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Old October 19th, 2018, 07:48 AM   #11
greg737
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Join Date: May 2009

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Posts: A lot.
Quote:
Originally Posted by oakleaf View Post
Question about adjusting valves... 7 of 8 gaps were under spec. I would think it would be the other way. I adjusted all intake valves to between .004" and .005" and all exhaust valves to between .005" and .006". What effect would gaps between .002 and 003 on all valves but one intake have had?
Getting "tighter" is what the valves naturally do. The aluminum alloy of the cylinder head is the softest metal in the whole mechanism (everything else involved is harder: the valves are steel, the camshaft lobes are steel, the rocker arms are some sort of alloy that's harder than the cylinder head) so they (the valves) slowly seat themselves a bit deeper into the head over time.

Some of the really high-mileage guys (like the Iron Butt Rally riders) used to have valves that ran completely out of the ability to be adjusted. I remember reading forum posts where they were experimenting with shaving the bottoms off of the rocker arms on the adjustment screw side to create more clearance.

What effect does a too-small gap have? I've heard that an EX250 becomes harder to start as the valve clearances get tighter. But this is just a "cold engine" effect, which isn't very important because the valve clearance is a setting that's intended to have its proper effect when the engine is at normal operating temperature.

The changes that take place as the different metal alloys in the valve train (valves, cylinder head, camshaft lobes, rocker arms) heat up and expand at differing rates are sure to create a really complex and dynamic situation that only an engine designer/metallurgist could understand/explain.

I've never found/read a truly complete analysis of what the overall effect of temperature change is on the EX250's valve clearance. Does the clearance gap stay the same as the metals of the various parts all expand with heat? Or is there an overall tightening or loosening as the engine reaches normal operating temperature?

But at the extreme end of the situation: bigger clearance gaps mean the valve will spend more time on the valve seat in the cylinder head. When the gaps are allowed to get smaller/tighter you're running the risk of a valve than never really seats that well on the cylinder heat valve seat. On the exhaust side valves this can result in overheated, deformed valves because they never get a chance to transfer enough heat away into the cylinder head.
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