Inconsistent 330000 exhaust valve clearance specs in manuals

[ianviney]

Thanks everyone for your comments!

 

[StarTech]

>> In my experience, exhaust valves are generally looser by at least 0.002" on air cooled engines. This allows the hotter valve to grow more. So that in use, when fully warm, both clearances become nearly the same.

In general that is true but there are a few engine that are spec'd differently.

>> IMHO, while it is important to achieve adequate "valve lift at fully open", for the average working engine (NOT racing engine) the clearance is there to allow some lubricating oil to get between the cam and lifter surfaces so that there is an oil film between the 2 parts when the lifter rides up the cam lobe. Otherwise cam lobe wear would be extremely excessive.

Therefore: 1) Some clearance is always better than NO clearance, and 2) quibbling over 0.001 or 0.002" clearance is like withholding water from a thirsty man to decide if flavored water is better/worse than Perrier. Sure, Indy car engine mechanics need to get it right, lawn tractor engine mechanics "close is good enough".

Correct these engines are not super critical adjustment wise. I have seen engine to come in where the spec is .005-.007 to be as much as .015 or more out causing only starting problems.

I can see where the 0.007" (the looser setting) might be the setting when the engine leaves the factory. Because the factory knows the valves will be seating themselves during the first 20 hours, and the clearance will be reduced. Not wanting the camshaft to fail would be the primary concern, and far more important than a little added noise. The exhaust valves can always be adjusted to the tighter measurement at the first tune up.

Just my 2 cents.

So far every new engine I have check are within the range given in the manuals and this only if I am having to check things on a new install. Generally here the first time an engine checked is when there is other problems. But of course I am not dealer so I see equipment after the warranties have expired.