Split Cotter Pins in Connecting Rods

(Rob Reilly) #1

Another Arctic cold snap in Chicago so I’m inside doing some research just to satisfy curiosity.

Studying parts catalogues, I find that SS and Jaguar used slotted nuts and split cotter pins in connecting rod bolts from before 1938 to about 1968, when they changed to a different bolt with a self-locking nut. Not sure about the side valve cars because the parts aren’t listed separately.

Counting up yearly production figures in Clausager’s Jaguar, a Living Legend, leaving out pre-'38 and post-'67, this adds up to a little over 316,300 engines, mostly six cylinders with some fours, or about 3,700,000 cotter pins.

For pushrod engines the slotted nut was part C.358 size 3/8"-20 BSF, and the split cotter pin was part L.103.5/8U for all 4 and 6 cylinder engines.

For the XK engines, the slotted nut was part C.2361 size 3/8"-24 UNF and the same L.103.5/8U cotter pin was used up until the change to self-locking nuts beginning with the Series 2 E-Type and XJ6. The 3.4 3.8 S-Type catalogues list both and mentions that the one supersedes the other.

We learn from the XK120 parts catalogue that the L.103.5/8U cotter pin was size 7/64" diameter and 1" long and called the “Drivopen” type, which was a brand name from Nettlefolds of Birmingham.

Cotter pin length is measured on the straight shank excluding the loop.

The hole in the C.3944 con rod bolt measures .120" diameter and the slot in the C.2361 nut measures .109" wide, so the pin is a snug fit.

Here are a couple of interesting rod bolts with a copper coating from a 140 engine.

The correct size cotter pins for SS and Jaguar engines can be obtained from McMaster-Carr www.mcmaster.com part number 98338A734 in a pack of 100 for under $5.

Here is the correct method to install them in con rods.
The slots in the slotted nut are a little difficult to see in this view, or it may be that this view taken from an aircraft manual is of a castlellated nut which is not quite the same as a slotted nut.

I wasn’t able to find an original patent for the slotted or castellated nut, so it may date from the mid 1800s. The earliest patent I found was #1283346 for an improvement in the slot shape dated 1918, which unfortunately does not refer back to previous patents.

I am merely passing along what I thought was interesting factual engineering information, not opinions intending to (nor interested in) starting any arguments.

(Paul Wigton) #2

Nicely-documented, and proper pins, and how to use them.

(Phil.Dobson) #3

fascinating. so what is the correct method when installing the nuts to get the hole to line up, i.e. a bit tighter or a bit loose from the torque setting.

(Dzia) #4

While I’m not here to challenge the research that Rob has done , I thought I’d just add a comment. When I was rebuilding a Rolls Royce V12 engines for the Spitfire I was working on, we used the “British” method for bending a cotter pin. The pin would lay on its side and the ends were bent to engage the slots in the castellated nuts vice the upand down noted above. Just thought it interesting in the 2 different ways things were done then.


(Rob Reilly) #5

For Gordon, I believe you are describing figure (a) here, the preferred method in aviation for nuts exposed with danger of catching on clothing and hands etc.
From helitavia.com

The reference in which I found this is CAP Civil Airworthiness Information and Procedures, section 2-5 page 2.1

Method ( c ) is used where clearance is critical, the tabs hugging the nut.

For Phil, this is approaching the precipice of giving an opinion, which I wanted to avoid, but the torque method is a secondary method of achieving what you really want, which is axial pre-load within a certain range, and an inaccuracy of 1/12 of a rotation of a lubricated fine thread nut tighter or looser to get the pin in is in most cases still within the range.

(Paul M. Novak) #6

Attached are two pictures showing what I found in the 3.4L XK engine from my 1957 MK VIII when I partially disassembled the engine to inspect it. Only seven of the 12 castleated nuts had cotter pins present. The five nuts circled in red had no cotter pins and the seven nuts circled in green did have cotter pins.

I checked in the sump and did not find any cotter pins. Bad day on the production line?


(PeterCrespin) #7

I doubt it Paul. Critical aspects were inspected by a foreman, I’m sure.

The main new thing for me is to see the term cotter pin on the package, which is a first for me in a Brit usage where they are typically called split pins. Perhaps it was an export package? Cotters are wedge-shaped solid pins for holding cranks or gears to shafts, such that modern bicycles are said to have ‘cotterless cranks’.

The Driveopen brand appears to use a subtly deformed shank where the section near the loop bulges outwards to provide a greater OD from the same thickness wire. This would ensure a snug fit when tapped in during the ‘drive’ stage. As the pin was compressed, the free ends would tend to spring open a fraction as they emerged, speeding up installation.

(Rob Reilly) #8

Agree, Paul’s green circled pin appears to be too long, installed wrong and not an original L.103.5/8U pin.

