If you have questions regarding the dry sump system (no longer available), the V12 valve springs (several sets available), or would just like to talk with Scott about Miuras in general, don't hesitate to contact him via email at miuressence@austin.rr.com .................... or call 512-415-8253.

Journal bearing theory and Miura oiling issues

Worn main and rod bearings

Most current internal combustion automobile engines use journal bearings on the crankshaft rods and mains because of their efficiency, reliability and simplicity. Roller bearings and ball bearings, which are often called anti-friction bearings, actually generate about ten times the friction of properly functioning journal bearings.

When properly functioning, journal bearings have a much longer and more predictable life than anti-friction bearings. The keys to this long life and low friction are correct shaft-to-bearing clearances and proper lubrication; when these factors are "within specification" the surfaces of the bearing and the shaft never come into contact with each other, being separated by a very thin film of oil. Proper clearances between the crank shaft journals and the journal bearing inserts are typically in the range of .001 to .003 inches.

Oil is pressure-fed by the oil pump to the journal bearings through ports in the engine block and crankshaft. Typically, the oil pressure is between 30 and 130 psi. As the crankshaft journals rotate in their bearings, internal engine loads and rotational forces cause the shaft to become biased to one side of the clearance (eccentricity of the shaft within the journal bearing), and thus, the film of oil that fills the clearance is squeezed. In the area where the oil film gets squeezed into the smaller clearance, the instantaneous oil pressure builds up to approximately 1,000 psi. This instantaneous area of high pressure, active through the viscosity of the very thin oil film (on the order of .0001 in.) keeps the crankshaft journals from contacting the journal bearing material.

System in Miura

Any loss of the oil supply during operation will allow the crankshaft to contact the journal bearings, causing wear. This wear causes the clearances to increase. As the clearances increase, the oil tends to squeeze out the sides of the bearings faster, lowering the localized oil pressure. With enough bearing wear, the localized pressure drops to the point that it can no longer support the loads being imposed upon it by forces produced within the engine, resulting in bearing-to-journal contact. When it reaches this point, wear-out occurs rapidly. This is why most engine wear in a properly functioning automobile engine occurs when the engine is first cranked over and before the oil pump begins providing oil to the bearings. The external indicators of this wear are a decrease of oil pressure, erratic oil pressure, and/or audible bearing knock. If not corrected, these conditions can lead to catastrophic engine failure.

The Lamborghini Miura has two major engine design flaws that cause premature bearing wear. The first problem is that the oil in the oil filter slowly drains back into the oil pan when the engine is shut down. Thus, when the engine is first cranked over after it has been off for a few days, the oil pump must first fill the empty oil filter before oil reaches the bearings. Miura owners have most likely noticed that it's approximately 5 to 10 seconds after start-up before the oil pressure starts building.

The second problem inherent in the Miura design has to do with the oil pan and oil tube pickup design. During relatively moderate cornering, oil sloshes away from the pickup tube, allowing air to enter the pump, resulting in a loss of oil delivery to the bearings. The oil pressure in Scott Reid's stock P400 Miura dropped to "0" at very moderate cornering speeds. The photo at the top of this section is of main and rod bearing inserts out of Scott's personal Miura, showing major wear.

The Lamborghini design team made improvements to later Miuras by slightly increasing the capacity of the lower pan casting. Some individuals have increased the oil capacity by placing a spacer between the upper and lower pan castings. Both of these solutions decrease ground clearance of the pan and increase the risk of damage due to contact, and neither correct all of the design flaws.