How Long Should a Splice Bury Be? Everyone Measures in Fids, Not Diameters

Search this question and you will be handed three numbers that appear to be in flat contradiction: 73.5 diameters, 72 diameters, and 30 diameters. That is a two-and-a-half-fold spread on a dimension that decides whether a spliced eye holds.

They are not three opinions. One is superseded, one is folklore, and one is not a recommendation at all.

What this page is, and is not. It reconciles the bury-length numbers you will see quoted. It is not a splicing tutorial — taper length, tuck sequence, measuring direction and lock-stitch technique all matter as much as bury depth, and none of them is covered here. They are in the instruction sheets linked at the bottom. If you do not have a sheet for your rope, get one before you cut anything.

The Unit That Goes Unconverted

Splice instructions are not written in rope diameters. They are written in fids — the tapered tool you use to draw the tail through the rope — and a fid’s length is itself a fixed multiple of rope diameter.

New England Ropes states it outright:

“A “Fid” length equals 21 times the diameter of the rope (Ref Fid Chart).”

New England Ropes states the same ratio a second time inside the splice steps themselves, adding the circumference equivalent: a fid length equals 21 times the diameter of the rope or 7 times the circumference.

The Oregon forestry chart confirms the same ratio from the other end, in a table rather than a sentence: 3/8-inch rope gets an 8-inch fid; 1-inch rope gets a 21-inch fid.

That is the whole key. 3.5 fids and 73.5 diameters are one instruction written two ways — 3.5 × 21 = 73.5. Most of the apparent disagreement in this topic is people quoting the same rule in different units and assuming they have found a conflict.

What the Manufacturers Actually Publish

Convert everything to diameters and the picture is a modest range, not chaos:

Samson’s current sheet, Step 3:

“Measure 4 tubular fid lengths from Mark 3, make Mark 4. Insert fid and tapered tail at Mark 3 and bring fid out beyond Mark 4.”

So the real published range is 63 to 84 diameters — three to four fids. Which end applies to you depends on whose rope is in your hand, and the answer is on that maker’s instruction sheet, not in an average.

If You Have No Instruction Sheet

Which, for aftermarket winch line, is the normal case. Rebranded rope arrives on a spool with no paperwork and often no traceable maker.

Default to the top of the range — 4 fid lengths, 84 diameters. Two reasons. It is the current published figure from the maker whose rope most synthetic winch line actually is. And the one revision we can observe went upward: Samson’s own spec moved from 3.5 fids to 4, not the other way. That is a single data point rather than a trend, so we are not going to claim specs never shorten — but it is the only direction of travel we have evidence for, and the cost of the longer bury is a few extra inches of rope.

If you can find out who made the rope, ask them for the sheet. A rope worth splicing is worth one email.

One nuance worth stating rather than smoothing over: Samson’s document explicitly scopes itself to Class II rope — “made in whole or part from any of the following high modulus fibers: HMPE, LCP, Aramid and PBO” — which is what synthetic winch line is. New England Ropes’ single-braid section does not name a fibre class the same way. So the cleanest evidence that makers genuinely differ is Samson against its own older self, which is a like-for-like comparison: same company, same rope, same splice, 3.5 fids then, 4 fids now.

Where 73.5 Comes From, and Why It Is Not Current

It is Samson’s, and it is real — it simply is not the number Samson publishes today.

The Oregon OSHA and OSU Extension forestry document tabulates a 3.5-fid bury section for every rope size, and does the arithmetic in inches so a crew in the field does not have to:

And it says where it got them:

“NOTE: Based on splicing instruction for Samson Rope Technologies AmSteel Blue ropes. Length may vary by rope manufacturer, rope construction and rope fiber.”

That is a properly sourced public document doing a genuinely useful job. It is simply working from an earlier revision. Samson now specifies more bury than 73.5 diameters, not less — so if you are working from the old figure, you are on the short side of the current spec.

30 Diameters Is a Danger Floor, Not a Target

This is the correction that matters most, and the reason this page is marked safety-critical. It also needs stating carefully, because the evidence on either side of it is not equal.

What we can show: we could not find 30 diameters recommended as a bury length in any manufacturer splice instruction we opened. The lowest primary figure we found anywhere is New England Ropes’ 3 fid lengths — 63 diameters, more than double it. Thirty diameters is about 36% of Samson’s current 84.

What we cannot show: where the number came from. It is widely repeated as a slip-risk threshold — the depth below which a splice without a locking Brummel starts to be at risk of the tail pulling through — attributed to a Marlow technical contact, with 50 diameters given as the working recommendation instead. We could not verify that against any primary document, and we are not going to present a relayed attribution as established sourcing on the page’s most consequential point.

The conclusion does not depend on resolving that. Whatever 30 diameters originally described, no splice instruction we read recommends it, and building to it would put you at about 36% of what the rope’s own manufacturer currently publishes.

One thing worth understanding about the failure mode either way: an inadequate bury does not fail as a clean break at a predictable load. The tail progressively pulls through, which means a splice that has begun to fail can still look intact from outside.

