Fit Bolts: CNC Turned and Ground Fitted Bolts

Q: Is a turned bolt the same as a fit bolt?

In most drawings, yes. Turned bolt describes how the shank is made, and fit bolt describes how it is used. A ground bolt is a fit bolt whose shank has also been ground, for a closer fit and finer finish.

A fit bolt has a shank machined to a close tolerance so that it fills a reamed hole with almost no clearance. The joint then carries load through the shank in shear and bearing, not through clamp friction alone, so the parts cannot slip against each other. That is why fit bolts are used in flange couplings, heavy machinery and steel structures where even a small movement is not acceptable.

At S R Forgings, Ludhiana, the head is forged in-house and the shank is then either CNC turned or ground, depending on the fit your drawing calls for. Threads are rolled, not cut.

Same bolt, different names. Buyers and drawings call it a fit bolt, fitted bolt, fitting bolt, turned bolt, turned and fitted bolt, reamer bolt, reamed bolt, reamed hole bolt, body-bound bolt, body fit bolt, close tolerance bolt, precision bolt, fitted shank bolt or dowel bolt. In couplings it is usually just a coupling bolt. In German it is a Passschraube, in Spanish a tornillo calibrado or tornillo de ajuste, and on GOST drawings a болт под развёртку.

Batch of CNC turned fit bolts with machined shanks and centre-drilled ends, S R Forgings Ludhiana — A production batch of turned fit bolts at our Ludhiana plant. The centre holes in the ends show that the shanks were turned between centres, which keeps the shank true to the thread.

Diameter: 20 mm to 75 mm (metric), and equivalent inch sizes
Length: To your drawing
Property class: 8.8, 10.9 and 12.9
Shank: CNC turned or ground, to the tolerance on your drawing
Thread: Rolled, in metric coarse, metric fine, UNC, UNF and BSW
Standards: DIN 609, DIN 7968, DIN 7999 / EN 14399-8, IS 3640, GOST 7817-80, or customer drawing
Supplied: Loose, or as sets with matching nuts and washers

Turned Fit Bolts or Ground Fit Bolts: Which Do You Need?

Both routes give you a fitted shank. The difference is how close the fit is and how fine the finish has to be. We choose the route from the tolerance and surface finish on your drawing.

CNC Turned Fit Bolts

The shank is turned on CNC between centres after forging. This is the usual route for structural fit bolts and for most rivet replacement work, where the hole is reamed to size and the shank has to match it.

Structural fit bolts to DIN 7968 and DIN 7999
Turned bolts for replacing rivets in old steel bridges
Machinery and base frame joints

Ground Fit Bolts

The shank is turned and then ground. Grinding gives a tighter diameter band, a better finish and a shank that is round along its full length. Use it where the drawing calls for a close fit, such as k6 or m6, or a fine surface finish.

Flange and shaft coupling bolts
Rotating equipment such as turbines, pumps, gearboxes and mills
Precision ground bolts and dowel bolts to drawing

Fit Bolt Standards

Standard	What it covers	Typical use
DIN 609	Hexagon head fit bolts with a long threaded portion	Machinery, couplings, general engineering
DIN 7968	Hexagon fit bolts for steel structures. The shank is 1 mm larger than the thread (an M20 bolt has a 21 mm shank).	Steel structures, bridges, crane girders
DIN 7999 / EN 14399-8	High-strength hexagon fit bolts for preloaded structural joints, class 10.9 (HV fit bolts)	Preloaded, non-slip structural connections
IS 3640	Hexagon fit bolts, Indian Standard	Indian machinery and structural work
GOST 7817-80	Hexagon bolts with reduced head for reamed holes	Equipment built to Russian and CIS drawings
Customer drawing	Any shank diameter, tolerance, grip length, head and thread	Coupling bolts and special fit bolts

If your drawing gives a shank tolerance that differs from the standard, we work to the drawing.

Fit Bolt Dimensions: DIN 609, DIN 7968 and DIN 7999

Nominal dimensions in mm, from M20 upward. d = thread size, d_s = fitted shank diameter, s = width across flats, k = head height. The three standards give the same thread size a different shank, head and tolerance, so always state which standard your bolt is to.

DIN 609: Hexagon Fit Bolts (shank k6)

d	d_s nominal	d_s min – max (k6)	s	k
M20	21	21.002 – 21.015	30	12.5
M22	23	23.002 – 23.015	34	14
M24	25	25.002 – 25.015	36	15
M27	28	28.002 – 28.015	41	17
M30	32	32.002 – 32.018	46	19
M33	34	34.002 – 34.018	50	21
M36	38	38.002 – 38.018	55	22
M39	40	40.002 – 40.018	60	25
M42	44	44.002 – 44.018	65	26
M45	46	46.002 – 46.018	70	28
M48	50	50.002 – 50.018	75	30

DIN 609 holds the shank to k6, a few thousandths of a millimetre above nominal, for a close fit in an H7 reamed hole. This is the tolerance where a ground shank is usually needed.

