Connecting Rod Bearings
The Connecting rod babbitt bearings are die cast into the connecting rod.
When a bearing is scored or burnt, either form lack of lubrication or from having them set up too tight and not property worked in, refitting the bearing is the only remedy.
The scored bearing is one whose surface has been slightly roughened, but where the babbitt metal has not been burnt or run. A bearing of this kind can be refitted.
A burnt bearing is one whose surface is badly roughened or where the babbitt has melted and started to run. This bearing cannot be used and a new rod should be fitted.
Connecting rods which are not otherwise damaged, can be exchanged at factory for rebabbitted rods for the cost of the babbitt only.
Fitting of Connecting Rod Bearings
The fitting of the connecting rod bearings is one of the most important repair operations that can he performed in a motor.
Misalignment produces knocks and causes vibration.
The connecting rod reaming fixture, as shown, assures you of securing a properly lined connecting rod, and perfect fitting bearings.
This fixture is so designed that the crank pin bearing end be accurately reamed to size exactly parallel with the piston. It will handle any connecting rod, being adjustable as to length. Martell reamer heads are used, having a wide range adjustment. A reamed bearing gives front, 90 to 95% true bearing surface and is by far the best and quickest method to use in fitting bearings.
If the connecting rod reaming fixture is not available, spread a very thin coat of Prussian blue on the crankshaft or special arbor.
Install the connecting rod on the crankshaft or arbor of the same diameter with the piston end hanging downward, as shown.
Draw the nuts tight so that the bearing is snug on the shaft. Swing the rod back and forth several times and then examine the bearing for blue spots.
The blue spots, or as they are termed high spots, indicate that the bearing and the crankshaft rub at these points only.
It is then necessary to remove the high spots on the bearing with a scraper, as shown in Fig 27.
Repeat operation until all high spots are removed, and the bearing surface is smooth, and touches the crankshaft at all points.
The tension of the rod on the shaft should be snug enough so that when the piston and rod are moved to a horizontal position they will of their own weight move to a vertical position with a slight drag.
All burrs and other obstructions must be removed from the oil holes.
When the bearing has been fitted to the shaft, lubricate it thoroughly.
After the bearing has been properly fitted, the connecting rod with the piston must be tested for alignment, which can be done in the connecting rod reaming fixture, as shown in Fig. 28 or testing on arbor with square as shown in Fig. 29.
CRANKSHAFT AND MAIN BEARINGSTightening Loose Main Bearings
To tighten the main bearings, care should be exercised in removing an equal number of metal shims from each side of the bearing cap.
The number and thickness of shims to be removed will depend upon how loose the bearings are.
If it is found there are no shims between the main bearing caps and the crankcase, the bearing cap should be removed, locked in a vise and with a mill file remove enough metal from the face of the bearing cap to allow the proper tension. (See Fig. 30.)
Considerable care should be exercised not to get the bearings too tight, as there is danger of scoring or burning them.
If more than one bearing is loose, each bearing should be tightened separately, and when the proper adjustment has been secured, loosen the castled nuts sufficient to take the pressure of the bearing from the crankshaft. Then proceed to the next bearing in the Same manner.
After bearings have been properly fitted, the motor should be allowed to run idle under its own power for sometime, which will have a tendency to work in the bearings properly.
Use plenty of lubrication during this process, as bearings which are set up too snug will heat readily at first. Therefore, the danger of scoring or burning is very great until the bearings have time to work in.
Care should be exercised in driving car for approximately one hundred miles after bearings have been tightened
Fitting Crankshaft Main Bearings
The upper half of the crankshaft bearings are installed in the crankcase. The lower half being die-cast in bearing caps.
Figure 31 shows the main bearing line reamer and support, designed for STAR motors.
With this equipment the three main bearings can be reamed at one time, giving 85% to 95% true bearing surfaces, and perfect alignment. With this equipment a set of main bearings can be installed, line-reamed and shaft set up for tension in less than two hours.
A reamed bearing, properly done, aside from being much faster than hand scraping, will give a higher degree of true bearing surface.
If line reaming fixture is not available, the bearings can be scraped. This can be accomplished by putting a thin coat of Prussian blue an the bearing surface of the crankshaft.
Place the crankshaft in its normal position in the case, and rock back and forth. Remove crankshaft and scrape the blue spots from the bearings. (See Fig. 32.)
It is necessary to repeat this operation several times to obtain the desired results.
The upper bearings must be scraped in first. After the desired bearing is obtained, the bearing caps can then be fitted. The bearing caps must be fitted to the crankshaft in the same manner as the upper bearings.
End Play in Crankshaft
The end play in a crankshaft is governed by the center crankshaft bearing.
A clearance of .006" should be allowed between the crankshaft and the bearing.
Too much end play will often cause a pound or knock when the motor is running idle.
A sprung crankshaft will cause the bearings to loosen quickly and if the motor has been run for some time with loosened bearings, the shaft should be tested to see if it is true before attempting to refit the bearings.
A test can be made by placing the crankshaft in the case as shown in Fig. 32 by first smearing the babbitt bearings with a thin coat of Prussian blue; then revolve the crankshaft and note whether the blue shows completely around all the main bearings.
If it is sprung to any extent, it will also pivot on the center main bearing in a certain position.
Another method of testing a crankshaft is to place it between centers in a lathe or straightening press as shown in Fig. 33, and by using a dial indicator, the exact amount it is out of true can be determined.
A crankshaft can be straightened in an arbor or straightening press by supporting the two end bearings with blocks and applying the pressure on the center main bearings.
Out-of-round Crankshaft Bearing
To determine whether main or connecting rod bearing are out of round, tighten each bearing cap separately and give the crankshaft a complete turn. If the bearing is out of round, the shaft will invariably turn free at one point and bind at another. Or measurements can be taken of the shaft bearings with a pair of outside micrometers.
If bearings are out of round, they should be reground or turned in a lathe and polished.