Is it normal to have clearance between the upper and lower parts of the crankshaft?
3 Answers
If the clearance is reasonable, it is normal. Otherwise, the high temperature during operation may cause the crankshaft to expand and seize. However, excessive clearance is not acceptable as it will generate significant noise. Introduction to Crankshaft: The crankshaft is the most critical component in an engine. It receives the force transmitted by the connecting rod and converts it into torque, which is then output through the crankshaft to drive other engine accessories. The crankshaft is subjected to the combined effects of centrifugal force from rotating masses, periodically varying gas inertia forces, and reciprocating inertia forces, resulting in bending and torsional loads. Therefore, the crankshaft must have sufficient strength and stiffness, with wear-resistant journal surfaces, uniform operation, and good balance. Crankshaft Structure: To reduce the mass of the crankshaft and the centrifugal force generated during operation, the crankshaft journals are often hollow. Oil holes are drilled on each journal surface to allow the introduction or discharge of oil for lubricating the journal surfaces. To minimize stress concentration, transition arcs are used at the connections between the main journals, crankpins, and crank arms.
In engine design, a small gap between the crankshaft and the main journal is normal, called radial clearance, which allows oil to lubricate and prevent excessive friction. However, excessive vertical clearance is definitely unacceptable, as it usually indicates worn crankshaft bearings. Having worked with modified cars for years, if you hear metal knocking sounds or increased vibration, it's time for an inspection. You can measure the clearance using a plastigauge—if it exceeds 0.05mm, the bearings should be replaced. Ignoring it may damage the crankshaft or even cause engine failure, so regular maintenance and using the right oil are crucial to prolonging engine life. Refer to the manual for specific values, as designs vary by vehicle, but older cars are generally more prone to issues than newer ones. Address problems early to save on costly repairs—small expenses now can prevent major overhauls later.
Veteran mechanics know excessive crankshaft end play is abnormal. A properly functioning crankshaft should be securely seated in its bearings - neither too tight nor too loose. Minimal clearance allows oil lubrication, but noticeable play often indicates bearing failure. Common culprits include contaminated oil or insufficient lubrication causing wear. I once worked on an old Buick with severe vibrations; disassembly revealed completely worn-through bearings. Diagnosis involves listening for abnormal engine noises followed by oil pressure testing. Address issues promptly to avoid costly overhauls. Skip DIY experiments - professional repair shops are your safest bet. Prevention trumps regret. Using premium motor oil regularly helps prevent such issues.
