GM Firebird 1967-1981 Repair Guide

Cylinder Block

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RECONDITIONING



Main Bearing Clearance Check

Invert the engine, and remove the cap from the bearing to be checked. Using a clean, dry rag, thoroughly clean all oil from crankshaft journal and bearing insert.

Plastigagereg; is soluble in oil; therefore, oil on the journal or bearing could result in erroneous readings.

See Figures 1 and 2

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Fig. Fig. 1: Apply a strip of gauging material to the bearing journal, then install and torque the cap



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Fig. Fig. 2: After the cap is removed again, use the scale supplied with the gauge material to check clearances

Place a piece of Plastigage along the full length of journal, reinstall the cap, and tighten it to specifications.

Specifications are given in the engine specifications earlier in this section.

Remove the bearing cap, and determine bearing clearance by comparing the width of Plastigage to the scale on the Plastigage envelope. Journal taper is determined by comparing the width of the Plastigage strip near its ends. Rotate the crankshaft 90ordm; and retest, to determine journal eccentricity.

Do not rotate the cranksahft with Plastigage installed.

If the bearing insert and journal appear intact, and are within tolerances, no further main bearing service is required. If the bearing or journal appear defective, cause of failure should be determined before replacement.

Remove the crankshaft from the block. Measure the main bearing journals at each end twice (90ordm; apart) using a micrometer, to determine diameter, journal taper and eccentricity. If journals are within tolerances, reinstall bearing caps at their specified torque. Using a telescope gauge and micrometer, measure the bearing I.D. parallel to the piston axis and at 30ordm; on each side of the piston axis. Subtract the journal O.D. from the bearing I.D. to determine oil clearance. If the crankshaft journals appear defective, or do not meet tolerances, there is no need to measure bearings; for the crankshaft will require grinding and/or undersize bearings will be required. If the bearing appears defective, cause for failure should be determined prior to replacement.

Connecting Rod Bearing Clearance Check

Connecting rod bearing clearance is checked in the same manner as main bearing clearance, using Plastigage. Before removing the crankshaft, connecting rod side clearance also should be measured and recorded.

Checking connecting rod bearing clearance, using a micrometer, is identical to checking main bearing clearance. If no other service is required, the piston and rod assemblies need not be removed.

Crankshaft Removal See Figures 3 and 4

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Fig. Fig. 3: Using a number punch, matchmark the connecting rods with their respective caps



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Fig. Fig. 4: If a number punch is not handy, use a scribe and mark the connecting rods and caps with the number of the cylinder cylinder No. 1 should get one scribe mark, cylinder No. 2 gets two, etc.

Using a punch, mark the corresponding main bearing caps and saddles according to position (i.e., one punch on the front main cap and saddle, two on the second, three on the third, etc.). Using number stamps, identify the corresponding connecting rods and caps, according to the cylinder (if no numbers are present). Remove the main and connecting rod caps, and place sleeves of plastic tubing or vacuum hose over the connecting rod bolts, to protect the journals as the crankshaft is removed. Lift the crankshaft out of the block.

Cylinder Bore Ridge Removal See Figure 5

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Fig. Fig. 5: Before removing the piston/connecting rod assembly, the upper ridge, created by piston ring wear on the bore, must be ground down with a ridge reamer

In order to facilitate removal of the piston and connecting rod, the ridge at the top of the cylinder (unworn area; see illustration) must be removed. Place the piston at the bottom of the bore, and cover it with a rag. Cut the ridge away using a ridge reamer, exercising extreme care to avoid cutting too deeply. Remove the rag, and remove cuttings that remain on the piston.


CAUTION
If the ridge is not removed, and new rings are installed, damage to rings will result.Piston and Connecting Rod Removal

Invert the engine, and push the pistons and connecting rods out of the cylinders. If necessary, tap the connecting rod boss with a wooden hammer handle, to force the piston out.


CAUTION
Do not attempt to force the piston past the cylinder ridge.Crankshaft Service

Ensure that all oil holes and passages in the crankshaft are open and free of sludge. If necessary, have the crankshaft ground to the largest possible undersize.

Have the crankshaft Magnafluxed, to locate stress cracks. Consult a machinist concerning additional service procedures, such as surface hardening (e.g., nitriding, Tuftriding) to improve wear characteristics, cross drilling and chamfering the oil holes to improve lubrication, and balancing.

Freeze Plug Removal

Drill a small hole in the middle of the freeze plugs. Thread a large sheet metal screw into the hole and remove the plug with a slide hammer.

Oil Gallery Plug Removal

Threaded plugs should be removed using an appropriate (usually square) wrench. To remove soft, pressed in plugs, drill a hole in the plug, and thread in a sheet metal screw. Pull the plug out by the screw using pliers.

Cylinder Block Cleaning See Figure 6

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Fig. Fig. 6: Place rubber hose over the connecting rod studs to protect the crank and bores from damage

See Figures 7, 8, 9 and 10

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Fig. Fig. 7: Carefully tap the piston out of the bore using a wooden dowel



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Fig. Fig. 8: Using a punch and hammer, the freeze plug can be loosened in the block



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Fig. Fig. 9: Once the freeze plug has been loosened, it can be removed from the block

Do not hot-tank aluminum parts.

Have the block hot-tanked to remove grease, corrosion, and scale from the water jackets.

Consult the operator to determine whether the camshaft bearings will be damaged during the hot-tank process.

Cylinder Block Inspection

Visually inspect the block for cracks or chips. The most common locations are as follows:



Adjacent to freeze plugs.
 
Between the cylinders and water jackets.
 
Adjacent to the main bearing saddles.
 
At the extreme bottom of the cylinders.
 

Check only suspected cracks using spot check dye. If a crack is located, consult a machinist concerning possible repairs.

Magnaflux the block to locate hidden cracks. If cracks are located, consult a machinist about feasibility of repair.

Oil Gallery and Freeze Plugs Installation

Coat freeze plugs with sealer and tap into position using a piece of pipe, slightly smaller than the plug, as a driver. To ensure retention, stake the edges of the plugs. Coat threaded oil gallery plugs with sealer and install. Drive replacement soft plugs into block using a large drift as driver.

Rather than reinstall lead plugs, drill and tap the holes, and install threaded plugs.

Cylinder Bore Inspection

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Fig. Fig. 10: The cylinder bore can be measured with a dial gauge

See Figures 11, 12 and 13

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Fig. Fig. 11: When inspecting the bore, measure in numerous positions to determine out-of-round or taper



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Fig. Fig. 12: If a telescoping gauge is used to measure the bore, the gauge must then be measured itself with a micrometer



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Fig. Fig. 13: A telescoping gauge may be used to measure the cylinder bore diameter

Visually inspect the cylinder bores for roughness, scoring, or scuffing. If evident, the cylinder bore most be bored or honed oversize to eliminate imperfections, and the smallest possible oversize piston used. The new pistons should be given to the machinist with the block, so that the cylinders can be bored or honed exactly to the piston size (plus clearance). If no flaws are evident, measure the bore diameter using a telescope gauge and micrometer, or dial gauge, parallel and perpendicular to the engine centerline, at the top (below the ridge) and bottom of the bore. Subtract the bottom measurements from the top to determine taper, and the parallel to the centerline measurements from the perpendicular measurements to determine eccentricity. If the measurements are not within specifications, the cylinder must be bored or honed, and an oversize piston installed. If the measurements are within specifications the cylinder may be used as is, with only finish honing.

Cylinder Block Bearing Alignment Inspection See Figure 14

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Fig. Fig. 14: Use a straightedge set into the crankshaft bearing bores to determine if align-boring is necessary

Prior to submitting the block for boring, perform the following operation(s).

Remove the upper bearing inserts. Place a straightedge in the bearing saddles along the centerline of the crankshaft. If clearance exists between the straightedge and the center saddle, the block must be align-bored.

Deck Height Inspection

The deck height is the distance from the crankshaft centerline to the block deck. To measure, invert the engine, and install the crankshaft, retaining it with the center main cap. Measure the distance from the crankshaft journal to the block deck, parallel to the cylinder centerline. Measure the diameter of the end (front and rear) main journals, parallel to the centerline of the cylinders, divide the diameter in half, and subtract it from the previous measurement. The results of the front and rear measurements should be identical. If the difference exceeds .005 in., the deck height should be corrected.

Block deck height and warpage should be corrected at the same time.

Block Deck Inspection

Using a straightedge and feeler gauges, check the block deck for warpage in the same manner that the cylinder head is checked (see Cylinder Head Reconditioning). If warpage exceeds specifications, have the deck resurfaced.

In certain cases a specification for total material removal (Cylinder head and block deck) is provided. This specification must not be exceeded.

Piston and Connecting Rod Cleaning and Inspection See Figure 15

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Fig. Fig. 15: Measure between the points shown to determine connecting rod length all rods should be the same length

See Figures 16, 17, 18 and 19

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Fig. Fig. 16: Use a ring expander tool to remove the piston rings



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Fig. Fig. 17: Clean the piston grooves using a ring groove cleaner



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Fig. Fig. 18: You can use a piece of an old ring to clean the piston grooves, BUT be careful, the ring is sharp

Using a ring expander, remove the rings from the piston. Remove the retaining rings (if so equipped) and remove piston pin.

If the piston pin must be pressed out, determine the proper method and use the proper tools; otherwise the piston will distort.

Clean the ring grooves using an appropriate tool, exercising care to avoid cutting too deeply. Thoroughly clean all carbon and varnish from the piston with solvent.


CAUTION
Do not use a wire brush or caustic solvent on pistons.

Inspect the pistons for scuffing, scoring, cracks, pitting, or excessive ring groove wear. If wear is evident, the piston must be replaced. Check the connecting rod length by measuring the rod from the inside of the large end to the inside of the small end using calipers (see illustration). All connecting rods should be equal length. Replace any rod that differs from the others in the engine.

Have the connecting rod alignment checked in an alignment fixture by a machinist. Replace any twisted or bent rods.

Magnaflux the connecting rods to locate stress cracks. If cracks are found, replace the connecting rod.

Piston-to-Cylinder Fitting

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Fig. Fig. 19: Measure the piston's outer diameter using a micrometer

Using a telescope gauge and micrometer, or a dial gauge, measure the cylinder bore diameter perpendicular to the piston pin, 2 1 / 2 in. below the deck. Measure the piston perpendicular to its pin on the skirt. The difference between the two measurements is the piston clearance. If the clearance is within specifications or slightly below (after boring or honing), finish honing is all that is required. If the clearance is excessive, try to obtain a slightly larger piston to bring clearance within specifications. Where this is not possible, obtain the first oversize piston, and hone (or if necessary, bore) the cylinder to size.

Piston-to-Connecting Rod Assembly See Figure 20

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Fig. Fig. 20: Install the lockrings to hold the piston (wrist) pins in their bore, if the engine is equipped with full-floating wrist pins

Inspect piston pin, connecting rod small end bushing, and piston bore for galling, scoring, or excessive wear. If evident, replace defective part(s). Measure the I.D. of the piston boss and connecting rod small end, and the O.D. of the piston pin. If within specifications, assemble piston pin and rod.


CAUTION
If piston pin must be pressed in, determine the proper method and use the proper tools; otherwise the piston will distort.

Install the lockrings; ensure that they seat properly. If the parts are not within specifications, determine the service method for the type of engine. In some cases, piston and pin are serviced as an assembly when either is defective. Others specify reaming the piston and connecting rods for an oversize pin. If the connecting rod bushing is worn, it may in many cases be replaced. Reaming the piston and replacing the rod bushing are machine shop operations.

Camshaft Cleaning and Inspection See Figures 21 and 22

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Fig. Fig. 21: The camshaft can be checked for run-out with a dial indicator gauge as shown



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Fig. Fig. 22: To determine camshaft lobe height, subtract measurement B from measurement A

Degrease the camshaft, using solvent, and clean out all oil holes. Visually inspect cam lobes and bearing journals for excessive wear. If a lobe is questionable, check all lobes as indicated below. If a journal or lobe is worn, the camshaft must be reground or replaced.

If a journal is worn, there is a good chance that the bushings are worn.

If lobes and journals appear intact, place the front and rear journals in V-blocks, and rest a dial indicator on the center journal. Rotate the camshaft to check straightness. If deviation exceeds .001 in., replace the camshaft.

Check the camshaft lobes with a micrometer, by measuring the lobes from the nose to base and again at 90ordm; (see illustration). The lift is determined by subtracting the second measurement from the first. If all exhaust lobes and all intake lobes are not identical, the camshaft must be reground or replaced.

Camshaft Bearing Removal and Installation See Figures 23, 24 and 25

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Fig. Fig. 23: To remove the camshaft bearings on Overhead Valve (OHV) engines, a special puller is necessary



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Fig. Fig. 24: Using a ball type cylinder hone is an easy way to hone the cylinder bore



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Fig. Fig. 25: A properly cross-hatched cylinder bore

This procedure applies only to OHV engines.

If excessive wear is indicated, or if the engine is being completely rebuilt, camshaft bearings should be replaced as follows: Drive the camshaft rear plug from the block. Assemble the removal puller with its shoulder on the bearing to be removed. Gradually tighten the puller nut until bearing is removed. Remove remaining bearings, leaving the front and rear for last. To remove front and rear bearings, reverse position of the tool, so as to pull the bearings in toward the center of the block. Leave the tool in this position, pilot the new front and rear bearings on the installer, and pull them into position: Return the tool to its original position and pull remaining bearings into position.

Ensure that oil holes align when installing bearings.

Replace camshaft rear plug, and stake it into position to aid retention.

Cylinder Honing

Chuck a flexible drive hone into a power drill, and insert it into the cylinder. Start the hone, and move it up and down in the cylinder at a rate which will produce approximately a 60ordm; crosshatch pattern.

Do not extend the hone below the cylinder bore.

After developing the pattern, remove the hone and recheck piston fit. Wash the cylinders with a detergent and water solution to remove abrasive dust, dry, and wipe several times with a rag soaked in engine oil.

Piston Ring End-gap Inspection See Figures 26 and 27

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Fig. Fig. 26: Before installing the rings on the piston, check ring end-gap as shown



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Fig. Fig. 27: Checking the ring-to-ring groove clearance

Compress the piston rings to be used in a cylinder, one at a time, into that cylinder, and press them approximately 1 in. below the deck with an inverted piston. Using feeler gauges, measure the ring end-gap, and compare to specifications. Pull the ring out of the cylinder and file the ends with a fine file to obtain proper clearance.


CAUTION
If inadequate ring end-gap is utilized, ring breakage will result.Piston Ring Installation

Inspect the ring grooves in the piston for excessive wear or taper. If necessary, recut the grooves(s) for use with an overwidth ring or a standard ring and spacer. If the groove is worn uniformly, overwidth rings, or standard rings and spacers may be installed without recutting. Roll the outside of the ring around the groove to check for burrs or deposits. If any are found, remove with a fine file. Hold the ring in the groove, and measure side clearance. If necessary, correct as indicated above.

Always install any additional spacers above the piston ring.

The ring groove must be deep enough to allow the ring to seat below the lands (see illustration). In many cases, a "go-no-go" depth gauge will be provided with the piston rings. Shallow grooves may be corrected by recutting, while deep grooves require some type of filler or expander behind the piston. Consult the piston ring supplier concerning the suggested method. Install the rings on the piston, lowest ring first, using a ring expander.

Position the ring as specified by the manufacturer.

Consult the engine service procedures earlier in this section for details concerning specific engines.

Camshaft Installation

This procedure applies only to OHV engines.

Liberally lubricate the camshaft lobes and journals, and install the camshaft.


CAUTION
Exercise extreme care to avoid damaging the bearings when inserting the camshaft.

Install and tighten the camshaft thrust plate retaining bolts.

See the engine service procedures earlier in this section for details concerning specific engines.

Camshaft End-play Inspection See Figures 28 and 29

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Fig. Fig. 28: The camshaft end-play can be checked with a feeler gauge ...



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Fig. Fig. 29: ... or with a dial indicator by pushing the camshaft back and forth and measuring the difference OHV engines

This procedure applies only to OHV engines.

Using feeler gauges, determine whether the clearance between the camshaft boss (or gear) and backing plate is within specifications. Install shims behind the thrust plate, or reposition the camshaft gear and retest end-play. In some cases, adjustment is by replacing the thrust plate.

See the engine service procedures earlier in this section for details concerning specific engines.

Mount a dial indicator stand so that the stem of the dial indicator rests on the nose of the camshaft, parallel to the camshaft axis. Push the camshaft as far in as possible and zero the gauge. Move the camshaft outward to determine the amount of camshaft end-play. If the end-play is not within tolerance, install shims behind the thrust plate, or reposition the camshaft gear and retest.

See the engine service procedures earlier in this section for details concerning specific engines.

Rear Main Seal Installation

See the engine service procedures earlier in this section for details concerning specific engines.

Crankshaft Installation See Figures 30, 31 and 32

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Fig. Fig. 30: The upper crankshaft bearing shells can be removed without removing the crankshaft itself by using a roll-out pin



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Fig. Fig. 31: A roll-out pin can be fabricated out of a common cotter pin, as shown



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Fig. Fig. 32: After installing the crankshaft or new bearings, align the thrust bearing as shown

Thoroughly clean the main bearing saddles and caps. Place the upper halves of the bearing inserts on the saddles and press into position.

Ensure that the oil holes align.

Press the corresponding bearing inserts into the main bearing caps. Lubricate the upper main bearings, and lay the crankshaft in position. Place a strip of Plastigagereg; on each of the crankshaft journals, install the main caps, and torque to specifications. Remove the main caps, and compare the Plastigagereg; to the scale on the Plastigagereg; envelope. If clearances are within tolerances, remove the Plastigagereg;, turn the crankshaft 90ordm;, wipe off all oil and retest. If all clearances are correct, remove all Plastigagereg;, thoroughly lubricate the main caps and bearing journals, and install the main caps. If clearances are not within tolerance, the upper bearing inserts may be removed, without removing the crankshaft, using a bearing roll-out pin (see illustration). Roll in a bearing that will provide proper clearance, and retest. Tighten all main caps, excluding the thrust bearing cap, to specifications. Tighten the thrust bearing cap finger-tight. To properly align the thrust bearing, pry the crankshaft the extent of its axial travel several times, the last movement held toward the front of the engine, and tighten the thrust bearing cap to specifications. Determine the crankshaft end-play and bring within tolerance with thrust washers.

Crankshaft End-play Inspection See Figures 33 through 40

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Fig. Fig. 33: The crankshaft end-play can be measured with a feeler gauge between the crankshaft throw and the thrust bearing face



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Fig. Fig. 34: A dial gauge may be used to check crankshaft end-play



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Fig. Fig. 35: Carefully pry the shaft back and forth while reading the dial gauge for play



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Fig. Fig. 36: A dial gauge may also be used to check crankshaft run-out



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Fig. Fig. 37: Mounting a dial gauge to read crankshaft run-out



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Fig. Fig. 38: Turn the crankshaft slowly by hand while checking the gauge



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Fig. Fig. 39: Most pistons are marked to indicate positioning in the engine (usually a mark means the side facing front)



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Fig. Fig. 40: Installing the piston into the block using a ring compressor and the handle of a hammer

Mount a dial indicator stand on the front of the block, with the dial indicator stem resting on the nose of the crankshaft, parallel to the crankshaft axis. Pry the crankshaft the extent of its travel rearward, and zero the indicator. Pry the crankshaft forward and record crankshaft end-play.

Crankshaft end-play also may be measured at the thrust bearing, using feeler gauges (see illustration).

Piston Installation

Press the upper connecting rod bearing halves into the connecting rods, and the lower halves into the connecting rod caps. Position the piston ring gaps according to specifications (see car section), and lubricate the pistons. Install a ring compressor on a piston, and press two long (8 in.) pieces of plastic tubing over the rod bolts. Using the tubes as a guide, press the pistons into the bores and onto the crankshaft with a wooden hammer handle. After seating the rod on the crankshaft journal, remove the tubes and install the cap finger-tight. Install the remaining pistons in the same manner. Invert the engine and check the bearing clearance at two points (90ordm; apart) on each journal with Plastigagereg;.

Do not turn the crankshaft with Plastigagereg; installed.

If clearance is within tolerances, remove all Plastigagereg;, thoroughly lubricate the journals, and tighten the rod caps to specifications. If clearance is not within specifications, install different thickness bearing inserts and recheck.


CAUTION
Never shim or file the connecting rods or caps.

Always install plastic tube sleeves over the rod bolts when the caps are not installed, to protect the crankshaft journals.

Connecting Rod Side Clearance Inspection

Determine the clearance between the sides of the connecting rods and the crankshaft, using feeler gauges. If clearance is below the minimum tolerance, the rod may be machined to provide adequate clearance. If clearance is excessive, substitute an unworn rod, and recheck. If clearance is still outside specifications, the crankshaft must be welded and reground, or replaced.

Timing Chain or Belt Inspection

Visually inspect the timing chain for broken or loose links, and replace the chain if any are found. If the chain will flex sideways, it must be replaced. Install the timing chain as specified. Be sure the timing belt is not stretched, frayed or broken.

If the original timing chain is to be reused, install it in its original position.

Timing Gear Backlash and Run-out Inspection See Figures 41 and 42

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Fig. Fig. 41: Use a dial indicator to check for timing gear backlash ...



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Fig. Fig. 42: ... and camshaft gear run-out

This procedure applies to OHV 250 engines only.

Mount a dial indicator with its stem resting on a tooth of the camshaft gear (as illustrated). Rotate the gear until all slack is removed, and zero the indicator. Rotate the gear in the opposite direction until slack is removed, and record gear backlash. Mount the indicator with its stem resting on the edge of the camshaft gear, parallel to the axis of the camshaft. Zero the indicator, and turn the camshaft gear one full turn, recording the run-out. If either backlash or run-out exceed specifications, replace the worn gear(s).

 
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