Screw Basics

Screws and nuts hold the motorcycle together – but only if the owner knows how to handle them correctly.

Screw Basics

Technical Info

Screws are the connection between a wide range of motorcycle components and come in all shapes, colours and materials. They can enhance or detract from the visual appearance of the bike and can be a real headache if they are stripped or can no longer be unscrewed: The screws. As a screw is not just simply a screw, you should check a few things when replacing it so that the result looks good, is capable of withstanding loads and is long-lasting.


Forms and Functions

Fig. 1 – Worn screws

Fig. 1: Worn screws

Screws

Larger screws and bolts on the vehicle that are exposed to high loads generally have a hexagonal head. For sizes of 8 mm and smaller, a hexagon socket or Torx head is often used. 

Conventional slotted or crosshead screws must always be handled gently using suitable screwdrivers, otherwise you will quickly strip them (Fig. 1).

It is therefore advisable to replace them with hexagon socket screws where possible – provided that they fit in with the style of the bike. 

Stark belastete, größere Schrauben und Bolzen am Fahrzeug haben in der Regel einen Sechskant-Kopf, ab 8 mm und kleiner kommt gern der Innensechskant- oder der Vielzahnkopf zum Einsatz. 

Herkömmliche Schlitz- oder Kreuzschlitz-Schrauben müssen immer mit Gefühl und gut passenden Schraubendrehern angefasst werden, sonst dreht man sie rasch rund (Abb. 1).

Daher empfiehlt sich wo möglich der Austausch gegen Innensechskantschrauben – wenn es zum Stil des Bikes passt. 

Fig. 2.1– Overview of screws

Fig. 2.1: Overview of screws

Where a pleasing appearance is what really matters, depending on the application, fillister head screws (Fig. 2.1 a) are often more elegant than conventional hexagon socket screws with a barrel head (Fig. 2.1 b). Hexagon socket screws with a conical head (Fig. 2.1 c) can be visually appealing. 

Fig. 2.2 – Overview of screws

Fig. 2.2: Overview of screws

It is often advisable to countersink the screw head flush into the material for visual or space-related reasons. This can be done using a countersink (Fig. 2.2 f) and a countersunk screw (Fig. 2.2 e). If the material is too thin for this, a flat-headed screw (Fig. 2.1 d) can be used as an alternative. Although it does not end up flush with the material, it only projects very slightly.

Round-head screws (Fig. 2.2 g) are more for classic motorbikes or a desired classic look. 

If the screw head cannot be held during the assembly process, it is worthwhile using a square neck underneath ("mushroom head bolt", Fig. 2.2 h) as anti-rotation element.


Fig. 3.1 – Overview of nuts

Fig. 3.1: Overview of nuts

Nuts

You can choose from different designs of nuts too. In addition to normal hex nuts, there are nuts with collars that distribute the crimping pressure more effectively (Fig. 3.1 c). Cap nuts are used for decorative purposes and protect the thread from dirt and weathering (Fig. 3.1 b). 

Fig. 3.2 – Overview of nuts

Fig. 3.2: Overview of nuts

Castle nuts can be secured with a cotter pin (not shown). Grooved nuts are used for the steering head, for example (Fig. 3.2 e). In most situations, it is more elegant to use a self-locking nut (Fig. 3.1 a) or threadlock adhesives (particularly to protect the screw connection against vibrations). Thin nuts (not shown) are a good solution if there is little space available (beneath the mudguard, for example). Wing nuts (Fig. 3.2 d) can be disassembled without having to use any tools. 


Fig. 4 – Long nut

Fig. 4: Long nut

Long nut

Weld nuts (not shown) with or without retainers can be welded to the object in such a way that they are fixed or moveable. Long nuts are used if a short one would be too inaccessible, for example, on the headlight clamping ring (Fig. 4).


Fig. 5 – Overview of washers and rubber bearings

Fig. 5: Overview of washers and rubber bearings

Plain washers, etc.

When it comes to slight crimping pressure distribution, screws/nuts with a collar are the ideal choice, or you use bigger washers instead.  

Spring washers (Fig. 5a) make it more difficult to loosen the screw connection, while toothed washers (Fig. 5b), which dig into the sub-surface, have an even tighter fit. Self-locking nuts or a drop-shaped screw locking device should be used on the thread in places that are prone to vibrations. Rubber bearings (Fig. 5e) can protect sensitive (e.g. electronic) components from vibrations. 

Place a nylon washer underneath (Fig. 5c) if you'd like to protect a painted or polished sub-surface. 

Screw connections that close a liquid or oil-filled casing require sealing. If there is no paper or rubber seal, place a copper or aluminium washer directly underneath the screw head as sealant (Fig. 5d). Copper and aluminium sealing rings should only be used once.   

If a riveted bolt needs to be replaced on a joint (e.g. footrest), in this situation a screw with a self-locking nut, which you tighten so the joint can still move, can be installed in most cases. 


Materials 

Fig. 6 – Steel screw with a tensile strength of 8.8

Fig. 6: Steel screw with a tensile strength of 8.8

Steel bolts

Standard screws on a motorbike are made of steel. Because steel rusts when untreated, these screws are generally zinc-coated or chrome-plated. While zinc coatings can also be black, yellowish or greenish in appearance, they are metallic silver in most cases. Burnished or black parkerised screws are only suitable for assembly in the engine, as they rust quickly when exposed to the elements. In Germany, steel screws are divided into classes according to their tensile strength/yield strength resulting from the steel quality. 

You should only use screws with a tensile strength/yield strength of 8.8 or better (9.8, 10.9 or 12.9) for motorbikes.

Strength class (Table)
Strength classTensile strength (N/mm²)Yield strength (N/mm²)
4.64 x 100 = 400 N/mm²4 x 6 x 10 = 240 N/mm² 
8.88 x 100 = 800 N/mm² 8 x 8 x 10 = 640 N/mm²
12.912 x 100 = 1,200 N/mm²12 x 9 x 10 = 1,080 N/mm²

Low-value screws, which are readily obtainable from a DIY store for example, are of an insufficient quality (too soft).

Steel nuts are identified by their tensile strength. The nuts should have at least the same tensile strength as the screws to create an optimum screw connection.


Identifying steel nuts

a = Engraving of strength class, b = or embossing in a clockwise direction

Identifying steel nuts

The following standard tightening torques can be used for metric steel screws/nuts: 

Tightening torques for screws according to strength classes – regular metric thread 

according to DIN ISO (general default values relate to the screw and not to the intended purpose – when used on a vehicle, the vehicle manufacturer's recommendation (if one has been made) must be followed, see workshop manual).

Tightening torques (Nm) for screw strength classes 5.6 to 12.9: (Table)
Threads5.66.88.810.912.9
M30,610,91,31,92,2
M41,362,12,94,14,9
M52,814,368,510,1
M64,77,3101416,9
M811,717,8253542
M102335496982
M12406286120145
M14-115135190230
M1698180210295354
M18136245290405489
M20192345410580692
M22-258550780928
M24-33371010001198
M27-492105015001772
M30-680145020002447

Tightening torques for screws by strength class – metric fine thread

Tightening torques (Nm) for screw strength classes 6.8 to 12.9: (Table)
Threads6.88.810.912.9
M8 x 123273845
M10 x 1,2544527388
M12 x 1,258095135160
M12 x 1,57690125150
M14 x 1,5125150210250
M16 x 1,5190225315380
M18 x 1,5275325460550
The following core hole dimensions and spanner sizes apply to ISO standard screws: (Table)
Nominal dia.PitchHSS drill dia.Hex socket wrench widthHexagon key width
M30,52,55,52,5
M40,73,373
M50,84,2804 (5)
M615105
M81,256,813 (12)6
M101,58,517 (16)8
M121,7510,219 (18)10

Fig. 7 – Overview of screws made of aluminium, stainless steel and titanium

Fig. 7: Overview of screws made of aluminium, stainless steel and titanium

Screws/nuts made of aluminium, stainless steel and titanium

It naturally makes sense to use non-rusting materials for screw connections on a motorbike and this enhances the bike's value when done correctly. 

