NUTS AND BOLTS

IN THIS SECTION, YOU WILL FIND THE FOLLOWING SUBSECTIONS:

TORQUE SPECIFICATIONS
TORQUE WRENCHES
ESTIMATING TORQUE REQUIREMENTS
IF YOUR FASTENER WILL BE USED IN CAST IRON OR STEEL….
AND IF YOUR FASTENER WILL BE USED IN ALUMINUM….
IF YOUR FASTENER IS AN OIL DRAIN PLUG….
BOLTS AND NUTS
INTERPRETING SAE GRADES
TORQUE VALUES CHART
NUT AND BOLT FAILURES

TORQUE SPECIFICATIONS

Many small engine components are affixed with nuts and bolts. When reassembling an engine, you must ensure that the fasteners being used are properly tightened; this way, you know that they will remain tight when the engine is operated. At the same time, a fastener should not be overtightened. This is where torque and tightening specifications come into play.

Many modern small engines are made of aluminum, a relatively soft metal that can be easily damaged. An engine manufacturer will normally specify the torque values that should be applied to certain critical fasteners. These values are known as fastening torque specifications. Torque is the amount of twisting or rotational force applied to an object. Large torque values are expressed in units called foot pounds(abbreviated ft lb, or lb ft); smaller torque values are expressed in inch pounds(abbreviated in lb). When you are working with metric measurements, you will use torque units known as Newton meters(N-m).

TORQUE WRENCHES

The most common cause of nut and bolt failure is inadequate tightening. Even if a torque wrench was used to tighten a fastener, the tightening can be inadequate. This is especially true if the torque wrench being used was incorrectly calibrated. Many engine bolts are meant to be tightened to a precise torque value, and any variation, even a slight one, can be the difference between success and failure. This means that a torque wrench must be treated, and stored, with care. The instrument should be shielded from damage; keep in mind that physical shocks or sharp vibrations can knock a torque wrench out of calibration.

The best type of torque wrench is whichever proves to be most accurate. Generally speaking, the more a tool costs, the more reliable is. The cheapest and least accurate torque wrench is the beam type. A torque value will be indicated by a needle that runs the length of the beam and ends in a scale located near the wrench handle.

A dial type torque wrench provides its torque value on a built in dial gauge. A click type torque wrench is set to a desired torque value before tightening begins; the wrench will click, alerting its operator to the fact that this preset torque value was reached. Dial and click type torque wrenches are more expensive than the rudimentary beam type, however they are also far more accurate. The dial type wrench best maintains its accuracy over time, and is generally preferred by experienced mechanics who are accustomed to working on expensive equipment.

Beam type torque wrenches are inexpensive enough to simply replace every so often. If one of these torque wrenches is used frequently, it should probably be replaced on a yearly basis. The dial and click torque wrench types should be sent to its manufacturer for calibration yearly, or once for every 10,000 torques. A torque wrench should also be calibrated if it was dropped, left out in the sizzling sun, or subjected to sharp vibrations.

ESTIMATING TORQUE REQUIREMENTS

Torque specifications for the fasteners used in an engine can normally be found in the service manual. If a service manual fails to list torque specifications, or the manual is simply unavailable, there are some general torque related guidelines that can be followed. The amount of torque recommended will depend on the grade or strength of a fastener, on its size, and on the type of metal to which it is being attached. Here are some general suggestions regarding torque values that you can follow:

IF YOUR FASTENER WILL BE USED IN CAST IRON OR STEEL….

SIZE GRADE 2 GRADE 5 GRADE 8
#8-32 20 in. lb. 25 in. lb.
#10-24 32 in. lb. 40 in. lb.
#10-32 32 in. lb. 40 in. lb.
¼-20 70 in. lb. 115 in. lb. 165 in. lb.
¼-28 85 in. lb. 140 in. lb. 200 in. lb.
5/16-18 150 in. lb. 250 in. lb. 350 in lb.
5/16-24 165 in. lb. 270 in. lb. 30 ft. lb.
3/8-16 260 in. lb. 35 ft. lb. 50 ft. lb.
3/8-24 300 in. lb. 40 ft. lb. 60 ft. lb.
7/16-14 35 ft. lb. 55 ft. lb. 80 ft. lb.
7/16-20 45 ft . lb. 75 ft. lb. 105 ft. lb.
½-13 50 ft. lb. 80 ft. lb. 115 ft. lb.
½-20 70 ft. lb. 105 ft. lb. 165 ft. lb.
9/16-12 75 ft. lb. 125 ft. lb. 175 ft. lb.
9/16-18 100 ft. lb. 165 ft. lb. 230 ft. lb.
5/8-11 110 ft. lb. 180 ft. lb. 260 ft. lb.
5/8-18 140 ft. lb. 230 ft. lb. 330 ft. lb.
¾-10 150 ft. lb. 245 ft. lb. 350 ft. lb.
¾-16 200 ft. lb. 325 ft. lb. 470 ft. lb.

AND IF YOUR FASTENER WILL BE USED IN ALUMINUM….

SIZE GRADE 2 GRADE 5 GRADE 8
#8-32 20 in. lb. 20 in. lb. 20 in. lb.
#10-24 32 in. lb. 32 in. lb. 32 in. lb.
¼-20 70 in. lb. 70 in. lb. 70 in. lb.
5/16-18 150 in. lb. 150 in. lb. 150 in. lb.

IF YOUR FASTENER IS AN OIL DRAIN PLUG….

SIZE INTO CAST IRON PANS INTO ALUMINUM PANS
¼ in. 150 in. lb. 100 in. lb.
3/8 in. 180 in. lb. 120 in. lb.
½ in. 20 ft. lb. 13 ft. lb.
¾ in. 25 ft. lb. 16 ft. lb.
3/8 in. hex nut
w/fiber gasket 20-25 ft. lb. 20-25 ft. lb.

CONVERSIONS FOR USE WITH ABOVE CHARTS
ft. lb. = in.lb. x 0.083
in. lb. = ft. lb. x 12
kgm = ft. lb. x 0.1383
Nm = ft. lb. x 1.3558

Bolt grades: most fasteners are available in different grades that correspond to different hardnesses or strengths. To determine the grade of a bolt, examine its head. You may observe on the bolt head several raised lines. The more of these lines there are, the harder or stronger a bolt is. The actual grade number for a bolt is the number of lines on its head plus two. So, if you observe three lines on a bolt head, you know that its grade is 5(3 + 2 = 5). If you know the grade of a given bolt, you can find its recommended torque value on the chart shown above.

If a bolt head contains no slashes at all, then the bolt is a grade 2(0 slashes + 2 = 2). If there are two slashes on a bolt head, the bolt grade is 4(2 slashes + 2 = 4). A bolt that harbors three slashes on its head is a grade 5(3 slashes + 2 = 5). The most common bolt grade in small engine applications is a 5. As pertains to metric bolts, there is no figuring necessary. Metric bolt strength will be indicated by a number stamped on its head.

Bolt sizes: the size numbers listed in charts tell you two things about a bolt: the outside diameter of its threaded pin or shaft, and the number of threads per inch along the shaft length. With very small bolts, those with diameters of less than ¼ inches, the symbol # is used along with a number to represent bolt diameter. The smallest of these # diameters is #0(equivalent to 0.060 inches). The largest of these # diameters is #12(equal to 0.216 inches). On the other hand, larger bolt diameters are expressed in fractions of an inch. In either system, the last number in the designation represents the number of threads per inch.

For example, consider a bolt of size 3/8-24; this bolt has a shaft diameter of 3/8 inches, and 24 threads per inch along its shaft length. A bolt with coarse threads will possess fewer threads per inch than a bolt with fine threads.

Aluminum versus iron: to use the above charts, you must determine whether a bolt is being applied to an aluminum component, or a component that is of steel or cast iron. When you have located the correct chart, look down its left hand column to find the applicable bolt size. Then go across the chart until you have reached the applicable bolt grade. The value listed will be the general torque specification recommended for a bolt of that particular size and grade.

Suppose that you are about to reinstall a grade 5 bolt of size 3/8-16. You check the engine service manual, only to discover that the torque value is not specified. However, you know that the bolt will be applied to a steel component. To find the correct torque specification, look at the chart that contains general specs for cast iron and steel. The recommended torque for this bolt is 35 ft. lb. Charts like these should only be used when recommended torque value does not appear in an engine service manual.

BOLTS AND NUTS

Bolts and nuts are designed to be used together. The bolt has external threads while the nut harbors internal threads. This much is common knowledge. What is less commonly understood is that a bolt and nut that will be used together must possess the same type of threads per inch to lock properly. In other words, a bolt with coarse threads should never be mated to a nut with fine threads, even though they can be forced together.

A special type of nut called a locknut boasts a locking feature inside of its hole that enables it to grip its bolt mega tightly. Aside from this locking feature, a locknut has the same dimensions as a regular nut. Locknuts are used to prevent a nut bolt combination from working itself loose during machine operation, as it is subjected to vibration and other motions.