DIN 7968: Fit Bolts for Steel Structures (shank h11)

d	d_s nominal	d_s min – max (h11)	s	k
M20	21	20.87 – 21.00	30	13
M22	23	22.87 – 23.00	34	14
M24	25	24.87 – 25.00	36	15
M27	28	27.87 – 28.00	41	17
M30	31	30.84 – 31.00	46	19

DIN 7968 is a structural standard with a much wider tolerance band (h11, up to 0.16 mm under nominal). A CNC turned shank meets it comfortably. The shank is always the thread size plus 1 mm.

DIN 7999 / EN 14399-8: HV Fit Bolts, Class 10.9

d	d_s nominal	s	k
M20	21	32	13
M22	23	36	14
M24	25	41	15
M27	28	46	17
M30	31	50	19
M36	37	60	23

DIN 7999 uses the large across-flats HV head, the same head as a DIN 6914 / EN 14399-4 structural bolt, so one spanner size serves both bolt types on the same job. M36 is covered by EN 14399-8.

Above M48, or a size not in these tables? We make fit bolts up to 75 mm diameter to drawing. Values above are nominal and given for reference. Check them against the current edition of the standard named on your drawing, and confirm lengths and thread lengths with us when you enquire.

Where Fit Bolts Are Used

Coupling Bolts

Flange and shaft couplings on turbines, pumps, compressors, gearboxes and rolling mills. The fitted shank passes torque from one flange to the other through bearing on the shank, and keeps the two shafts aligned.

Steel Structures and Bridges

Non-slip joints in bridges, crane girders and heavy steel frames. On old riveted bridges, turned fit bolts in reamed holes are the standard way to replace rivets that have worked loose.

Heavy Machinery

Joints that have to locate parts precisely as well as clamp them, such as machine bases, housings and split frames. Here the fit bolt also does the job of a dowel.

Fit Bolt Finish

Coating thickness matters on a fitted shank. Hot dip galvanising adds too much zinc for a close fit, so we do not recommend it on the fitted portion. Plain oiled is the most common finish. Zinc plating or zinc flake can be used when the drawing allows for the coating thickness. Tell us the finish when you send the drawing and we will machine the shank to suit it.

How to Order Fit Bolts

Send us your drawing or the standard, with:

Thread size and pitch, and shank diameter
Shank tolerance, or the reamed hole size it has to fit
Grip (shank) length, overall length and thread length
Property class or material
Finish
Quantity, and whether you need nuts and washers

We quote per piece, with the weight per piece shown. Weights for the plain bolt can be checked on our bolt weight calculator. For machined bolts without a fitted shank, see CNC machined bolts. For structural bolts to IS 3757, see HSFG and high tensile bolts.

Questions Buyers Ask

What is a fit bolt?

A fit bolt is a bolt whose shank is machined to a close tolerance to fill a reamed hole. Because there is almost no clearance, the joint carries shear load on the shank and the connected parts cannot slip. It is also called a fitted bolt, reamer bolt or turned bolt.

What is the difference between a fit bolt and a normal bolt?

A normal bolt sits in a clearance hole, usually 1 to 3 mm larger than the bolt, and holds the joint by clamping. A fit bolt has a shank machined to match a reamed hole, so the joint holds in bearing even if clamp force drops. Fit bolts cost more because every shank is machined.

Is a turned bolt the same as a fit bolt?

In most drawings, yes. "Turned bolt" describes how the shank is made, and "fit bolt" describes how it is used. A ground bolt is a fit bolt whose shank has also been ground, for a closer fit and finer finish.

Should I specify a turned or a ground fit bolt?

For structural fit bolts and rivet replacement, a turned shank is normally enough. For coupling bolts and rotating equipment, where the drawing calls for a close fit such as k6 or m6, specify a ground shank. If you are unsure, send the drawing and we will advise.

Can you make fit bolts to our drawing?

Yes. Most fit bolts we make are to customer drawing: shank diameter and tolerance, grip length, head form and thread are all taken from your drawing.

Can you supply nuts and washers with the fit bolts?

Yes. We supply fit bolts loose or as complete sets with matching nuts and washers, in the same property class.

Send Your Fit Bolt Drawing

Share the drawing or the standard, the quantity and the finish. We reply with a per-piece rate and the weight per piece.

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