Quote:
Originally Posted by Dave in MN
Brent,
Specifically: Why raise and lower the engine rpm when breaking in?
If done rapidly, from a high rpm by totally closing off the carb., it will draw oil into the rings and promote lubrication of the upper cylinder wall and rings thus reducing the heat being generated between these components.
I follow this practice.
I do have another purpose for running the long-blocks I build longer, as I am verifying them to be leak free well past normal driving loads by using a water brake/Dyno to load them.
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As others have attested to above, I do like thought provocation as it does make me think of better ways of doing things.
Dave, I do like your thinking regarding snapping the throttle closed to induce oil movement however I am unsure if it will work that way. Work with me to establish how this works. If the piston is at TDC, the oil that is on the cylinder walls is typically below the bottom oil ring as slinging it around is always under that control ring. If a quality job of boring & honing has been done, the two oil rings should be the first to seal (i.e.: re-shape themselves) to the wall's cylindrical shape simply because of their thickness. Next I would want to figure the volumetric capacity of the bore & chamber at TDC and compare it to the capacity of the manifold draw tube, both sides of the upper tube section, and the port size of both the cylinder and the opposite port to see how much piston movement would be required to create enough vacuum to draw that oil past the oil rings. In other words, my initial thoughts would be that the amount of vacuum draw from the piston moving downward will first be used to pull the air/fuel mixture out of the manifold & ports prior to it creating enough vacuum to draw past the piston rings. Since volumetric efficiency numbers are low on a L-head anyway, I would think the oil would not be sucked by the new rings. Adding to this, one would need to consider the oil rings are already scraping some oil away from the walls as the piston is moving downward which would be less oil to draw anyway. What am I missing??
I guess my thinking is that the only oil that would reach the top rings is what oil is held in the cross-hash pattern on the walls since this would be below the surface of the compression rings and the oil rings. Do you think if compression ring lubrication is a concern, then maybe adding some upper end lube such as Marvel Mystery oil or 2-cycle oil to the gas would be a better option when you are doing the initial start.
One other thing for us all to consider is why do we hone the cylinders and put the cross-hash into the freshly bored cylinder?? In reality, if we could bore the cylinder to a truly perfect diameter, and install a piston with rings that exactly matched the shape of that perfect bore, we wouldn't need to hone. So in other words, the worse the shape of the bore, then the more the rings will need to be cut to meet that bore shape.
Now with that said, an engine rebuilder can put too much/deep of a cross-hash into the walls during the machining process. When this occurs, the mechanical honing away of the piston's ring's metal continues unnecessarily and shortens the life expectancy of that ring. It also causes premature wear on the piston. So how much does this premature ring wear shorten the engine's life? Good question but I am thinking that 15%-25% of the life shortened is believable. I wish Pete Samuelson, Bill Stipe, or Dudley Moordigan would jump in with their opinions on this topic.
Dave, is your running of the engine really a 'break-in' or just running it to verify there are not any leaks and put enough heat in it so you can retighten the head nuts??