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newSchoolWorks.com › VoTechWorks › Transportation › Small Engine & Power Equipment › Capstone Project: Small Engine & Power Equipment Overhaul and Documentation › Lesson 2
TRN-SMAL74-07 · Lesson 2 · 30 clock hours · lab

Disassembly, Precision Measurement, and Parts Evaluation

Objective. Following the approved project plan, the student safely disassembles all involved systems, records precision measurements for every wear component against OEM specifications, and produces a parts disposition list that correctly classifies each component as reuse, resurface, or replace.

⚠ Pending professional review

This lesson, its worked examples and its practice questions has not yet been verified by a certified examiner or master technician in the trade or by a certification or approval body. It is a study draft and may contain errors. Do not use it as instructions for work on a job site or on live equipment. Always follow your instructor, your supervising professional, and current official references and regulations.

VoTechWorks program approval and accreditation are still in progress. Credentials are issued only by the certifying bodies named in each program, after their own exams.

Why Disassembly Sequence and Documentation Matter

A capstone overhaul is not simply taking something apart — it is a controlled teardown that preserves evidence, protects parts, and builds the paper trail a professional shop depends on. Every measurement you record and every decision you justify becomes part of a legal work order. Employers and EETC evaluators expect technicians to follow OEM service manual procedures exactly, document findings in writing, and make defensible parts decisions. Sloppy teardown destroys torque patterns, mixes fasteners, and loses the information needed to diagnose root cause. Do it right from the first bolt.

Lockout/Tagout and PPE Before You Touch Anything

Small engines store energy in multiple forms: chemical (fuel), mechanical (springs, flywheel inertia), electrical (ignition coil charge), and pressure (compression). Before disassembly begins, apply lockout/tagout to every energy source.

Your instructor will conduct at least one unannounced PPE and organization check. Failure to comply stops the lab.

Controlled, Sequenced Disassembly

Open the OEM service manual to the disassembly section for your specific engine model before removing anything. OEM manuals specify removal sequences to prevent warping mating surfaces and to avoid damaged threads. Follow these steps:

1. Photograph every assembly before removal. Use your phone or shop camera. Photos document wire routing, linkage positions, and the relationship of parts that are hard to describe in words.

2. Tag and contain all fasteners. Place bolts, nuts, and washers from each assembly into a labeled parts tray or zip-lock bag. Write the location on the label (e.g., "cylinder head bolts — quantity 4"). Never mix fasteners from different assemblies.

3. Follow torque sequences in reverse. When removing a cylinder head or side cover, loosen fasteners in the reverse of the specified tightening sequence to release stress evenly. The manual shows the numbered sequence diagram.

4. Clean parts before measuring. A film of oil or carbon on a journal surface produces an inaccurate reading. Use the parts cleaning tank with approved solvent, then dry thoroughly with compressed air before measuring.

Precision Measurement: Tools and Techniques

Every critical wear dimension must be measured with the correct tool and recorded immediately on your two-column spec sheet.

Outside micrometer — used for crankshaft journal diameter and piston pin diameter. Select the range (0–1 in or 1–2 in) that covers the nominal dimension in the manual. Clean the anvil and spindle faces, zero the micrometer on a standard, then measure at two orientations 90° apart on each journal to detect out-of-round wear.

Telescoping gauge and outside micrometer — used together to measure cylinder bore diameter. Expand the telescoping gauge inside the bore until it locks at the bore diameter, withdraw it, and transfer the measurement to the outside micrometer. Measure at the top of ring travel, middle, and bottom of the bore, and at two orientations, to detect taper and out-of-round.

Feeler gauge — used for piston ring end gap, valve stem-to-guide clearance, and armature air gap. For ring end gap, square the ring in the bore using the piston crown, then slide the feeler gauge into the gap until a slight drag is felt. For armature air gap, set the gap per the OEM specification between the flywheel magnet and the armature legs.

Carburetor float height — measured with a caliper or specific go/no-go gauge per the service manual. With the carburetor body inverted and the float resting on the needle seat (spring not compressed), measure from the gasket surface to the float. Adjust the tang if the height is outside spec.

