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Engintel: Hydraulic Lifter Summer School Session

Let's take a quick run through Hydraulic Lifter 101 and at the end I'll leave you with a valuable PBT (Practical Builder Tip)

By Roy Berndt

It's summer, it's hot and there are more things going on in the world of motor-powered products than you can shake a stick at. Whether it's NASCAR, Indy Car, hot rods, motorcycles, boating, personal watercraft, you name it: if it has an engine, it is being used and used often.

But we are in July and those of you with kids know school is just around the corner. I have a daughter getting ready to go back to college, so the school thing is on my mind as well. Based upon some of the questions that I have been getting lately - particularly in the areas of valve train lifter noise and bearing clearances - I thought it was time to go back to engine school.

Let's take a quick run through Hydraulic Lifter 101 and at the end I'll leave you with a valuable PBT (Practical Builder Tip). There are, of course, a number of different hydraulic lifter forms today. The most popular are flat lifters and roller lifters, both typically seen in overhead valve engines. The latest technology is Displacement on Demand (DOD) or whatever buzzword each manufacturer has come up with for having an electronically controlled oil manifold that can shut oil on and off to different lifters and thus deactivating cylinders.

The bottom line is that all hydraulic assemblies are going to operate on the same basic principle. When on the base circle of the camshaft the plunger spring takes up the clearance in the valve train. Oil under pressure will enter the body of the hydraulic unit through an orifice of some type to feed the plunger. The oil will then pass through the plunger past the check valve and fill the void below it. As the camshaft rotates and the lobe ramp and lift takes place the pressure of the valve train weight and valve  spring pressure apply load against the oil below the check valve which forces it to seat and in essence becomes a solid unit. A metered amount of oil does pass between the plunger and body and that is the controlled leakdown. Once the valve opens and closes we are at the base circle and the entire cycle starts all over again (see Figure 1).

Now for the PBT. Every hydraulic unit of any type in an internal combustion engine has the same optimum operating range; the center third of its total travel. So if you have a flat hydraulic lifter that typically will have .210" travel your optimum operating range is .070"-.140" (.210/3=.070). However if you have a HLC with a travel range of .045" your optimum operating range is .015"-.030". Are you starting to see where I am going with this?

What controls whether your lifter is in the operating range? Valve stem height! So if you only have a .015" window of optimum range in your HLC but you have a valve stem height spec range of .030" what do you think is going to happen?

Coupled with the fact that very few OE manufacturers provide a maximum/minimum stem height spec you are at the mercy of what is alleged to be the correct information. Do not take everything that you find out there to be true and correct. Verify dry lash before deciding that a problem you have is a component failure - it may actually be a bad specification.

Unless you verify the correct information (I strongly recommend recording it for future use) you could be building your own problems into an engine and not even know it. Stem height that's too low can result in a clattering valve train and stem height that's too high can result in a lifter that pumps up and holds the valve open resulting in a miss and/or misfire code. Neither of these are going to make you happy.

Now on to bearing clearance. The good thing is that this is a specification that is readily available from the OE manufactures. However, remember that they operate with new components that are all standard and have the option of select fit bearings that allow them to adjust for any variances in either housing bore or shaft diameter. As the builder/reman, you are going to be using under/oversize machined components. They can all be to specification tolerances but may still render unsatisfactory results for maximum bearing oil clearance.

Say for instance you have line honed a block and the main bearing housing bore is at the low limit of tolerance. You have a crankshaft that is reground and the main bearing journals are at the high limit of tolerance. You are seeking .0015-.002" vertical oil clearance.

Prior to installing the crankshaft you install the main bearings in the block with main saddles torqued to spec and use a dial bore set to the crankshaft size and find that you have .003" to .0032" vertical oil clearance. Not nightmarish, but certainly more than what you want and, as it turns out, beyond maximum specification. You remove the main bearing and use a ball micrometer and find that bearings are .0005" smaller than the maximum wall thickness listed at the crown.

You get another bearing set and another, try three different manufacturers and find that they are all consistent in thickness. Now if you lose .0005" per bearing shell that is .001" in clearance and if you took away .001" you would be perfect.

The quandary? Deciding what is going to be an acceptable scenario and not cause damage. You could make the main bearing housing bore .001" smaller but then you need to worry about bearing crush being excessive. Since you have tried three different bearing manufacturers with the same result, I consider the only logical solution is to grind the crankshaft beyond the maximum specification to render the correct clearance. Sometimes being wrong is the right thing to do.

I also propose that some of these scenarios may actually be a conversion factor between metric and standard dimensional measurement. The conversion factor is .03937 but no one generally goes that far out in a conversion. For instance .25 mm is considered .010" undersize yet if you convert .25 mm it comes out to .0098425" and .50 mm is actually .0019685" and .75 mm is .0295275". You can see that when you get out to .75mm or .030" you are almost a full half thousands off dimensionally. If you convert .030" to a pure metric size it comes to .07620 mm. I know that nothing is going to change and if you talk to bearing manufacturers they will tell you that they have compensated…but which way? All I am saying is that out in the real world there are instances particularly in main bearing clearance that end up being excessive which can result in low hot idle oil pressure or worse a hot idle main knock.

I am not trying to cry wolf and be an alarmist but today's engines are operating under higher cylinder pressures and trace detonation. Higher temperatures and more horsepower with smaller displacement. The luxury of wider tolerance ranges in many of today's engines does not exist and you either have it right or you are out of the game. I hope that these two situations keep you aware of what can happen and that sometimes thinking out side the box is what you have to do today if you're going to be successful.

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