1-3.1 Cylinder model 1GM

Cylinder wear is measured with a cylinder gauge. The amount of cylinder wear becomes greater as the piston nears the top, and it becomes greatest at the portion of the top ring when the piston is at top dead center. The reason for this is that when the piston is at the top position, lateral pressure is high due to the high explosive pressure, and lubrication is very difficult due to the high temperature. Therefore, the amount of wear must be measured in the at least three positions, namely the top, middle and bottom of the cylinder.

Although the greatest wear is at the top of the cylinder, the piston ring does not side with the cylinder at the topmost position. Therefore, a step-like pattern is formed between the worn part and the non-worn part.

Furthermore, wear is liable to occur along the rotating direction of the crankshaft due to the lateral pressure of the piston on the other hand, wear occurs in the direction of the crankshaft center line due to the thrust of the crankshaft and the angle of the connecting rod.

Therefore, the amount of wear must be measured in both directions of the crankshaft rotation and crankshaft centerline. When the difference of these two values (i.e. circularity wear) is large, the cylinder must be repaired.

When the result indicates that eccentric and circularity wear exceed the specific limit, the cylinder must be rebored.

1-3.2 Boring the cylinder

When wear on the inside of the cylinder is excessive, rectify by machining. This is what is known as boring.
(1) Dimension to be bored

The Cylinder must be bored to the same dimension as an over-size piston.

O.D. of standard piston	O.D. of over-size piston
Ø72 (2.3846)	Ø72.25 (2.8445)
mm (in.)

(2) Limit of cylinder's expanded I.D.

Never bore the cylinder beyond the limit of expanded inner diameter, because no over-size piston is available for that dimension, besides which there is danger in having too thin a will thickness.

I.D. of standard cylinder	I.D. of over-size cylinder
Ø72 (2.3846)	Ø72.25+0.03 (2.8445~2.8457)
mm (in.)

(2) Honing

The inside surface of the cylinder must be honed after being bored in order to remove machine tool marks.

As the cylinder is subjected to thermal expansion and high pressure repeatedly, it will not recover its original shape after the engine has stopped and cooled down and will be distorted. The Distortion is mainly caused by construction and material differences of the cylinder, but may arise from the cylinder head bolts being tightened in the wrong order or an uneven tightening torque of the bolts when assembling. If there is any distortion a the upper surface of he cylinder, it will cause compression pressure leakage, gas leakage or water leakage as a clearance is formed around the cylinder head even though the cylinder head is thoroughly secured.
(1) How to measure distortion of the upper surface of the cylinder:

The amount of distortion is measured by placing a straight edge on the upper surface of the cylinder and inserting a thickness gauge between the upper surface of the cylinder and the straight scale.

Measurement is to be carried out on the 4 sides and the 2 diagonal lines as shown in the figure, and the largest value of clearance for each measurement is to be taken as the amount of distortion.

1-5.1 Purpose of cup plug

In order to minimize the danger of cylinder block breakage caused by the cooling water freezing, a cup plug is provided at the side of the cylinder block to prevent damage by frost.

In the event the freezing cooling water has caused the cup plug to come out repair the following way.

In cold weather it is necessary to drain the cooling water completely fro the inside of the cylinder block through the cooling water drain pipe.

3.2.1 Measurement of carbon build-up at combustion surfaces and intake and exhaust ports

Visually check for carbon build-up around the combustion surface and the port near the intake and exhaust valve seats, and remove any build-up.

When a large amount of carbon has built up, check the top of the chamber combustion for oil flow a the intake and exhaust valve valve guides, and take suitable corrective action.

3-2.2 Deposit build-up in water passages

Check for build-up deposit in the water passages, and remove any deposit with a deposit remover. When a large amount of deposit has build up, check each part of the cooling system.

3-2.3 Inspection of corrosion in water passages and anti-corrosion zinc

Inspect the state of corrosion of the water passages, and replace the cylinder head when corrosion is severe.

Corrosion and pitting limit: 2mm (0.0787in.)

Inspect the anti-corrosion zinc on the cylinder head cover and replace the zinc when it has worn over the wear limit.

Anticorrosion zinc wear limit: Volumetric ration with new zinc = 1/2

3-2.4 Cracking of combustion surface

The combustion surface is exposed to high temperature, high pressure gas and low temperature air, and is repeatedly flexed during operation. Moreover, it is used under extremely severe conditions, such as the high temperature difference between the combustion surface and cooling water passages.

Inspect the combustion surface for cracking by the color check (see 1-2.5), and replace the cylinder head if any cracking is detected. At the same time, check for signs of overloading and check the cooling water flow.

3-2.5 Cylinder head distortion

The Distortion of the cylinder head causes gasket packing damage, compression leakage, change in compression, etc.

Measure the distortion as described below, and replace the cylinder head when the wear limit is exceeded. Since distortion of the cylinder head is caused by irregular tightening forces, faulty repair of the mounting face, and gasket packing damage, these must also be checked.

Cylinder head distortion
	Wear limit
1GM	0.007 (.0028)
mm (in.)

(1) Clean the cylinder head tightening surface.
(2) Place a straightedge across two symmetrical paints a the four sides of the cylinder head, as shown in the figure.
(3) Insert feeler gauges between the straight edge and cylinder head combustion face.
(4) The thickness of the largest feeler gauge that can be inserted is the amount of distortion.

3-2.6 Cylinder head valve seat

The valve seats become wider with use. If the seats become wider than the maintenance standard, carbon build-up a the seats will cause compression leakage. On the other hand, if the seats are too narrow, they will wear quickly and heat transmission efficiency will deteriorate. clan the carbon and other foreign matter from the valve seats, and check that the seats are not scored or dented.

Measure the seat width with vernier calipers, and repair of replace the seat when the wear limit is exceeded.

When the valves have been lapped and/or ground, measure the amount of valve recess, and replace the valve when the wear limit is exceeded

(1) Lapping the valve seat.

When scoring and pitting of the valve seat is slight, coat the seat with valve compound mixed oil, and lap the seat with a lapping tool.

At this time, be sure that the compound does not flow into the valve stem and valve guide.

(2) Correcting valve seat width.

When the valve seat is heavily pitted and when the seat width must be corrected, repair with a seat grinder.

1) Repair pitting of the seat face with a 45 degree grinder.
2) Since the valve seat is larger than the initial valve, correct the seat width to the maintenance standard by grinding the inside face of the seat with a 70 degree grinder.
3) Grind the outside face of the valve seat with a 15 degree grinder, and finish the seat width to the standard value.
4) Mix the lapping compound with oil, and lap the valve.
5) Finally, lap with oil.

NOTE: When the valve seat has been corrected with a seat grinder, insert an adjusting shim between the valve spring and the cylinder liner.

3-2.7 Measuring valve sinkage

When a valve has been lapped many times, the valve will be recessed and will adversely affect combustion performance. Therefore, measure the valve shrinkage, and replace the valve and cylinder head when the wear limit is exceeded.

3-2.8 Rocker arm support positioning pin

Check if the guide pin is damaged or if the hole is clogged, and replace if the pin is faulty.

When dismounting and remounting the cylinder head, the mounting bolts must be removed and installed gradually and in the prescribed sequence to prevent damaging the cylinder head. Since the tightening torque and tightening sequence of the mounting bolts when remounting the cylinder head are especially important form the standpoint of the engine performance, the following items must be strictly observed.

3.3.1 Cylinder head assembly sequence.

(1) Check for loose cylinder head stud bolts, and lock any loose bolts with two nuts and then tighten to the prescribed torque.

The Cylinder head is fitted to the engine with 4 stud bolts in the model 1GM.

	1GM
Stud bolt diameter of cylinder head	M10 (0.3937in.)
Cylinder head stud bolt tightening torque	2.5~3.0 kg-m (18.1~21.7 ft-lb)
mm (in.)

(2) Checking the gasket packing mounting face.

Confirm correct alignment of the front and rear of the gasket packing, and install the packing by coating both sides with Three Bond 50.

Assemble the gasket packing keeping the flat surface upward (Cylinder head side). Make sure that the gasket hole aligns with the drilled hole in the cooling water passage in the cylinder block.

(3) Installing the cylinder head assembly

Position the cylinder head assembly parallel to the top of the cylinder block, and install the assembly on the block. being careful that the cylinder head assembly does not touch the threads of the cylinder head bolts.

3-3.2 Tightening the cylinder head bolts and nuts

(1) Cylinder head fixing nuts and bolts, tightening torque, tightening sequence.

3.4.1 Inspecting and measuring the intake and exhaust valves

(1) Valve seat wear and contact width.

Inspect valve seats for carbon build-up and heavy wear.

Also check if each valve seat contact width is suitable. If the valve seat contact width is narrower than the valve seat width, the seat angle must be checked and corrected,

	1GM
Intake valve diameter	Ø32 (1.2598in.)
Exhaust valve diameter	Ø26 (1.0236in.)
Valve seat width	3.15 (0.1240in.)
Valve seat angle	90°
mm (in.)

NOTE: Note that the intake valve and exhaust valve have a different diameter.

	1GM
Intake valve diameter	Ø32 (1.2598in.)
Exhaust valve diameter	Ø26 (1.0236in.)
Valve seat width	3.15 (0.1240in.)
Valve seat angle	90°
mm (in.)

 

(2) Valve stem bending and wear.

Check for valve stem wear and strain, and repair when such damage is light. Measure the outside diameter and bend, and replace the valve when the wear limit is exceeded.

(3) Valve seat hairline cracks

Inspect the valve seat by the color check, and replace the seat if cracked.

3-4.2 Inspecting and measuring valve guides

The same valve guide is used both for the intake and exhaust valves in the model 1GM engine. It has a gas blow opening cut in the inner face at the bottom.

(1) Floating of the intake and exhaust valve guides.

Check for intake and exhaust valve guide looseness and floating with a test hammer, and replace loose or floating guides with guides having an oversize outside diameter.

			Maintenance standard	Clearance at assembly	Maximum allowable clearance	Wear limit
1GM	Intake	Valve guide inside diameter (after assembly)	Ø7 (0.2756)	0.045~0.070 (0.0018~0.028)	0.15 (0.0059)	Ø7.08 (0.2787)
		Stem outside diameter				Ø6.9 (0.2717)
	Exhaust	Valve guide inside diameter (after assembly)	Ø7 (0.2756)	0.045~0.070 (0.0018~0.028)	0.15 (0.0059)	Ø7.08 (0.2787)
		Stem outside diameter				Ø6.9 (0.2717)
mm (in.)

 

(2) Measuring the valve guide inside diameter.

Measure the valve guide inside diameter and clearance and replace the guide when wear exceeds the wear limit.

	1GM
Valve guide protrusion	7 (0.2756)
mm (in.)

 

(3) Replacing the intake / exhaust valve guide.

1) Using a special tool for extracting and inserting the valve guide, extract the valve guide.

2) Using the above tool, drive the valve guide into position by starting from the valve spring side and finish the inside diameter with a reamer.

 

	1GM
Amount of interference of valve guide	0.005 ~ 0.034 (0.0002 ~ 0.0013)
mm (in.)

.

As the valve guide for the model 1GM does not have a groove, fit the intake and exhaust valve guides after checking its dimension (valve guide protrusion) and marking it.

3-4.3 VaIve spring

(1) Valve spring inclination.

Since inclination of the valve spring is a direct cause of eccentric contact of the valve stem, always check it at disassebly.

Stand the valve upright on a stool, and check if the entire spring contacts the gauge when a square gauge is placed against the ouside diameter of the valve spring.

If there is a gap between the gauge and spring, measure the gap with a feeler gauge.

When the valve spring inclination exceeds the wear limit, replace the spring.

	Maintenance standard
Valve spring free length	38.5 (1.5157)
mm (in.)
Allowable tilt value (B/A) is less than 0.035

(2) Valve spring free length

Measure the free length of the valve spring, and replace when the spring wear limit is exceeded.

Also, measure the tension of the spring with a spring tester. If the tension is below the prescribed limit, replace the spring.

	1GM
	Maintenance standard	Wear limit
Valve spring free length	38.5mm (1.5157in.)	37mm (1.4767in.)
Length when attached	29.2 mm (1.1496in.)	---
Load applied attached	16.16kg (35.63 lb)	13.7kg (30.20 lb)

3-4.4 Valve stem seal

A valve stem seal is assembled at the top of the valve guide and the valve stem chambe to limit oil is sucked into the combustion chamber through the valve guide (oil down) and thus prevent an increase in oil consumption.

The valve stem seal must always be replaced whenever it has been removed.

When assembling, coat the valve stem with engine oil before inserting.

3-4.5 Spring retainer and spring cotter pin

Inspect the inside face of the spring retainer and the ouside surface of the spring cotter pin, and the contact area of the spring cotter pin inside surface and the notch in the head of the valve stem. Replace the spring retainer and spring cotter pin when the contact are is less than 70% of when the spring cotter pin has been recessed because of wear.

3-5.1 Precombustion chamber

Remove the packing and insulation packing at the precombustion chamber's front and rear chambers, and inspect. Check for burning at the front end of the precombustion chamber front chamber, and acid corrosion at the precombustion chamber rear chamber, and for burned packing.

Replace if faulty.

3-5.2 Insulation packing

The insulation packing prevents transmission of heat from the precombustion chamber to the nozzle valve and serves to improve the nozzle's durability.

Always put in new insulation packing when it has been disassembled.

3-5.3 Top clearance

Top clearance is the size of the gap between the cylinder head combustion surface and the top of the piston at top dead center.

Since top clearance has considerable effect on the combustion performance and the starting characterictic of the engine, it must be check periodically.

(1) Top clearance measurement

1) Check the cylinder head mounting bolts and tightening torque.

2) Remove the fuel injection valve and precombustion chamber.

3) Lower the piston at the side to be measured.

4) Insert quality of fuse wire [plastigauge] (Ø1.2mm, 0.472in.) through the nozzle holder hole. (Be carful that the wire does not enter the intake and exhaust valve and the groove in the combustion surface.)

5) Crush the fuse wire [plastigauge] moving the piston to top dead center by slowly cranking the ingine by hand.

6) Lower the piston by hand cranking the engine and remove the crushed fuse wire [plastigauge], being careful not to drop it.

7) Measure the thickness of the crushed part of the fuse wire [plastigauge] with vernier calipers or a micrometer.

	1GM
Top clearance	0.5 (0.0276)
mm (in.)

 

When the top clearance value is not within the above rance, check for damaged gasket packing, distortion of the cylinder head combustion surface, or other abnormal conditions.

Since the intake and exhaust valve rocker arm shaft and bushing clearance and valve head and push rod contact wear are directly related to the valve timing, and have an effect on engine performance, they must be carefuly serviced.

3-6.1 Components of valve rocker arm

The same part is used for both intake valve rocker arm and exhaust valve rocker arm. The bushing is not fitted to the valve rocker arm.

It has a simple construction as the valve rocker arms are fitted to the valve rocker arm support from both side without using the retainer. In the place of a retainer, ther rib of the bonnet cover prevents the rocker arm from comming out.

NOTE: Take care that the valve rocker arms do not get detached from the valve rocker arm shaft when dismantling or assebling. Replace the bonnet carefully when assebling.

3-6.2 Measureing the valve rocker arm shaft and bushing clearance

Measure the ouside diameter of the valve rocker arm shaft and the inside diameter of the bushing, and replace the rocker arm or bushing if the measured value exceeds the wear limit.

Replace a loose valve rocker arm shaft with a new bushing. However, when there is not tightening allowance replace the valve rocker arm.

3-6.3 Valve rocker arm and valve top retainer contact and wear

Check the valve rocker arm and valve top retainer contact, and replace then there is any abnormal wear or peeling.

3-6.4 Valve clearance adjusting screw

Inspect the valve clearance adjusting screw and push rod contact, and replace when there is any abnormal wear or peeling.

3-6.5 Classification of the intake and exhaust valve rocker arms

The same part is used for both the intake valve rocker arm and exhaust valve rocker arm.

Adjustment of the intake and exhaust valve head clearance governs the performance of the engine, and must be performed accuratly. The intake and exhaust valve head clearance must always be checked and readjusted, as required, when the engine is disassembled and reassebleed, and after every 300 hours of operation. Adjust the valve head clearance as described below.

3-7.1 Adjustment [valve lash]

Make this adjustment when the engine is cold.

(1) Remove the valve rocker arm cover.

(2) srank the engine and set the piston to top dead center (TDC) on the compression stroke. [when both the intake and exhaust valve rocker arms are "loose" it is the compression stroke.]

The matching mark is made at the setting hole of the starter motor on all models.

With respect to the 1GM model, a projection which serves as the matching mark is provided in the cast hole of the clutch housing.

NOTE: Set to the position at which the valve rocker arm shaft does not move even when the crankshaft is turned to the left and right, centered around the matching mark.

(3) Check and adjust the intake and exhaust valve head clearance of the piston.

Loosen the valve clearance adjusting screw lock nut, adjust the clearance to the mainetnance standard with a feeler gauge, and retighten the lock nut.

3-7.2 Ajdusting without a feeler gauge

Set the head clearance to zero by tightening the adjusting screw, being careful not to tighten the screw too tight.

Then adjust the valve clearance to the maintenance standard by backing off the adjusting screw by the angle given below.

NOTE: Calculating the backoff angle.

calculate the 2mm advance angle from 1.25mm advance at one turn = 360°

0.2 / 1.25 x 360° = 58°

One side (60°) of the hexagonal nut should be used to measure.

The decompression mechanism is used when the starter motor fails to rotate sufficiently because the battery is weak, and to facillitate starting in cold weather. When the decompression lever is operated, the valve is pushed down, the engine is decompressed, the engine turns over easily and the flywheel inertia increases, thus making starting easy.

3-9.1 Disassembling the cylinder head

When disassembling the cylinder head, group the parts separatly according to cylinder, intake or exhaust to avoid confusion.

(1) Disassembling the rocker arm ass'y

1) Remove the rocker arm ass'y mounting nuts.

2) Remove the rocker arm ass'y.

NOTE: A retainer is not used for the valve rocker arm on the model 1GM and is kept free, therefore the rocker arm can be removed directly.

(2) Removing the precombustion chamber

1) Remove the rear precombustion chamber and packing.

2) Remove the front precombustion chamber adn packing.

(3) Removing the intake and exhaust valve ass'y

1) Set the special tool aht intake and exhaust valve ass'y and depress the valve spring by turning the lever.

2) When the special tool is not available, depress the valve with a spring wrench.

3) Remove the spring cotter pin.

4) Turn the lever of the special tool in the loosening direction, release the valve spring retainer and valve spring.

5) Pull the valve from the cylinder head.

6) Remove the valve stem seal

7) Remove the valve guide.

3-9.2 Re assembling the cylinder head

Before reassembling the cylinder head, wash all the parts, inspect and measure the dimensions of each part, and repair or replace any parts that are abnormal. Be careful not to confuse the parts grouped by cylinder and intake or exhaust.

(1) Assembling the intake and exhaust valves

1) Press the valve guide into the cylinder head.

2) Install the valve stem seal. (Always replace the valve stem seal with a new seal.)

3) Install the valve in the cylinder head.

4) Install the spring and valve spring seat.

5) Install the split collar.

• Using the special tool.

• Using a wrench.

(2) Installing the valve arm ass'y

1) Install the intake and exhaust rocker arms on the rocker arm support.

2) Install the rocker arm support on the cylinder head, then tighten with nuts.

(3) Installing the precombustion chamber

1) Install the from precombustion chamber and packing.

2) Install the rear precombustion chamber and packing. (Always replace the insulation packing.)

2 3-9.1(3) Removing Valves Ass'y

The pistons are made of LO-EX (AC8A-T6) for the lightness and are designed for reduced vibration. The outside of the piston is machined to a special oval shape. During operation, Thermal expansion is small, the optimum clearance between the piston and cylinder wall is maintained, and a stable supply of lubrication is assured.

A complete set of piston rings consists of two compression rings and one oil ring.

To improve the regidity of the piston skirt no ring is installed on the skir itself so that the piston seldom becomes deformed and retains stable contact.

The piston pin is of the floating type. Both ends are fastened with circlips.

Grooves called heat dams are cut fround the top section of the piston. These grooves help dissipate heat and prevent scuffing.

4-2.1 Inspection

(1) Measuring important dimensions.

Measure each important dimension, and replace the piston when the wear limit is exceeded.

(2) Measure the clearance between the piston ring or oil ring and the groove with a thickness gauge.

(3) Piston pin outside contact and ring groove carbon buildup.

Check if the piston ring grooves are clogged with carbon, if the rings move freely, and for abnormal contact around the outside of the piston. Repair or replace the piston if faulty.

4-2.1 Replacing a piston

If the dimension of any part is worn past the wear limit outside or outside of the piston is scored, replace the piston.

(1) Replacement

1) Install the piston pin circlip at one side only.

2) Immerse the piston in 80°C oil for 10 ~ 15 minutes.

3) Remove the piston from the hot oil and place on a bench with the piston head at the bottom.

4) Insert the small end of the connecting rod into th piston, insert the piston pin with a rotating motion, and install the other piston pin circlip.

Use a wooden hammer if necessary.

(2) Precautions

1) Before inserting, check whether the piston pin is in the connecting rod.

2) Coat the piston pin with oil to facilitate insertion.

3) Check that the connecting rod and piston move freely.

4) Insert the pin quickly, before the piston cools.

4-3.1 Piston pin

Measure the dimensions of the piston pin, and replace the pin if it is worn past the wear limit or severely scored.

4-3.2 Piston pin bushing

A copper alloy wound bushing is pressed into the piston pin.

Since a metalic sound will be produced if the piston pin and piston pin bushing wear is excessive, replace the bushing when the wear limit is exceeded.

The piston pin bushing can easily be removed and installed with a press. However, when installing the bushing, be careful that it is not tilted.

NOTE: "Piston pin bushing inside diameter" is the dimension after pressing onto the connecting rod.

4-1.1 Disassembling the cylinder head

5-1.1 Disassembling the cylinder head