Recording Data: The Two-Column Spec Sheet

For every dimension, the spec sheet has three entries: the OEM specification (pulled from the service manual), your measured value with units, and the calculated deviation. Deviation = Measured − OEM nominal. A negative deviation means the part is undersized; positive means oversized. Compare the deviation to the OEM wear limit to decide: Reuse (within spec), Resurface (beyond nominal but within resizable limit, e.g., a crankshaft that can be reground to an undersize), or Replace (at or beyond wear limit, cracked, scored, or no resizing option available).

Never guess a disposition. Every replace or resurface decision must be backed by a measured value that falls outside the OEM specification.

Parts Disposition List and Revised Work Order

With your spec sheet complete, produce a parts disposition list: one line per component, showing part name, measured value, OEM limit, disposition (reuse/resurface/replace), and — for replace — the exact OEM part number pulled from the current parts lookup system. Add quantity, current price, and estimated labor time to reassemble, then update the work order. This document is what the shop uses to order parts and bill the customer. Accuracy here is professional and ethical responsibility.

Key terms10

Lockout/Tagout (LOTO)A safety procedure that physically isolates all energy sources on a machine — fuel, electrical, mechanical, and pressure — and applies a lock or tag so the machine cannot be started or energized while work is in progress.
Telescoping gaugeA transfer measuring tool with spring-loaded plungers that lock at the internal diameter of a bore; the locked dimension is then measured with an outside micrometer to obtain the bore size.
Outside micrometerA precision hand tool that measures the outside diameter or thickness of a part to 0.001-inch resolution (or finer on a vernier or digital version) using a calibrated spindle and anvil.
Feeler gaugeA set of thin metal blades of known thickness used to measure small gaps, such as piston ring end gap, valve clearances, and armature air gap.
Piston ring end gapThe clearance measured between the two ends of a piston ring when the ring is squared inside the cylinder bore; compared to OEM specification to determine whether the ring provides adequate sealing.
Armature air gapThe specified distance between the ignition coil armature legs and the flywheel magnets; setting this gap correctly ensures the coil generates sufficient voltage for reliable ignition.
Parts dispositionA written classification assigned to each disassembled component — reuse, resurface, or replace — based on measured dimensions compared to OEM wear limits.
OEM service manualThe official repair and specification document published by the original equipment manufacturer, giving torque values, clearance specifications, disassembly sequences, and wear limits for a specific engine or equipment model.
Deviation (measurement)The numerical difference between a measured value and the OEM nominal specification; calculated as Measured minus OEM nominal, used to determine whether a component is within acceptable limits.
Float heightThe vertical distance of the carburetor float from the carburetor body gasket surface when the float is resting on the needle valve seat; determines the fuel level in the bowl and affects the air-fuel mixture.