Keep in mind that steel bolts are actually elastic; they stretch as they are tightened down. If a bolt is stretched carefully, it will return to its original length when released, and should be in good enough condition for reuse. If a bolt is excessively stretched, it will become distorted, and will either break or not return to its original length. In other words, treat bolts(and other fasteners for that matter) with consideration and forethought.

INTERPRETING SAE GRADES

The Society of Automotive Engineers(SAE) has established eight grades of nuts and bolts; ratings are based on the elasticity of the steel of which they are formed. As you ascend from a grade 1 rating to a grade 8 rating, the bolt becomes harder and less elastic, and also progressively more brittle. Certain grades of nuts and bolts will be compatible with certain torque specifications. For this reason, you should never substitute a nut or bolt of one grade for the nut or bolt of a different grade. Failure can easily occur, for example, when a grade 8 bolt is substituted for a grade 5 bolt, for two reasons: first, the grade 8 bolt will not enjoy the elasticity to stretch with the two engine components it secures during routine operation; second, the grade 8 bolt will not provide adequate clamping force when tightened to the torque value specified for a grade 5 bolt. Even supposing that the grade 8 bolt was tightened to the torque value recommended for a grade 8 bolt, the lack of necessary elasticity may well bring about its failure anyway. As the joint between the two engine components it holds together begins to expand and contract, the bolt will be unable to follow. If the joint being secured by this incorrect bolt was placed under a heavy enough load, the hard metal of the bolt could yank the threads right out of the engine piece into which it was threaded!

Substituting a softer bolt for a hard bolt is also inappropriate. A grade 2 bolt, for example, will likely not provide the clamping force needed to secure engine parts when a heavy work load is applied to the joint. The moral of the story is: that you should never interchange bolts or nuts of different grades when repairing or reassembling an engine; the incompatibility of their elasticities and clamping forces will make them prone to malfunction.

Below you will see an example of ¾ inch bolts and nuts from three SAE grades that are frequently used in outdoor power equipment. Remember that a grade 8 bolt will have six lines or bars on its head, while the head of a grade 5 bolt will have just three bars, and the head of a grade 2 bolt will be completely bare:

TORQUE VALUES CHART
For a 3/8 inches bolt or not of:

SAE Grade 8 Torque Value Should Be: 36 ft lb
SAE Grade 5 Torque Value Should Be: 26 ft lb
SAE Grade 2 Torque Value Should Be: 16 ft lb

A similar system(not identical) of markings is used to identify nuts by SAE grade. The side of a grade 8 nut will have two grooves, while the side of a grade 5 nut will sport a single groove. A grade 2 nut will be characterized by no marks on its side at all. A different torque value, expressed in foot pounds(ft lb), is recommended for each grade of fastener; the larger the SAE grade, the more tightening force is required to obtain adequate clamping.

NUT AND BOLT FAILURES

Two of the engine components commonly secured by bolts are the cylinder head and engine block. The torque or turning force exerted by these cylinder head bolts must exceed the explosive or spreading force of combustion. If the torque on cylinder head bolts was too small, the head would lift and stretch the bolt each there was a power stroke, and each time the piston reached top dead center or TDC. Over time, this would cause the bolt to harden and finally sever at its first thread. This would immediately compromise the cylinder head gasket, causing a significant reduction in engine power.

To ensure that bolts are tightened to their appropriate torque values, try following these general rules:

1)Consult the engine service manual for recommended torque values, and tightening sequences; follow these recommendations religiously

2)Use quality replacement nuts and bolts that exactly match the originals in terms of SAE grade, thread type, and bolt length

3)Install bolts only in holes that are clean and free of debris. Tightening a bolt into a hole filled with liquid can create extreme hydraulic pressure. Hole threads should also be clean; the bolt should turn freely as it goes in, without the imparting of force. Resistance to turning is actually an indication that clamping power of a fastener has been reduced

4)During reassembly, lubricate the bolt threads if necessary. As the bolt head turns, some friction will be created along its threads. This will result in a certain amount of twisting or wrenching of the bolt shank as tightening occurs. Over time, a twisted bolt will tend to unwind. Lubricating its threads as you are preparing to tighten a bolt will reduce friction between the bolt threads and hole threads, and lessen the chances of bolt twisting. An engine service manual may not specify any lubricant, however lubricant can normally be used unless such practice is strictly prohibited. On the other hand, a service manual may call for the use of some special lubricant, or ordinary engine oil, on all engine bolts

5)Never use an impact wrench to remove or install an engine bolt