|May 2000 News||www.virtualindian.org|
|Home / Features / Rod bearings|
most abused bearing in a 4 cylinder motorcycle engine are the rod bearings.
The upper and lower halves of these bearings are the first to show wear,
and it's the upper half more so than the cap half. Babbitt metal used for
years in motor bearing applications work very well, but even babbitt has
Heat and pressure destroy bearing surfaces, and cause changes in the diametrical clearance. Diametrical clearance is the difference between the diameter of the shaft and the inside diameter of the bearing surface. In order for a bearing to operate in optimum conditions this space must be filled with the engine oil, fed by pressure or splash to each bearing location.
Babbitt metal is an ideal metal for a rotating shaft such as a crank shaft main bearings, however babbitt is not well designed for constant pounding caused by the power stroke especially during the initial startup before oil pressure has time to reach maximum. When the motor reaches maximum temperature the rod bearings run hotter as heat from the piston travels to the rods, to the bearings. Oil helps carry away excess heat and bearings will also carry heat to the journals. Heat causes the crankshaft to expand also causing additional wear. The slight changes over time due to heat and pounding during the power stroke have a tendency to cause cracks in the bearing surfaces, bearing separation and alter the diametrical clearance.
Once the clearance changes, typically oval shaped pressure is lost and wear rate is compounded. When the bearing wears to the point where the thickness of the oil exceeds the clearance the motor begins to knock. So careful attention to rod bearings is needed. Babbitt metal for rod bearings has been the norm for many years. It is a well known fact that babbitted rod bearings do not last as long as the main bearings.
But modern metallurgy has come to the rescue for rod bearings, replacing the old babbitt type. Providing similar characteristics this new material will outlast babbitt, provide better heat transfer and resist distortion. After 30,000 miles of testing the new surfaces show no signs of wear other than extremely high polished surfaces and no shaft wear. A tighter diametrical clearance can be achieved with the bearing as well.
I start with a material blank chucked up in the lathe and cut the thrust face ends and the basic inside rod diameter size. After a rough bore the bearing is parted off and split in two halves forming the upper and lower. They are fit to the rod halves and the faces are machined to accept the rod shims. Once installed in the rods, they are bolted together and torqued to the correct value and set up in the boring jig. This will ensure that there is a 90 degree angle between the rod and shaft otherwise the piston will end up cocked in the cylinder. Fillets are then cut on the inside diameter, front and back sides of each bearing to match the crankshaft. The rods can now be fitted to the shaft and checked for clearance.
Far more durable than babbit for rod bearings the new material will outlast babbitt in the same application, providing longer life, better heat transfer and no distortion.
Boring the bearing blank stock
Machining the bearings
Bearings ready to be split and mounted in rods
Bearings mounted in rods ready for additional machining
Connecting rod boring jig. Bearings are rough bored, then reamed and lapped ready for the crankshaft final fit
As some of you know, Rob has a shop in the Ontario (Canada) area, and if you are in the market for bearings or other services for Four Cylinder bikes (or anything to do with Hendersons -including new cylinders!), you can contact Rob at: