Metric Fastener Tips for the
Mitsubishi 3000GT and Dodge Stealth

by Jeff Lucius

Introduction

This short note discusses how to identify the metric bolt or screw used on your Mitsubishi 3000GT and Dodge Stealth. It introduces the topics of common metric thread types, fastener designation, and property class. References are listed at the end of this note for additional reading. The Mitsubishi 3000GT and Dodge Stealth are manufactured by Mitsubishi at their Nagoya, Japan plant. Accordingly, with the exception of a few pipe threads, metric fasteners are used throughout the car.

Designation

Metric fasteners are designated with the letter "M" followed by the major (or outside) diameter in millimeters (mm) of external threads, an "x", and the pitch of the threads. Bolts have external threads; nuts have internal threads. Thread pitch is the distance in mm between adjacent thread crests or adjacent thread roots. Threads are divided into coarse types (larger numbers) and fine types (smaller numbers). The "standard" or "preferred" thread is the coarse pitch. Fine pitches are used due to space or wall thickness considerations. Bolts are differentiated from screws in that a bolt is held in place using a nut; screws thread directly into the part being fastened.

The pictures below show an example of a metric fastener and how to determine the designation. This particular fastener, an M12 x 1.25, happens to be the banjo bolt that connects the top of the fuel filter to the high-pressure fuel line that goes to the front fuel rail. The head of the bolt is stamped with a "7". A 19-mm wrench is needed on the hex head. The threads have a 12-mm major diameter. To determine the thread pitch, count the number of threads in 10 mm (or other convenient size) of the threaded length. There are 8 roots in this case, noted by the small yellow circles, in 10-mm of length. Ten mm divided by eight yields 1.25 mm between roots (or crests): a 1.25 pitch.

Metric thread 1    Metric thread 2

Selected Metric Threads
Diameter Coarse Pitch Fine Pitch "Common" hex (wrench) size
6 1.0   10 mm
8 1.25 1.0 12 or 13 mm
10 1.5 1.25 14, 15, 16, or 17 mm
12 1.75 1.25 (1.50) 17, 18, or 19 mm
14 2.0 1.5 21 or 22 mm
16 2.0 1.5 24 mm
18 2.5 1.5 27 mm
20 2.5 1.5 30 mm

Property Class

Bolts and screws are designed to perform only one function: to clamp parts together. Therefore, the tensile strength and yield strength of a fastener are important. The tensile strength is the property of a fastener that resists tensional stress, or a force that causes physical lengthening of the fastener. Yield strength is a measure of how much tensional stress a fastener can withstand before it fails (breaks). Because bolts and screws are designed to clamp, compressive, shear, and bending strength and plasticity are not important. Well, at least they are not important when a bolt or screw is used as it is designed to be used, that is, not as a pivot or axle, or to locate parts (to keep them from shifting with respect to each other). The tensile and yield strengths are described for every fastener using a Property Class.

The Property Class for steel fasteners using Unified Screw Threads (the "Imperial" system) is described using SAE (Society of Automotive Engineers) Grades. The Property Class for Metric threads is described using Classes. Each metric Class symbol consists of two numbers separated by a period. The first number is 1/100 of the nominal tensile strength in N/mm2 (newtons per square millimeter; 1 N/mm2 = 145.038 psi). The second number is 10 times the ratio of nominal yield stress to nominal tensile strength. For example, a Class 8.8 bolt has a nominal tensile strength of 800 N/mm2 (or about 116,000 psi) and a nominal yield stress of 640 N/mm2 (8 times 800 divided by 10).

SAE Grade 8 (150,000 psi tensile strength) is equivalent to metric Class 10.9. SAE Grade 5 (120,000 psi tensile strength) is equivalent to metric Class 8.8. Metric Class 4.6 and 4.8 are equivalent to "mild steel".

Tightening Torque

The torque values in the tables below are taken from the 1999 Mitsubishi 3000GT Service Manual (p. 00-31). Check the service manual for your particular model to verify the correct recommended torques for the bolts used on our cars. The tables show standard values when the following conditions are true.
(1) Bolts, nuts, and washers are all made of steel and plated with zinc.
(2) The threads and bearing surface of bolts and nuts are in dry condition.

The table values are not applicable:
(1) If toothed washers are inserted.
(2) if plastic parts are fastened.
(3) If bolts are tightened to plastic or die-cast inserted nuts.
(4) If self-tapping screws or self-locking nuts are used.

The head mark is a number molded into the head of the bolt or screw. The marks approximately correspond to the following property classes:
Head Mark "4": 4.6, 4.8, 5.6, 5.8, 6.8
Head Mark "7": 8.8
Head Mark "8": 9.8
When I looked around in the engine bay, most bolts were marked with either a "4" or a "7".

Tightening Torque for Standard Bolts and Nuts
Diameter
(mm)		 Pitch
(mm)		 Torque Nm (ft.lbs.)
Head Mark <4> Head Mark <7> Head Mark <8>
M5 0.8 2.5 (1.8) 4.9 (3.6) 5.9 (4.3)
M6 1.0 4.9 (3.6) 8.8 (6.5) 9.8 (7.2)
M8 1.25 12 (8.7) 22 (16) 25 (18)
M10 1.25 24 (17) 44 (33) 52 (38)
M12 1.25 41 (30) 81 (60) 96 (71)
M14 1.5 72 (53) 137 (101) 157 (116)
M16 1.5 111 (82) 206 (152) 235 (174)
M18 1.5 167 (123) 304 (224) 343 (253)
M20 1.5 226 (166) 412 (304) 481 (354)
M22 1.5 304 (224) 559 (412) 647 (477)
M24 1.5 392 (289) 735 (542) 853 (629)


Tightening Torque for Flange Bolts and Nuts
Diameter
(mm)		 Pitch
(mm)		 Torque Nm (ft.lbs.)
Head Mark <4> Head Mark <7> Head Mark <8>
M6 1.0 4.9 (3.6) 9.8 (7.2) 12 (8.7)
M8 1.25 13 (9.4) 24 (17) 28 (20)
M10 1.25 26 (19) 49 (36) 57 (42)
M10 1.5 24 (17) 44 (33) 54 (40)
M12 1.25 46 (34) 93 (69) 103 (76)
M12 1.75 42 (31) 81 (60) 96 (71)


Below is an example of a thread gauge kit, which can simplify determining fastener designation.

Thread gaufe kit

References

Forbes Aird, 1999, High Performance Hardware - Fastener Technology for Auto Racers and Enthusiasts: HPBooks, 201 p.
Carroll Smith, 1990, Carroll Smith's Nuts, Bolts and Fasteners and Plumbing Handbook: MBI, 223 p.
Screw Threads by www.efunda.com
Threaded Fasteners by Engineered Racing Products
Fastener Training Manual by James Glen (588-KB Adobe PDF file)

Back Home Forward

Except for the small gif and jpg images, the content, images, photographs, text, and multimedia displayed are Copyright ©2000-2003 by Jeff Lucius and K2 Software. All rights reserved. No part, section, image, photo, article, or whole of this site may be reposted or redisplayed without permission of the authors.
Page last updated August 22, 2006.