C link small

Connecting lengths of chain together is relatively easy if the joint does not have to pass through a windlass. The biggest problem to be faced may be finding a shackle with an end sufficiently small to pass through the eye of the chain, so that sometimes it may be necessary to use two shackles fitted back-to-back.

Lok-a-Loy A few patent connectors are available for the job, such as the Mid-link, Quick-link, Lok-a-Loy and others.   This photo shows one of this type. Although strong enough for anchoring they will not pass through a gypsy and tend to rust quickly

However, if the chain is used with a windlass none of these solutions is possible. The only method available is to use a pair of specially designed C-links that can be riveted together, forming a link of the same dimensions as the chain. During the testing of anchor connectors, see YM May 2006, two C-links were tested to destruction but their Ultimate Tensile Strength (UTS) was disappointing, being less than half the value of the chain. Both links were bought cheaply at UK chandleries.

In view of this poor performance we decided to investigate whether C-links bought from specialist sources, both in UK and abroad, would give better reliability than the standard items. The Internet was able to provide a number of sources, both from yachting or lifting and hoisting suppliers, who supplied a variety of links. Many were made in USA, so postage costs need to be added if following this route.

A problem that immediately became apparent was that, although only 5/16 in or 8 mm links were purchased, they were not the same size as DIN 766 calibrated chain. The wire size was correct but the link size was often larger than a chain link. Fortunately, most gypsies can cope with a single link that is bigger than the chain links. During research for this article I discovered that my own anchor chain was joined using a larger C-link. It had been in use with two windlasses for many years.

DIN 766 chain dimensions

DIN 677 dimensions

Nominal wire thickness d (mm) Internal length t (mm) External width b (mm) External length e (mm)
Chain 8 24 26 36
Crosby link 8 23.88 29.5 42.9
Suncor link 8 25.4 28.58 44.45
ACCO link 8 Similar to Crosby


What was tested

Description Supplier Internet address Supplier’s code Maker Internet address Maker’s code Safe Working Load Manufacturing method
5/16 in. Connecting link.


West Marine www.westmarine.com





1950 lbs

(886.4 kg)

Forged, heat treated carbon steel
5/16 in. G-335 ‘missing link’


Selby Eng and Lifting Safety, Leeds



Crosby Group



1950 lbs

(886.4 kg)

Forged, quenched and tempered carbon steel
5/16 in. Suncor 316-NM Bosun Supplies www.bosunsupplies.com


Suncor, USA



2000 lbs

(909.0 kg)

Forged, stainless steel
Maillon a river Inox Uship



 Bosun, USA



1000 lbs

(454.5 kg)

 Cast, bright stainless steel
China G30 Chandlery, France Unknown, China

(G 30 refers to Grade 30)

1900 lbs

(863.6 kg)

Cast, low carbon steel
Italy, 5/16 in. Chandlery, Holland  Unknown, Italy ?  Cast, low carbon steel
Unmarked Chandlery, Holland  Unknown ?  Cast, low carbon steel
8 mm chain Bussells, Weymouth





BS 6405 Grade 30 6156-30

(795 kg) Low carbon steel

Note that these links and the chain have a safety factor of 4. The Safe Working Load (SWL) is therefore one quarter of the Ultimate Tensile Strength (UTS).  Use of this equipment at loads higher than the SWL is not recommended and C-links should generally not be used in lifting equipment, although the Crosby links came with a certificate from the supplier approving their use in lifting applications.

All links were corrosion resistant, either by being galvanised or by being manufactured from stainless steel.

Test procedure

We made up test pieces using 8 mm links joining two short lengths of 10 mm chain, to ensure that the link would break before the chain did. In one case only, the single un-named 8 mm link, we used 8 mm chain because the link was too small to pass over the 10 mm wire thickness. Each test piece was then pulled to destruction in a 50 tonne Dennison tensile test machine, at a constant strain rate of 1 mm per minute.

Most links were tested in duplicate. We also pulled a length of 8 mm chain for reference purposes.

All links were tested with a magnet to confirm their material of manufacture.


Test Results

Description Test with Magnet Test 1 UTS (tonnes) Test 2 UTS  (tonnes) Mean SWL measured



ACCO chain connector Magnetic 4.7 4.6 1.16 tonnes

(1160 kg)

Crosby G-335  ‘missing link’ Magnetic 4.5 3.8 1.04 tonnes

(1040 kg)

Suncor 316-NM Non-magnetic 3.4 3.1 0.81 tonnes

(810 kg)

Uship Maillon a river inox very slightly magnetic 2.5 2.5 0.625 tonnes

(625 kg)

China G30 5/16 Magnetic 3.3 5.4 1.09 tonnes

(1090 kg)

Italy 5/16 Magnetic 2.9 N/A 0.73 tonnes

(730 kg)

Unmarked 8 mm Magnetic 1.3 N/A 0.33 tonnes

(330 kg)

Previous test

YM May 2006

1.75 1.95 0.46 tonnes

(460 kg)

8 mm chain 4.9 1.23 tonnes

(1230 kg)

Technical terms

Cast Molten metal poured into a mould the shape of the final object. Lowest mechanical properties.
Forged Hot or cold metal bar is stamped or pressed into final shape. Mechanical properties improved.
Quenched and tempered A two-stage heat treatment process that maximises strength and toughness.
Heat treatment Various processes that may include quenching and tempering, that improve toughness and strength.
Low carbon steel Also known as mild steel. General purpose steel with carbon content below 0.2%. Not hardenable by heat treatment.
Carbon steel Carbon content above 0.2 %. Heat treatable.
Stainless steel Non-magnetic grades are not heat-treatable. Cold forging can induce slight magnetism in some grades.