An advantage of pins on the assembly line, the inspector seeing pins can know that the nuts have been tightened.

I found those pix of Nettlefolds pins on ebay.uk by searching Drivopen so probably not export; a clever concept to spring open like that, but apparently the competing idea of one leg longer than the other prevailed in the market.

(PeterCrespin) #9

Could have been corporate fortune not technical merit, I suppose. Nettlefolds fate was to become the N in GKN.

(Rob Reilly) #10

I guess this British company in Northampton couldn’t make up their mind what to call them.

Here is the other kind of cotter pin, holds the crank on the shock absorber of my '38 SS.

(Roger Payne) #11

For what its worth, there were originally three different British Standards - BS190-1924, BS191-1924 and BS193-1924 for BSW, BSF and BSWS respectively that as well as bolts, setscrews, nuts, washers and studs, also covered/specified SPLIT PINS. No reference to Cotter Pins, solely Split Pins, with a line drawing illustrating and a full table of the ‘standard’ sizes.
In 1942 these three standards were withdrawn, due to the US/British confusion regarding SAE versus Whitworth, with an interim new wartime emergency BS1083-1942 Standard was introduced for BSW and BSF Bolts, Nuts and Setscrews only, now deleting coverage of washers, studs and Split Pins - with later updates of standard still not covering.

The BROWN BROS catalogue referring to Split Cotter Pins suggests a compromise amalgamated name, and you have to wonder about Nettlefolds using Cotter Pin - maybe some patent or licensing origin of the special ‘Drivopen’ version, which I really cannot imagine would have had much acceptance in the auto industry, for critical applications such as con-rod bolts.

Having said that, my understanding is there is only one correct way of installing a split-pin in any critical applications, and that is as per Robs illustration (b).
The head sits securely sideways within the slotted-nut or castellated-nuts slot, and the slots and splin pin head sizes are specified accordingly, and the bending of the feet ensure tight fitting, and no movement of the split-pin.

A split pin fitted as per (a) or especially ( c) will move so in an engine running at 5000rpm, can easily fatigue fracture thus the used engines found with split-pins missing.

Maybe there are special applications for (a) and ( c) methodology - but not for automotive con-rod bolts.

(Rob Reilly) #12

Quite right, Roger, the (a) and ( c ) methods are used in aircraft and not for their con rods. ( c ) seems to have the loop inside the slot, but the legs are twisted 90 degrees as they wrap around.

BTW I discovered that you have to leave a space when typing c in parentheses or the computer thinks you want a copyright symbol.

Here is the relevant page out of the XK120 parts catalogue. This and the Mark V and Mark VII are the only ones I found that mentioned Drivopen.
Mark IV has a code SP.55/G.1 but I don’t know what that means.
We could take that a couple of ways. Did Jaguar always buy Drivopen from Nettlefolds, or did they buy from other people like Brown, but just used the same part numbering system for any and all? Maybe we’ll never know, and maybe the brand name doesn’t matter as long as we get the right size and install them right.

(Roger Payne) #13

I would suspect in the early days, given use of “Drivopen” trade-mark name, good chance initially purchased was from Nettlefolds.
I do note that in my 1947 Edition “The Trader Handbook” (forty-first edition, the earliest I have of this invaluable annual) does not show ‘Drivopen’ as a current proprietary name, nor indeed still show Nettlefolds as a current Trader. It does however show Guest, Keen & Nettlefold of 24 Heath Street Birmingham (the Nettlefolds factory address), later generally known as GKN. So I suspect this ‘Drivopen’ reference may have pre-war origins, as most certainly Jaguar was using the more usual design split pin for the XK engine. Maybe a Mark IV or SS-Jaguar owner can comment.

Brown Bros. was what the British refer to as a ‘factor’, as in a retailer of items bought in from other manufacturers, and not actually a manufacturer themselves. So who made the Split Pins offered by Brown Bros. is anyone’s guess, and I cant say I have seen too many period advertisements for Split Pins. The Brown Bros. BOOKS - not merely a catalogue, are amazing, and expensive to buy and their postage - only have three (different XK relevant years - another annual)

(Roger Payne) #14

British Standard BS.1574-1949 is now a standalone standard on SPLIT COTTER PINS of the more familiar design.
This was revised to be BS.1574-1958 still called SPLIT COTTER PINS.
The next revision BS.1574-1994 is now titled SPECIFICATION FOR SPLIT PINS (inch series), so now formalising the more acceptable/common use of just ‘Split Pin’

As far as I am aware BS.1574 was first introduced in 1949, so it would seem Nettlefolds was still trading under their individual name and not necessarily yet as GKN, but it would also seem supplying/making BS.1574 compliant Split pins and not ‘Driveopen’, but still calling them Cotter Pins.