Metric Mechanic - The Engine Block - The Bottom End

Metric Mechanic engines come as a "Long Block" (Head & Block). They include everything from the valve cover to the oil pan but exclude the intake and exhaust manifold and outside accessories.

Overview
Major Changes

1. Slipper Skirt Forged Alusil Pistons with Lightweight Piston Pins
2. High Silicon content Aluminum Alloy, lightweight, low expansion rate.
3. Lightweight rods - Deep "I" Beam (2200, 3600, 2500, 3800 & 4000 engines)
4. Stroker Crank
   • 82 mm - 2200 Engines
   • 86 mm - 3500 Engines
   • 88 mm - 3600 Engines
   • 94 mm - 2500, 3800 & 4000 engines
5. "Blue Printed" Oil Pump
6. Cornering Baffle (All Sport Engines, 4 Cylinder)
7. Improved Water Flow into the 6 cylinder Block

Parts Replaced

1. Rod Bearings & Main Bearings
2. Front & Rear Seals Plus Gaskets
3. Front Pulley or Drive Flange if unacceptably grooved by the seal
4. Single Roller Cam Chain
5. Oil Pump Chain

Assembly

1. Pistons & Rods Balanced to within 1 gram
2. "Blue Printing" done to low side of factory tolerances. A .0001" micrometer or a dial bore gauge is used for measuring and fitting.
3. Painting Blocks are painted black. All hardware, nuts & bolts plus freeze plugs are zinc, gold iridite plated.

Metric Mechanic Forged Alusil Pistons
i. Design & Description
ii. Piston Update
After designing the 2500, 3800 and 4000 HiFlo ST Engines, much of their success was due to piston design. These slipper skirt forged Alusil (Aluminum Silicon) Pistons were extremely light weight. By using a slipper skirt design, we were able to narrow the bridge between the pin bosses for better pin support. This allows us to use a shorter lightweight piston pin without sacrificing strength. We also add an oil feed system to lubricate the piston pin bores. Other minor improvements include a tougher alloy "zero" pin offset and stronger skirts.
By way of complementing these "new" pistons, we add "moly" (molybdenum) filled upper compression rings to help reduce piston ring and cylinder wall wear.

We are proud to announce that this technology has been added to the 2200 and 3500 HiFlo ST Sport Engines. All the above improvements are now standard in all our engines. They only differ in weight and shape but not design (see Picture). By now you've probably noticed a small dome on top of the piston. This was done to up the compression to 10.0:1

How do we manage to run such high compression ratios in our engines? Well, most of it is due to Surface Turbulence but some of it is due to the forged Alusil Pistons we use. This type of piston allows us to up the compression ratio about 0.5 to 0.7 over a conventional cast piston without raising the level of detonation. Two things account for this:

1. The lighter weight of the forged piston means less heat retention
2. Because a forged piston is formed under 1500 tons of pressure, the piston ends up with a very dense grain structure which accommodates better heat transfer. A cast piston, made by pouring molten aluminum into a mold, creates a porous piston (lots of small air pockets) with less conductivity. Simply put, a dense molecular structure transfers heat better.

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.

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.

So, 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.