The difference between heat-treated and cast links was immediately apparent when we riveted them to join the two halves together. Whereas the cast ones required only a couple of taps with a hammer to press the rivets down into their countersunk holes, the heat treated links took considerable effort and a number of heavy blows using a steel drift. This is not a job that should be done on board.

The C-links we tested fall into several distinct groups.

  •  Firstly, there is the fully identifiable industrial type, made of carbon steel using the optimum metallurgical processes of forging and subsequent heat treatment. The ACCO and Crosby fall into this group. The industrial C-links came with documentation that included the SWL and other information. Our test programme has found these links to be virtually as strong as the chain and therefore perfectly acceptable for anchoring use.
  •  Secondly, the stainless steel types. These appear to have been aimed at the leisure marine market. The emphasis has been put on their corrosion resistance, rather than their strength and thus the stainless steel links were weaker than the carbon steel types. The Suncor links failed to reach their advised strength although the values achieved were reasonable and repeatable. They were easier to assemble than the carbon steel links and could be an acceptable compromise for the yachtsman.
  • Thirdly, various miscellaneous low-carbon, cast types. Many are made as cheaply as possible and are often the ones on sale in chandleries. They can be difficult for the yachtsman to identify. Two of them, ‘China G30’ and ‘5/16 Italy’ were probably made to match the Grade 30 specification for low-carbon chain. The two ‘China G30’ links were bought in a chandlery with no documentation. The China link that failed at the lower load appeared to be rather brittle, suggesting that heat treatment had not been carried out correctly. Although the mean UTS was good the lower value was below the required strength.  The two links tested in the YM May 2006 tests, and our only unmarked link, failed at an unacceptably low figure. The blackening of the unmarked link’s fracture face is almost certainly due to poor quality material, containing slag and oxides. The single link marked Italy was slightly disappointing, although still acceptable for the majority of anchoring applications.

This C-link was particularly poor, failing at a low load and appearing to be largely composed  of slag.

Failed C link

On inspecting the failed links we can see immediately that their fractures differ. The more ductile stainless steel links deformed but none fractured across the wire diameter, whereas the steel links all fractured in a more brittle manner. If all links had the same strength, the more ductile ones would be better in shock loading, such as snatching in waves. Taking the strength into account, a chandlery-bought stainless steel link is probably better than a chandlery-bought steel one but not as good as a specialist heat-treated one.

The reference chain failed in a ductile manner, with considerable deformation before fracture. This behavior augurs well for snatch loading and shocks.


Heat-treated, carbon steel C-links that are marked with a load standard are virtually as strong as the chain which they join. Provided they will pass through the gypsy, which most will, they are fully acceptable for yachting use.

Stainless steel C-links are less strong than carbon steel equivalents but are adequate for general use. Although they will not rust in normal anchoring use, they can promote increased rates of zinc loss from attached chain, by galvanic action. Use them with caution.

Links of the type often seen in chandleries, unmarked and of unknown origin, should be avoided. Many seem to be badly made, of inappropriate materials, and to use them for anchoring would be risky.

Further information

Since this work was carried out there have been some changes so far as manufacturing is concerned. It appears that ACCO have been bought out by Peerless and manufacture has been transferred to the Far East. The C-links now sold by West Marine appear to be fully satisfactory. I intended to test them until I found that delivery would cost $70.

I have found a UK supplier who is prepared to sell single items by mail order, no minimum order. The company is Tecni-cable, based in Somerset. See http://www.tecni-lift.co.uk/Products…5-Missing-Link. Listed under Crosby products or search G335 from the home page. They will carry sufficient stock for the likely numbers required by yacht owners, in 8, 10 and 13 mm sizes.

Some buyers have reported that the Crosby links will not articulate correctly in the final links of the chain being joined. This is probably because the Crosby links are made to a USA standard, not a metric one. The solution is simple. Squeeze the final link of the chain in a vice to open it up a little. Grades 30 and 40 chain are in the normalised condition and will not be harmed by this minor deformation. The chain will still pass through a windlass.

8 mm chain with internal width 10 mm. Crosby C-links require slightly more, maybe 10.5 mm. Final link of the chain being squeezed in a vice. I needed to slip a length of pipe over the vice handle to deliver sufficient torque. Final link width is now 12 mm. Damage to the chain is minimal, although this sample chain is well used and some surface deposit has flaked off.


Some C-links are extremely susceptible to corrosion. This does not appear to be for galvanic reasons but simply because the cheaper versions are made from poor quality steel without any alloying elements that might protect them. Frequent inspection is advised, especially for chain that may have spent a winter in a wet anchor locker. Two examples to illustrate this:

It might be supposed that some form of galvanic reaction between  the C-link, galvanised and stainless steel chains might be responsible for corrosion of the C-link. However, in this case both chains are galvanised and the zinc on each appears to be intact. The corrosion is simply due to the poor electroplated zinc coating on the C-link and highly vulnerable steel of manufacture.