Titanium is easily one third lighter than steel, has a beautiful high-tech finish and has approximately the same tensile strength as an 8.8 screw. It can therefore be put to very good use on a motorbike for most purposes (Fig. 7c). 

Aluminium is even lighter (being 1/3 of the weight of steel). However, depending on the aluminium quality, its tensile strength only roughly corresponds to that of a 5.6 steel screw – which is too low for all the loaded assembly points on a bike! Aluminium screws are ideal for the engine cover, as they have the same thermal expansion coefficients as the aluminium casing, which protects the threads. You can also use them for all fittings that do not have to withstand much (Fig. 7a).

While non-rusting stainless steel screws made of V2A or V4A (Fig. 7b) are slightly heavier than steel screws, they have a very pleasing effect due to their beautiful and valuable appearance. When polished, they shine almost as if they were chrome-plated, and they are a real eye-catcher on black-painted surfaces in particular. Their tensile strength is slightly lower than that of steel screws. 

Tightening torques for V2A/V4A screws in Nm (Table)

The following tightening torques result depending on the strength class:

ThreadsPitchStrength class
 507080
M50,81,73,54,7
M61,0368
M81,257,11622
M101,5143243
M121,75245675
M162,059135180
M202,5114280370

A2-50 and A4-50: Values apply without length limitation. A2-70 and A4-70 as well as A2-80 and A4-80: Values only apply to screw lengths up to eight times the thread diameter. 

The thermal expansion coefficient of stainless steel screws is very different to that of aluminium screws. Caution must therefore be exercised when using these in engine cases – if the cast aluminium is of a moderate quality, the threads can tear out!  

In any case, you should install aluminium, titanium and VA screws on the thread with ceramic paste as a release agent. This prevents seizing and contact corrosion. Ceramic paste is also highly heatproof.

Brass is a fantastic material for setting visual standards on your bike, particularly when you're after a vintage look. A brass screw with good alloying can achieve just under 2/3 of the tensile strength of an 8.8 screw and would thus be similar to V2A of the medium strength class. But since the tensile strength is not specified on the screw, you should exercise caution and stick with screw connections that do not have to withstand much, just to be on the safe side. 


Inch screws

Fig. 8 – Centric bar code

Fig. 8: Centric bar code

Imperial screws can be found on Harley V-Twins and English classic motorbikes, the most widespread being the American standard thread UNC for coarse thread and UNF for fine thread. The strength is indicated on the screw head by means of a centric bar code (Fig. 8). 

If there is no bar code, the strength is below the German DIN ISO comparative value of 8.8. Three bars (see Fig. 8) indicate "Grade 5" screws with a tensile strength roughly in the region of 8.8. These screws are well-suited to most connections on a motorbike. "Grade 7" and "Grade 8" screws of an even higher quality have five or six bars; screws with the highest tensile strength are those identified with L9 ("Grade 9") or ASTM A574. Rustproof inch screws are often marked with 304 or 316. In terms of tensile strength, they are below Grade 5 screws (with three bars).