72 Diameters: Not Wrong, But Not a Spec

The 72-diameter figure appears in marine-rigging discussion and gets repeated widely. We could not trace it to any manufacturer splice instruction we opened — not Samson’s, not New England Ropes’, not Yale’s, and Marlow’s own splicing page publishes no bury figure at all.

It lands inside the genuine 63-to-84 range, so following it would not put you anywhere dangerous. But it is rigging lore rather than a published specification, and it should not be cited as though a rope maker stands behind it.

You Cannot Check a Finished Splice

Worth saying plainly, because most people reading this bought their line pre-spliced rather than splicing it themselves.

Once a bury is made, it is inside the rope. The tail is concealed along its whole length. You cannot measure it, see it, or feel it, and no amount of inspecting the finished eye will tell you whether it was built to 3 fids or 4 — or to something shorter than either.

So if you did not make the splice, you are trusting the maker’s process, not judging the rope in your hand. What you can check is limited and none of it establishes bury length:

• Whether it is lock-stitched — visible as stitching through the buried section.
• Whether the eye and throat look symmetrical and evenly tapered rather than lumpy or abruptly stepped.
• Whether the seller will tell you what spec it was built to — a fid count or a diameter multiple. A supplier who cannot answer that has not told you the rope is bad; they have told you that you have no way to know.

That is the honest position, and it is the reason the numbers on this page are more useful before you buy than after.

Why the Channel Is Longer Than the Tail

A detail worth understanding, because it looks like an error until you know the reason: the bury channel you measure is longer than the tail you put into it. Samson buries a 3-fid tail into a 4-fid channel.

The Oregon document explains it in one line, from the shop floor:

“Note length shortens, thus the need for 3-1/2 fids to bury the 3 fid tail”

The rope compresses and shortens as the fid is drawn through it. Measuring the channel to the tail’s length would leave you short at the end of the pull.

The same document is also blunt about what the lock stitch is for:

“Lock-stitching the buried tail section with braided nylon twine and yarn needle. THIS IS NOT A STRENGTH ELEMENT. It prevents the manual removal of the buried tail section.”

And it adds a bedding-in step most people skip: “Prior to use, load (pull-on) the line to 10 percent of it’s breaking strength to remove the construction stretch in the rope and eye splice.”

Why Any of This Matters

Because the splice is the reason to use rope this way at all. Samson — and the second half of this sentence matters as much as the first:

“the splice can retain from 90% to 100% of average new rope strength and in used rope up to the same proportion of residual used rope strength.”

Marlow puts the same idea as a ceiling on the loss: “A good splice using the recommended method should not reduce the strength of a rope by more than 10%.”

Against the Cordage Institute on the alternative:

“A splice is always preferable to a knot, which can reduce rope strength by approximately 50%.”

A correct splice costs you almost nothing. A knot costs you half the rope. The bury length is what decides which one you have actually tied.

On used rope, read that Samson clause carefully. The 90–100% applies to residual strength — a rope that has already lost capacity to use, UV or a previous overload keeps its splice efficiency, not its original rating. New England Ropes adds a practical warning for anyone re-splicing a line that has already been in service: “Through use and exposure to the elements, the fibers in a used rope have probably lost some of their original strength. In addition, some shrinkage has probably occurred making this splice the most difficult to perform.”

If you are re-splicing a rope because the last splice let go, that is the paragraph to sit with.

Don’t Confuse These

• Fids versus diameters. Nearly every “conflict” on this topic is a unit conversion. 1 fid = 21 diameters.
• The tail versus the channel. They are different measurements and the channel is deliberately longer.
• A slip-risk floor versus a specification. 30 diameters is the first; nothing published treats it as the second.
• Rope class. Samson’s Class II sheet covers HMPE. Yale’s outwardly similar 12-strand instruction covers polyester. Check that a figure is even about your rope before comparing it to another.
• A lock stitch is not structure. It stops the tail being pulled out by hand. It is not what holds the load.
• These are general rigging specs, not winch-tested ones. Samson’s, Yale’s and New England Ropes’ instructions are written for splicing generally; the Oregon document is forestry. A winch line takes repeated shock loading through a hook or thimble, which is a different stress profile from a static rigging splice, and none of these sources quantifies whether that changes the right bury length. Treat the figures as the published floor for a synthetic splice, not as numbers validated against winch loads.

How We Chose

Three primary documents were downloaded, confirmed as real PDFs by content type rather than by status code alone, and read directly: Samson’s current Class II eye splice sheet, the Oregon OSHA and OSU forestry document, and the New England Ropes splicing guide. Every fid figure quoted here was checked in the document itself, and the 21-diameter fid ratio was confirmed independently in two of them — once as a sentence, once as a table.

Two figures are reported here as unverified against any manufacturer document: the 72-diameter rule and the 30-diameter slip threshold. Both are labelled as such in the text rather than mixed in with the manufacturer specs. Where the origin of a number could not be established, this page says so instead of picking the version that reads best.

We also did not find an independently published bury specification from the winch-rope brands themselves — Warn’s rope page carries none, and several other brands’ documents were unreachable rather than silent. That is a limit on what we could open, not a finding about what those companies publish. Full sourcing and rating methodology: review methodology.