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Technical
Lard's engine rebuild (* was "tight/on it's way out valve")
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<blockquote data-quote="Geeze" data-source="post: 281470" data-attributes="member: 15843"><p>The barrels and the pushrods expand at different rates.</p><p>Although they have a gap when cold, once the engine is fully warmed up, they should have (theoretically) zero lash, ie. no gap. That's why when properly tuned and valve clearances set, aircooleds are notoriously rattly when cold but much quieter once all warmed up 8) </p><p></p><p>If the valve clearance is zero when the engine is dead cold, then the valve is still shutting and everything is ok but once the engine is hot, the pushrod is now holding the valve very slightly off the seat. Causes the poor running and means some of the full heat of combustion seeps round which gets to the valve stem which then fails...</p><p>Happens more on exhaust valves if they're tight because the inlet gets cooled by the intake charge.</p><p></p><p>Also all means that compression at cold will be fine (but it's still a good test, confirms it's likely to be valve related, not something else).</p><p></p><p>Good news is that if the compression is good and the bottom end is tight with no end float, then a top end rebuild isn't a big job and much cheaper than a complete new motor. But it's showing signs that would make me want to have the head off for an inspection. If you want to do another check before pulling the motor then one other test is to see if the exhaust port on that cylinder is getting much hotter than the other three.</p></blockquote><p></p>
[QUOTE="Geeze, post: 281470, member: 15843"] The barrels and the pushrods expand at different rates. Although they have a gap when cold, once the engine is fully warmed up, they should have (theoretically) zero lash, ie. no gap. That's why when properly tuned and valve clearances set, aircooleds are notoriously rattly when cold but much quieter once all warmed up 8) If the valve clearance is zero when the engine is dead cold, then the valve is still shutting and everything is ok but once the engine is hot, the pushrod is now holding the valve very slightly off the seat. Causes the poor running and means some of the full heat of combustion seeps round which gets to the valve stem which then fails... Happens more on exhaust valves if they're tight because the inlet gets cooled by the intake charge. Also all means that compression at cold will be fine (but it's still a good test, confirms it's likely to be valve related, not something else). Good news is that if the compression is good and the bottom end is tight with no end float, then a top end rebuild isn't a big job and much cheaper than a complete new motor. But it's showing signs that would make me want to have the head off for an inspection. If you want to do another check before pulling the motor then one other test is to see if the exhaust port on that cylinder is getting much hotter than the other three. [/QUOTE]
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EarlyBay Forums
Technical
Lard's engine rebuild (* was "tight/on it's way out valve")
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