Worked examples4

1. The OEM service manual specifies a cylinder bore diameter of 2.5625 in with a wear limit of +0.005 in over nominal. You measure the bore at the top of ring travel and obtain 2.5658 in. Is the bore within specification, and what is the deviation?
  1. Identify OEM nominal bore diameter: 2.5625 in.
  2. Identify the wear limit: bore must not exceed 2.5625 + 0.005 = 2.5675 in.
  3. Calculate deviation: 2.5658 − 2.5625 = +0.0033 in (bore is oversized by 0.0033 in).
  4. Compare deviation to wear limit: 0.0033 in < 0.005 in maximum allowable wear.
  5. Record disposition based on additional checks (taper, out-of-round); based on this single reading the bore is within the wear limit.
Answer: Deviation = +0.0033 in. The bore is 0.0033 in over nominal but within the 0.005 in wear limit. Disposition at this location: reuse (pending full bore measurement at all positions).
2. The OEM spec for piston ring end gap on a small engine is 0.010–0.016 in. You insert a feeler gauge into the ring gap and find that a 0.014-in blade passes with slight drag and a 0.016-in blade does not pass. What is the measured end gap and what is the disposition?
  1. The largest blade that passes with slight drag is 0.014 in — this is the measured end gap.
  2. OEM specification range: 0.010–0.016 in.
  3. Check: 0.014 in is greater than 0.010 in and less than or equal to 0.016 in.
  4. The measurement falls within the specified range.
Answer: Measured end gap = 0.014 in. This is within the OEM specification of 0.010–0.016 in. Disposition: reuse.
3. The OEM nominal crankshaft main journal diameter is 1.1800 in with a wear limit of −0.002 in (minimum 1.1780 in). You measure the journal diameter with an outside micrometer and read 1.1773 in. Calculate the deviation and determine the disposition.
  1. OEM nominal: 1.1800 in; minimum allowable (wear limit): 1.1780 in.
  2. Calculate deviation: 1.1773 − 1.1800 = −0.0027 in (journal is undersized by 0.0027 in).
  3. Compare to wear limit: 1.1773 in < 1.1780 in minimum — the journal exceeds the allowable wear.
  4. Determine whether regrinding to an undersize is available per the OEM manual; if no undersize option is listed, disposition is replace. If an undersize bearing set is available, disposition is resurface (regrind) and note the required undersize.
Answer: Deviation = −0.0027 in. The journal is below the OEM minimum of 1.1780 in. Disposition: resurface (regrind) if an undersize bearing is listed in the OEM manual, or replace the crankshaft if no undersize option exists.
4. You are measuring armature air gap. The OEM specification is 0.006–0.010 in. You insert a 0.006-in feeler blade — it slides through freely with no drag. You insert a 0.008-in blade — slight drag felt. You insert a 0.010-in blade — it will not enter the gap. What do you record and what action is needed?
  1. The 0.006-in blade passes freely, meaning the gap is larger than 0.006 in.
  2. The 0.008-in blade passes with slight drag — this is the measured gap: 0.008 in.
  3. The 0.010-in blade does not pass, confirming the gap is less than 0.010 in.
  4. OEM specification: 0.006–0.010 in. Measured: 0.008 in. 0.008 in is within this range.
Answer: Measured armature air gap = 0.008 in. This is within the OEM specification of 0.006–0.010 in. No adjustment required; record as within spec.
In class: Students perform a controlled, sequenced disassembly using the OEM service manual torque and sequence references, photographing assemblies before removal, tagging all fasteners and small parts in labeled containers, and applying lockout/tagout at every stage; the instructor spot-checks PPE compliance and component organization at least once during this lesson block. · Students measure all critical wear components — cylinder bore diameter, piston ring end gap, crankshaft journal diameter, valve stem clearance, armature air gap, carburetor float height — using feeler gauges, outside micrometers, and telescoping gauges as appropriate, recording every measurement on a two-column spec sheet (OEM spec vs. measured) and calculating the deviation. · Using the completed spec sheet, students compile a parts disposition list and prepare a revised parts order with actual part numbers pulled from the OEM parts lookup, then update the work order with the final labor estimate; the instructor verifies measurement technique by observing the student take at least three measurements live and asking the student to state the OEM specification from memory or by correctly locating it in the service manual.
Materials: OEM service manuals and parts microfiche/online lookup, outside micrometers (0–1 in, 1–2 in), telescoping gauges, feeler gauge sets, bore gauges or small-hole gauges, digital calipers, torque wrenches, impact tools, parts cleaning tank with approved solvent, parts trays, labeling tape, two-column spec sheets, revised work order forms, PPE
Check: Instructor reviews the completed two-column spec sheet and parts disposition list; all measurements must be recorded with correct units, each measurement must be compared to the OEM spec with a clear pass/replace notation, and measurement technique must be observed as correct on at least three live checks — pass requires all measurements recorded correctly and technique rated satisfactory by the instructor with no unsafe practices observed.

Practice check8

Answer each question, then check it. Nothing is recorded; this is your own practice.

Safety and Environmental Practices
1. Before beginning disassembly of a small engine on the workbench, which action MUST be completed first?
Engine Systems — Disassembly and Reassembly
2. When removing a cylinder head with multiple fasteners, the correct loosening procedure is to:
Engine Systems — Precision Measurement
3. A technician measures cylinder bore diameter with a telescoping gauge and outside micrometer. At the top of ring travel the reading is 2.5700 in; at the bottom of the bore the reading is 2.5665 in. This difference indicates:
Engine Systems — Parts Evaluation
4. The OEM specification for piston ring end gap is 0.008–0.014 in. You measure 0.019 in. The correct parts disposition is:
Engine Systems — Precision Measurement
5. Which tool combination is correct for measuring cylinder bore diameter?
Engine Systems — Precision Measurement
6. On a parts disposition spec sheet, a technician records an OEM crankshaft journal nominal of 1.3750 in and a measured value of 1.3742 in. What is the deviation and what does a negative value indicate?
Work Order and Documentation
7. What is the PRIMARY purpose of photographing assemblies before disassembly?
Ignition Systems
8. When measuring armature air gap, which tool is used and where is the gap measured?
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