UNC thread table
ThreadsThreadsOutside dia.Core hole dia. in mm 
 inchesinchesmmBoltNut
N1- 64641,851,551,58
N2- 56562,181,851,87
N3 - 48482,512,12,14
N4 - 44442,842,352,38
N5 - 40403,182,652,7
N6 - 32323,52,852,9
N8 - 32324,173,53,53
N10 - 24244,833,93,95
N12 - 24245,494,54,59
1/4"- 20 201/4"6,355,15,25
5/16"- 18185/16"7,946,66,68
3/8"- 16 163/8"9,5388,08
7/16"-14147/16"11,119,49,44
1/2"- 13 131/2"12,710,810,88
9/16"- 12129/16"14,2312,212,3
5/8"-11115/8"15,8813,513,69
3/4"-10 103/4"19,0516,516,62
7/8"- 997/8" 22,2319,519,52
1"- 8 81"25,422,2522,34
1 1/8"-7 71 1/8"28,572525,08 
1 1/4"-771 1/4"31,752828,26
1 3/8"- 661 3/8"34,9230,7530,85
1 1/2"- 661 1/2"38,13434,03
1 3/4"- 551 3/4"44,4539,539,56
2"- 4 1/2 4,52"50,84545,37
2 1/4"- 4 1/24,52 1/4"57,15 51,551,72
2 1/2"- 442 1/2"63,557,2557,39 
2 3/4"- 4 42 3/4"69,8563,563,74
3"- 443"76,27070,09
3 1/4"- 443 1/4"82,5576,276,44
3 1/2"- 443 1/2"88,982,682,79
3 3/4"- 4 43 3/4"95,2588,989,14
4"- 444"101,695,2595,48
UNF thread table
ThreadsThreadsOutside dia.Core hole dia. in mm
 inchesinchesmmBoltNut
0 - 80801,521,251,30
1 - 72721,851,551,61
2 - 64642,181,901,91
3 - 56562,512,152,19
4 - 48482,842,402,45
5 - 44443,172,702,74
6 - 40403,502,953,01
8 - 36364,163,503,59
10 - 32324,824,104,16
1/4"- 28 281/4"6,355,505,56
5/16"- 24245/16"7,946,906,99
3/8"- 24 243/8"9,538,508,56
7/16"- 20207/16"11,119,909,94
1/2"- 20 201/2"12,7011,5011,52
9/16"- 18189/16"14,2312,9012,97
5/8"-18185/8"15,8814,5014,55
3/4"-16 163/4"19,0517,5017,54
7/8"- 14147/8" 22,2320,4020,49
1"- 12 121"25,4023,2523,36
1 1/8"- 12 121 1/8"28,5726,5026,54
1 1/4"- 12121 1/4"31,7529,5029,71
1 3/8"- 12121 3/8"34,9232,7532,89
1 1/2"- 12121 1/2"38,1036,0036,06

Which manufacturers use which thread?

The table on the right specifies the thread types predominantly used by the major motorbike manufacturers. There is always the possibility of deviations among these manufacturers if they use components provided by international suppliers. If in doubt, always look carefully at the screw in front of you, and use a calliper gauge and a thread gauge to precisely determine the diameter and the pitch respectively. 

The screw length is only measured including the head for countersunk screws; otherwise, the shaft length applies. 

Classic motorbikes are not mentioned here – special threads that deviate from standard UNC and UNF threads, for instance, can be found on old Harleys and British bikes too. Relevant thread tables can be found online.  

Brand-specific screws

ManufacturerMetric Imperial Special features
APRILIA

X

    
BMW

X

    
BUELL

X

 

X

 Fittings sometimes metric
CAGIVA

X

    
DUCATI

X

    
HARLEY-DAVIDSON

X

 

X

 V-Rod models sometimes feature metric connecting elements, (Harley also featured special threads until approximately 1977.)
HONDA

X

   Identical to DIN ISO up to and including M8, then fine thread
INDIAN (Polaris)

X

    
KAWASAKI

X

   Identical to DIN ISO up to and including M8, then fine thread
KTM

X

    
MOTO MORINI

X

    
MOTO GUZZI

X

    
MV AGUSTA

X

    
PIAGGIO/VESPA

X

    
SUZUKI

X

   Identical to DIN ISO up to and including M8, then fine thread
TRIUMPH as of 1990 (Hinckley)

X

    
TRIUMPH up to 1983 (Meriden)  

X

 British imperial thread and special Triumph thread
VICTORY (Polaris)

X

    
YAMAHA

X

   Identical to DIN ISO up to and including M8, then fine thread

Bonus tips for real DIY mechanics

Secure correctly

Threads to which liquid hardening threadlocker was previously applied, must be thoroughly cleaned and fresh material applied. Never replace self-locking nuts with normal nuts. They can also be used two or three times, providing they offer enough resistance to turning.


The Louis Technical Centre

If you have a technical question about your motorbike, please contact our Technical Centre, where they have endless experience, reference books and contacts.

Please note!

These tips for DIY mechanics contain general recommendations that may not apply to all vehicles or all individual components. As local conditions may vary considerably, we are unable to guarantee the correctness of information in these tips for DIY mechanics.

Thank you for your understanding.