets was missing. Now I have put a clip on, and sprayed some WD40 on the valve.|
Hope it wont stick again.
Thanks Pete and Barney for your interventions.
See you later Frans
|Hope you solved the problem with the new collets retaining clip. Call me after 4 p.m.|
This thread was discussed between 06/11/2011 and 07/11/2011
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