Piston Rings, Rods, Crankshaft & Oil Pumps

Piston Rings
The number one wear problem in an engine that dictates the length of an engine's life, is the beating out of the upper compression ring groove. Once this happens, cylinder pressure leaks into the crankcase and blow-by (oil vapor) exits the vent tube. High oil consumption and rough idling become a problem. We have done two things to retard this wear process.

1. Using a stronger Forged Alusil Piston which doesn't tend to beat out so quickly.
2. Using a lighter 1.5 mm upper compression ring so it doesn't hammer on the ring groove so hard.

The upper compression ring groove is filled with a high strength ductile "moly" Ring. These rings are made by adding a groove to the face of a high strength ductile cast iron ring and then filling the groove with "Moly" (molybdenum), a very tough porous metal. The ring is then lap finished to the bore size with a barrel face for quick seating. "Moly", due to its porous nature, tends to impregnate the face of the ring with oil. This helps reduce cylinder wall and ring wear. "Moly" rings also seat rather quickly. On the second ring, we use a ductile cast iron ring. The oil ring is a more modern 3 piece design composed of two chrome rails separated by a zig zag spacer. This improves oil control. BMW uses a one piece ring.

Stroker Crankshafts
Over the years, we discovered that the combination of a lightweight reciprocating mass (piston and rod weight) and a stroker crankshaft will produce a broad power band yet still be free revving. Our 2500, 3800 and 4000 engines push this technology to the upper limits. These engines have 94 mm stroked crankshafts with larger rod bearing journals for greater load support. To give these long stroke engines a free revving nature, we reduced the reciprocating mass by 28% (over stock). All stroked crankshafts we use in our engines have been stress tested under racing conditions. We have twisted up the 2200 and 2500 crankshaft to 7200 rpms with burst speeds just under 8000 rpms with no crankshaft breakage or bearing failures. Now, we're NOT telling you that it's safe to take one of our engines and rev the __ out if it, because it's NOT! The race engines we build are modified at the #4 main bearing to deliver more oil to the #3 and #4 bearings. Without this modification, the #3 and #4 rod bearings will starve for oil at these rpms and knock out a rod bearing. The point here is that these cranks are built with a good margin of safety.

Metric Mechanic Blue Printed Oil Pump
The factory oil pump body is made out of magnesium and uses an Eaton type gear set up to move oil. The major problem with the stock pump is with the pressure relief valve system. The pressure relief valve piston is made out of steel (which has a low expansion rate) and it rides in a magnesium bore (which has a high expansion rate). Now, in cold weather (below 0°F) this bore shrinks and sticks the pressure relief valve piston. When the pressure relief valve can't kick off, the oil pressure skyrockets and the oil filter "O" ring blows out. Another thing that happens is that after a number of miles the steel piston grooves the bore badly at the bottom of the piston stroke. This not only creates an oil pressure loss, but also causes the piston to cock in the bore and lock up (this can also cause the oil filter to blow). In either case, when the oil filter "O" ring blows out, you only have a few seconds before all the oil is pumped out of the engine. If you don't observe your oil light, your engine can be destroyed in a matter of a few minutes!

The root of this whole problem is the magnesium bore. To cure this problem, we bore out the pressure relief valve hole and re-sleeve it with a silicon bronze bushing. Silicon bronze has a very low expansion rate and is very wear resistant. The net results are:

1. That we can fit the piston much tighter in the bore (.0015" versus .004-.0045" for the factory pump). This helps to increase idle oil pressure and lubricate the valve train better.
2. On earlier 6 cylinder pumps they have a small hole drilled from the pressure relief valve area to the pressure side of the oil pump. This hole is used to burp air out of the pump during initial start up to kick off the oil light faster. The only problem with this hole is that once the engine is warmed up oil pressure is being bled off at idle. On later 6 cylinder oil pumps the factory eliminated this burp hole. When we sleeve the pump, we cover up this hole. We also use a pin to set up the upper stroke position of the piston. The factory uses a step that can sometimes wear out and stick the piston in the up stroke.
3. Since the sleeve we use is made out of silicon bronze (instead of magnesium), the bore will be very resistant to wear.

Cornering Baffle
The right side of the oil pan has a horizontal baffle to keep oil from climbing up the side of the inclined BMW Block on hard left turns.

Think About This
Imagine how very little wear a valve guide gets even when it is being starved for oil by the valve guide seal. Now, imagine if you used magnesium for a valve guide. I believe it wouldn't last past a few days.

In Summary
we now have a tight fitting, wear resistant bore for the pressure relief valve piston to ride in. Before the pump is assembled, we replace worn pump gears, pump bodies, and surface the pickup housing. We now have a Metric Mechanic rebuilt oil pump that we feel is better than new.

NOTE: BMW Dual Pressure Relief Valve Oil Pumps
Many of you may be aware of the late model dual pressure relief valve oil pump. A secondary pressure relief valve has been added to override the primary pressure relief valve should it stick. This is somewhat like taking a Band Aid approach to the fix because it really doesn't address the sloppy fit and wear problem of the primary pressure relief valve.