Chronicles of a Community College Engine Rebuilding Class
Week 6
Day 12 - The instructor started the morning lecture by discussing crankshafts. Inspection, measurements and repair of the journals are covered during the classroom session. During the disassembly of the engine earlier in the semester, we did a quick cursory exam of the crank and there appeared to be no discoloration or deep scoring of the journals. There may have been some very light scratches or scoring on one or two of the main journals.I’ll have the instructor make an evaluation on the crank next week or so. The scratches are not deep enough to catch a fingernail and so I’m hoping a crank journal polish will be enough and not a crankshaft regrind..According to the instructor crankshaft journal grinding is a very specialized skill that not too many auto machine shops have the equipment or machine operator to perform.
During the lab session, the instructor plans on doing a couple of demonstrations. If there is enough time after the demonstrations, my partner and I will try to get the crank journals measured. The lab session is all about connecting rods. The first demonstration is the proper way to torque the end caps to the connecting rod without damaging or stressing the con rod beam using a dedicated fixture. Before the big end can be measured for roundness the end cap nuts have to set to the factory torque specs (Pic1).
The next demonstration is using the built in micrometer of the rod honing machine. The micrometer is set to the big end journal spec and the rod is set on the measuring pegs and rotated about 180 degrees in both directions.Note that the micrometer measures, within an accuracy of 0.0005”, any deviation from the preset nominal diameter of the big end journal, not the actual big end diameter of the con rod. Any deviation greater than 0.001” will require the big end to be resized which essentially grinds down the end cap a few thousands and the rod assembly re-torqued and manually honed in a connecting rod honing machine to achieve original diameter and maintain roundness to within specification (Pic 2).
With the end of the planned demonstrations, there is just enough time for my partner and I to measure the crankshaft journals. Each journal, both main and big end, are measured in four specific locations. We start at the front of the crank, normally where the crank pulley and keyway are located. Each main journal (5) and big end (4) are numbered sequentially from front to back for measuring purposes. For the mains the keyway is used as the reference point and designated as the 12:00 position. Each main journal is measured at the 12:00 and 6:00 o'clock position and the 3:00 and 9:00 o’clock which will determine roundness of the journal. To determine taper both the front of the journal and the rear of the journal are measured at the same four points.
The connecting rod journals are measured essentially the same way. The only difference is the location of the designated reference point. I believe we used the casting lines on the crank as the reference. These are the specifications per the workshop manual:
Main: 2.1255”/2.1260” (53.987mm/54.000mm)
Crankpin: 1.9370”/1.9375” (49.199mm/49.211mm)
These are our crankshaft journal measurement points.
Pt A = 12:00 & 6:00 position/front
Pt B = 12:00 & 6:00 position /rear
Pt C = 9:00 & 3:00 position /front
Pt D = 9:00 & 3:00 position/rear
#1 main journal - Pt A = 2.1259///Pt B = 2.1258///Pt C = 2.1256///Pt D = 2.1256
#2 main journal - Pt A = 2.1258///Pt B = 2.1259///Pt C = 2.1256///Pt D = 2.1256
#3 main journal - Pt A = 2.1256///Pt B = 2.1258///Pt C = 2.1258///Pt D = 2.1258
#4 main journal - Pt A = 2.1259///Pt B = 2.1259///Pt C = 2.1259///Pt D = 2.1258
#5 main journal - Pt A = 2.1257///Pt B = 2.1256///Pt C = 2.1257///Pt D = 2.1257
#1 crankpin journal - Pt A = 1.9370///Pt B = 1.9371///Pt C = 1.9371///Pt D = 1.9371
#2 crankpin journal - Pt A = 1.9370///Pt B = 1.9371///Pt C = 1.9370///Pt D = 1.9371
#3 crankpin journal - Pt A = 1.9369///Pt B = 1.9369///Pt C = 1.9370///Pt D = 1.9370
#4 crankpin journal - Pt A = 1.9369///Pt B = 1.9369///Pt C = 1.9370///Pt D = 1.9370
From the above reading for the mains, the journals are well within spec for roundness, taper and diameter. The crankpin readings are also within spec for roundness and taper but all of the journals appear to be on the low end of the spec. Journals 3 & 4 are slightly under the minimum specification. Re-measuring of the crankpin journals and consultation with the instructor will be required. This ends Day 12.
One day after Day 12, a stay at home order due to the Covid-19 outbreak was issued for all of California and the Community College immediately announced the cessation of all classes for the next two weeks.Anticipating that this could turn into a longer break, I immediately email my instructor to see if I can pick up my engine block. He designates next Monday as the only day students can pick up their equipment if desired. If I had thought ahead, I should have picked up all of the disassembled bits but only ended up picking up just the block and piston/rod assemblies which I then dropped off at a local auto machine shop not far from the school.I asked them to clean the block with their pyrolytic oven, jet wash the oil galleries, and re-measure the cylinder bores to double check our measurements.They also recommend shot peening the block to relieve the metal of any residual stresses.. Since the block needed a re-bore, I agreed. While the block is out with the machine shop, I am on the lookout for a replacement jackshaft.
Pic 1 & 2 - The con rod fixture is secured in a vise with the rod big end clamped securely but not tight in order to torque the end cap to the rod. The rod needs to be at correct torque for the big end measurement checks.
Pic 3 - The instructor is demonstrating the measuring micrometer of the rod honing machine.