Plastigage, a regestered trade mark, is used to determine the clearance between a bearing insert and its journal. For the occasional engine builder, using this product is a much cheaper alternative to purchasing sets of OD and ID micrometers, which will easily run into a thousand dollars or more.
The proper way a machinest would determine bearing clearance would be to measure the OD of the journal diamater of a shaft and subtract from that the ID of the installed bearing insert, the difference being the clearance.
The clearance provides space for several events, first shafts and blocks are seldom if ever exactly straight so the clearance provides mechanical space where the shaft can find the center between it and the block. The clearance is then taken up by the oil pumped into the clearance such that the shaft is supported on a wedge of oil all the way around the journal. The clearance also provides a leakage rate for the oil thru the bearing to remove heat and debris, and keeps lubrication on the bearing surfaces which would be worked out by the mechanical forces if these were lubed with grease.
If the clearance is too tight, the shaft and block cannot find a center that provides an oil wedge all around the bearing, then there will be metal to metal contact. This problem is made worse by expansion of parts when they heat up and expand into the clearance. If the clearance is too wide an oil wedge cannot form becasue the leakage rate out of the bearing is too high, again metal to metal contact will occur.
The factory and bearing manufacturers specify a range of acceptable clearances for your engine. Running on the lower side is good for commuter street use as it minimizes oil loss thru the bearing which reduces the work the oil rings have to do in-order to keep excessive oil thrown from the crank off the cylinder walls. It also allows the factory to run a smaller hence cheaper oil pump, this also reduces parasitic power losses from the crankshaft which improves fuel milage on their goverment mandated CAFE tests. Performance use dictates that clearances be set toward the high side to allow for greater thermal expansion that comes with the heat of competition. These wider clearances are usually accompanied with a high volume oil pump to insure enough oil is pumped thru the engine to provide wedge development inside the bearings. This always results in tremendous amounts of oil being slung off the crankshaft which if left uncontrolled will overcome the oil rings ability to scrape it off the cylinder walls. To control this oil, competition engines will include scrapers and windage trays to peel this extra oil off the crank and return it to the pan.
Clearance between the crank's rod journal and bearing, while lacking the long alignment issue of the main journals to block, are subject to many of the same problems in developing an oil wedge between the journal and the bearing insert. There are additional problems with the rods in that there are alternating loads between power stroke and the other strokes that the oil wedge must protect the journal and bearing from. This is made more difficult by the fact the bearings are rather narrow making it easy for the forces on the power stroke to blow the wedge out. Plus these bearings carry the greatest loads which always hit in exactly the same place, so impact and heat are big problems for these bearings requiring a lot of oil flow thru them to cool the working surfaces. Again the manufacturer specifies clearances and the same rules apply here as with the main bearings regarding street to competition use.
The choice of bearing material also affects what clearances should be used. Typically copper/lead, copper/tin, copper/babbitt bearings should run toward the low side to the middle of clearances, aluminum bearings should be set from the middle to upper band of the clearances.
If you're building a full out race motor you can run wider clearances than recommended, but this is the ozone of pros who usually can afford to replace the motor if their clearance choices are wrong. Don't go there unless you've got big bucks and intend to run with the big boys.
Plastigage comes in several size ranges, so you need an idea of where your clearances should be falling in-order to get the right stuff. If the clearances are wrong the only choice is machining. Too tight and the shaft may have to have one or more journals turned down. If the clearance is too wide then either the shaft has to be replaced or it can be turned to a standard undersize with new specific bearings for that undersize. If the shaft is a little too large or small, most bearing companies and the factory include a selection of bearings that range from +/- .0001 to .00015 or .0002 inch around the nominal dimension so you can dial the clearance in pretty well.
Carb spacers sometimes work and sometimes don't, this is a case of cut and try to see if your engine likes them or not. They don't need to be super expensive, all it takes is enough to see if added height and or volume improves carb metering response or the flow patterns in the manifold. Swirl grooves and stuff like that are not effective and money for such stuff is largely wasted. Some spacers keep individual throttle bores all the way into the manifold or pair the primary and secondary bores on each side, for a 180 degree this keeps it functioning as originally intended with half the carb feeding half the engine. Some are open under the throttle body which allows any port to draw from the entire carb. This will often improve high RPM performance but always at the cost of some lower thru mid range torque. If you have a stick gear box you can make this work for you, but automatics don't like it.
Edited 10/24/2006 4:48 pm by oldBogie