Standard Bolt Torquesat Barrington Diesel Club

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Imperial High Tensile Bolts

Imperial High Tensile Bolts are widely found on older American, Canadian and British engines and machinery.

Largely though, newer engines and machines are gradually adopting metric ISO bolt standards.

However while practical, technical and other difficulties to complete the conversion to the metric system still remain, I think the Imperial system may still be with us for some years to come.

Imperial High Tensile Bolts - Table of Standard Torques
	IMPERIAL Grade 5 bolts, allow a torque variance of ± 5% either way			.		IMPERIAL Grade 8 bolts, allow a torque variance of ± 5% either way
Bolt size	Thread	Ft lbs	Nm	.	Bolt size	Thread	Ft lbs	Nm
1/4 inch	NC	6	8	.	1/4 inch	NC	8	11
1/4 inch	NF	7	10	.	1/4 inch	NF	11	13
5/16 inch	NC	12	16	.	5/16 inch	NC	16	21
5/16 inch	NF	13	17	.	5/16 inch	NF	17	24
3/8 inch	NC	25	32	.	3/8 inch	NC	33	44
3/8 inch	NF	28	37	.	3/8 inch	NF	37	50
7/16 inch	NC	37	48	.	7/16 inch	NC	48	65
7/16 inch	NF	45	60	.	7/16 inch	NF	59	80
Bolt size	Thread	Ft lbs	Nm	.	Bolt size	Thread	Ft lbs	Nm
1/2 inch	NC	54	71	.	1/2 inch	NC	73	100
1/2 inch	NF	65	90	.	1/2 inch	NF	88	120
9/16 inch	NC	70	95	.	9/16 inch	NC	95	130
9/16 inch	NF	85	115	.	9/16 inch	NF	112	152
5/8 inch	NC	108	145	.	5/8 inch	NC	142	193
5/8 inch	NF	130	175	.	5/8 inch	NF	173	235
3/4 inch	NC	185	250	.	3/4 inch	NC	245	332
3/4 inch	NF	220	300	.	3/4 inch	NF	295	400
7/8 inch	NC	310	422	.	7/8 inch	NC	415	563
7/8 inch	NF	360	490	.	7/8 inch	NF	480	650
1 inch	NC	440	595	.	1 inch	NC	585	795
1 inch	NF	518	700	.	1 inch	NF	690	930
Bolt size	Thread	Ft lbs	Nm	.	Bolt size	Thread	Ft lbs	Nm
	IMPERIAL Grade 5 bolts, allow a torque variance of ± 5% either way			.		IMPERIAL Grade 8 bolts, allow a torque variance of ± 5% either way

Metric Bolts - Standard torques

Metric system (SI).SI is the abbreviation for the metric system, the International System of Units.
Metric Bolt UsageFor many reasons standardization of various aspects of engines, like fasteners, is an ideal many are aiming at. Ongoing standardization for materials through ISO (International Standards Organization) has resulted in many popular engine manufacturers largely changing from imperial fasteners to metric ones. The use of metric bolts and nuts by engine manufacturers like Detroit Diesel and Cummins on their newer models of engines has become common place.

Difficulties with metric and ISO systems

From the outside, talking about anything ISO, is fraught with circles of technical jargon. e.g. try finding the correct way to write a bolt's size down on a piece of paper and you'll wind up having to buy an ISO nomenclature manual costing hundreds of Swiss marks in order to do so. Then if you do find out before you're bankrupt, you may find that no one on earth except the real technical boffins understand what it all means anyway.

Other Standards authorities e.g. DIN, JIS, SAE, may and do make their own standards on an 'ad hoc' basis when the situation demands it or they feel the situation demands it, and those standards may or may not agree with an ISO standard for a particular thing, if ISO has a standard for that particular thing in the first place.

For ISO to succeed as a system, or any other system to succeed for that matter, it needs simplification. Begin at something like an ABC base, where terms, manuals and protocols are defined and made cheaply and easily available and usable by the majority of normal people during the progress of normal living, and activity.

Metric Bolt Terms

Normally metric fastener sizes are stated in mm (millimeters) and begin with a capital M which is followed by the fastener's nominal diameter and possibly the bolt's thread pitch, separated by an "x". If the pitch is not mentioned it defaults to the 'standard metric coarse pitch' for that size bolt or nut.

For practical purposes we write a bolt's size as follows:

M12 x 1.75 x 60

giving M for metric,
12 for the bolt's diameter in mm,
1.75 the pitch of the bolt in mm,
60 for the length of the working bolt in mm.

When you order bolts you must specify the grade, check the old bolt for the grade on the head of the bolt. If you do not see the grade on the head of the bolt, it may be a cheap import, be careful not to use ungraded bolts, they break. In any case, for engines I suggest you never use ungraded bolts, and then never use a lower grade than 8.8.

Metric standard torques do not consider the pitch of a bolt but do consider wether the bolt is installed dry or if it is oiled before insertion and then there is a different torque value for the same diameter bolt.

Metric Bolts - Table of Standard Torques (± 5% Nm)
Metric Grade				.	Metric Grade
Diameter	Dry Threads - torque			.	Diameter	Oiled Threads - torque
M5	6	9	10	.	M5	4.5	6.5	7.5
M6	10	15	18	.	M6	7.5	11	13
M8	25	35	45	.	M8	18	25	30
M10	50	75	85	.	M10	35	55	65
M12	85	130	150	.	M12	65	95	110
M14	140	205	240	.	M14	100	150	175
M16	215	315	370	.	M16	160	235	275
M20	435	620	725	.	M20	320	455	535
M24	750	1070	1255	.	M24	555	790	925
Diameter	Dry Threads - torque			.	Diameter	Oiled Threads - torque
Metric Grade				.	Metric Grade