Interpret dial indicator readings, calculate shim corrections, and verify bearing fits for the Red Seal Industrial Maintenance Mechanic (433A) exam — Blocks C & F.
mwredsealprep.ca · Industrial Maintenance Mechanic 433A · Blocks C & F
Precision measurement and fit calculations are core millwright skills tested in Block C and Block F. The exam expects you to interpret dial indicator readings, calculate required shim corrections, and determine whether a bearing fit is acceptable based on measured dimensions.
These are not difficult calculations — but they require a clear understanding of what you are measuring and why. Terminology is where most candidates lose marks.
Rotating inner ring: interference on shaft, clearance in housing
Interference Fit (Press Fit)
Shaft is larger than bearing bore. Must be pressed or thermally installed. Used on the rotating ring.
Interference = Shaft OD − Bearing bore
Clearance Fit (Sliding Fit)
Shaft is smaller than bore (or housing larger than OD). Ring slides in. Used on the stationary ring.
Clearance = Housing bore − Bearing OD
ISO 286 Fit Codes — Quick Reference
Shaft (lowercase)
k, m, n → interference
g, h → clearance
js → transition
Housing (uppercase)
K, M, N → interference
G, H → clearance
JS → transition
Try to solve each problem before revealing the solution.
Worked Example
InterferenceProblem
A bearing has a bore of 40.000 mm. The shaft measures 40.018 mm. What is the interference?
Worked Example
ClearanceProblem
A bearing outer ring has an OD of 80.000 mm. The housing bore measures 80.025 mm. What is the clearance?
Worked Example
Thermal InstallProblem
A bearing with a 75 mm bore needs 0.020 mm of interference on the shaft. Using a thermal expansion coefficient of 0.000012 mm/mm/°C, what temperature rise is required?
Can you identify fits and read dial indicators under exam pressure?
Test yourself with timed Red Seal–style precision measurement questions.
Offset (Parallel)
Shaft centrelines are parallel but not coincident. Corrected by adding or removing shims equally at all feet.
Angular
Shaft centrelines meet at an angle. Corrected by adding or removing more shim at far feet than near feet.
Reading Convention
Zero the indicator at the top (12 o'clock), rotate to bottom (6 o'clock). Positive reading = indicator tip pushed in = shaft high. Negative = tip extended = shaft low.
Soft foot must be corrected before any alignment work
Tighten all feet to final torque
Loosen one foot bolt at a time
Watch a dial indicator on the machine frame near that foot
Movement greater than 0.002" (0.05 mm) = soft foot
Shim the affected foot to eliminate movement, then re-check all feet
Worked Example
Rim & FaceProblem
A rim indicator reading shows a TIR of +0.012" at the bottom. The coupling is 10" from the near feet and 22" from the far feet. What is the actual offset, and what correction is needed?
Worked Example
Reverse DialProblem
Reverse dial setup: Indicator A (stationary) reads +0.008" at bottom. Indicator B (moveable) reads -0.004" at bottom. Distance between indicators: 8". Near feet: 4" from indicator B. Far feet: 12" from indicator B. Calculate the shim corrections.
TIR vs actual offset
High RiskTIR is the full indicator swing top-to-bottom. Actual offset = TIR ÷ 2. Using TIR directly doubles your shim calculation — a very common exam error.
Sign convention matters
High RiskPositive readings typically mean the indicator tip is being pushed in (high). Negative means the tip extended (low). Establish your convention before reading results.
Soft foot must come first
High RiskCorrect soft foot before any alignment work. Shim corrections made while soft foot exists are invalid and will shift when tightened.
Thermal growth
Watch OutIf the exam mentions operating temperature, add the manufacturer's thermal growth value to your cold alignment target. Machines typically grow upward when hot.
Max bearing heating temperature
Watch OutNever heat a bearing above 120°C. Higher temperatures can damage the heat treatment of the bearing rings and permanently reduce load capacity.
Randomized bearing fit and alignment problems — try to solve before revealing the answer.
Practice Problem
Randomized every time
Solve This
A bearing with a 40 mm bore needs 28 µm of expansion for installation. Using α = 0.000012 mm/mm/°C, what temperature rise is required?
Last updated: June 2026 · Based on the Red Seal Occupational Standard (RSOS) for Industrial Maintenance Mechanic (Millwright) 433A
Bearing fits and alignment are two of the most practical skills on the 433A exam. Keep building across all six MWAs.
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