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Gib Keys

This article was originally published in the August and September editions of the Stationary Engine Magazine and is reproduced here with the kind permission of both Kelsey and the author Eric Brain.

It serves as an excellent guide to all things "Gib-Head Key" related.

A simple and very vital component, there is at least one fitted to virtually every stationary engine, but due to the ever-decreasing training in engineering skill, it is often the most maligned, least known about, most misused and often most badly fitted item in a restoration.

These are the critical dimensions of a standard key. W is the width, also the depth of the head: (and preferably the distance behind the head when fitted correctly) L is the length over the tapered portion: H1 is the thickness directly behind the head. Note that the standard USA taper is 1 in 100. (UK 1 in 96).

Eric's involvement

I was indeed fortunate to have an engineering apprenticeship and even luckier, it was in an old- style engineering factory where pride in personal skill still existed into the 1960s and 1970s.

It sounds impossible today, but I worked with engineers who could recall the time when keyways were cut by hand with a grooving chisel, filing to exact size and fitting hand-made keys. The company products were centred around high speed machinery for the paper and packaging industry. Those of you who know this industry will be aware the machinery is very fast-running and also with many reciprocating mechanisms, hence most rotational components were fitted using gib-head or taper keys for maximum security.

The trade-test for new fitters applying for a job in our works inevitably involved fitting a simple gib-head key to a shaft; many applicants were from the aircraft industry just up the road, where “assembling” meant taking components from a bin and screwing them together. They often had little or no “fitting” skill and were quickly rejected, many could not even recognise a key from a drawing. I well recall our foreman shouting at one such aspirant “That’s not a KEY – it’s a f****** WEDGE!” I also recall one such fitter on trial who thought “Loctite” was an essential for key fitting; he was very soon on his way out the door!

Some of our components had two gib-head keys, usually at 90degrees; if the machining was slightly out, these really did require special skills indeed and with such keys often up to about 1¼ inches square we really did know who our best fitters were! Often the inspectors would do a spot check; ask you to remove a key you had fitted and check all round before stamping with an individual mark before replacing in the assembled component. Having been a time-served fitter and later service engineer, I guess I must have fitted hundreds in my time, both new and re-used.

What is a Gib-Head Key?

A gib-head key (pron. with a hard G as in ‘give’) is a British Standard component and is manufactured to BS 46 (imperial) and BS4235 (metric) and later standards, but we will only concern ourselves here with imperial. The taper is 1 in 100 or ⅛ inch per foot (1 in 96). The keys are made from Keysteel (EN6 or if unavailable, EN8 or later spec.), which has high shear resistance, certainly not from “gauge plate” or, heaven forbid, from ordinary EN32 mild steel. The key is gauged for width and for height, the latter being the thickness at the large end under the head and is available in either square or rectangular section. The length is selected to suit the width of the component, being a pulley, a cam, or mainly in our hobby, a flywheel, and it is measured from the smaller end to the head or to be more precise, the width of the head’s distance away from the head to allow space for extraction. Therefore the depth of the small end is a function of the length and the dimension of the large end, and plays no part in the designation of key size. Any new component must have a new key to match the taper machined in it.

This diagram shows a correctly fitted gib head key.


Turning to the process of restoring an engine (I will use the flywheel as an example throughout as it is a major component although the principle applies to any application where a gib head key is used); if the flywheel needs to be removed from the crankshaft, this is best done, and indeed refitted, while it is still on the engine itself. Often the head of the key, if it is not shielded by a pulley, has a neat sheet-metal cover over it, fitted tightly to the flywheel boss to prevent the rotating head catching in someone’s clothing or a cleaning rag.

A is the gib head key, B is the boss of the flywheel, C shows the taper of the key fitting all down its length while D is a pressed steel cover fitted to many engines to prevent clothing or cleaning rags being caught up in the rotating gib-head. E is the crankshaft. The sketch to the right speaks for itself!!

(sketch: RS MacKay)

If the gib-head appears to be undamaged with no visible signs of a “hammer-fitter” having been at it, then you are in with a good chance.

A typical gib key seen here fitted to a Ruston Hornsby HR, there is plenty of room left behind the head for an extractor

(photo: Lefteris Hadjizacharias).

Clean the keybed in the shaft thoroughly and the shaft itself, to make sure there are no burrs or rust or anything which may impede the key coming out or indeed prevent the flywheel sliding along the shaft. At this stage – stop and think – give it all a good spray with WD40, PlusGas or similar lotion and go and have a cup of tea.

This Lister D is ready to have its key removed. Given a good soak in WD40 or similar penetrating fluid, it should come out easily.

“A little bit of early thought saves a lot of struggling later” we were always told. [1]

What have you got in the way of extractors?

An example of the most common purpose-made gib-key extractor is similar to that found in the tool box of Claas farm machinery and is a curved flat plate about ten inches in length and about 8mm thick, like a cow’s horn in shape.


A few types of key extractors from the author’s tool box. The small H-shaped one, designed and made “in-house”, came in a number of sizes, the large red one at the front is for much larger keys – above ¾ inch square

The red extractor at extreme bottom left is for heavier keys, over say 1 inch wide. It is hooked behind the head and uses the component boss as a fulcrum. A sharp blow with a hammer then levers the key loose along the keyway.

 The sketch shows the correct use of the curved key extractor, H being the end where you Hit it!

(sketch: RS MacKay)

It is inserted between the flywheel boss and the key head and carefully hammered in like a wedge until hopefully the key gives up its struggle to remain in place. The curve prevents damage to the flywheel boss as it turns the hammer forces away from it and into the direction of the removal of the key.

Use of a non-curved wedge shaped extractor such as a Morse taper drift can result in damage to the flywheel or component boss.

(sketch: RS MacKay)

It is advised to loosely clamp a G-clamp over the extractor and around the shaft to prevent the extractor rising up with the blows and jumping off or damaging the key head.

If the “Claas type” extractor is held down with a loosely fitted G-clamp, it cannot ride-up and damage the key head.

This type of extractor is paramount in the actual process of fitting a new key where the key constantly needs removing to allow the fitting process to take place. In our factory, nearly all fitters had one - or two, because they should come in ‘handed’ pairs. The inside of the curve was not square in cross-section but on an angle by a few degrees to allow it to bite into the gib-head at the very base of the head, the strongest part. The opposite-handed tool looks the same but the inside of the curve slants the other way so that if the key head does begin to burr up, it can be tackled from the other hand, giving another and possibly better, chance of removing the key and thus lessening the damage. These useful tools were a simple exercise for engineering apprentices to make, by band-sawing and filing the angle and then taking to the hardening shop to be heat-treated; another aspect of learning engineering which is becoming sadly lost in time.

There are many other types of extractor,

Home made Puller. (photo: Mike Milestone)

This simply-made plate-type puller goes over the head of the key by the slot, is turned through 90 degrees and the bolts tightened evenly thus pulling out the key. (Photo: Mike Milestone)

some easily custom-made, after a little thought, to suit the actual job in hand.

The “H-type” puller is very effective and came in a number of sizes. In our works they were given a ‘jig & tool’ number and could be drawn from the tool stores. The square hole fits over the key head and the nut tightened to withdraw the key.

A key puller which works entirely in conjunction with a slide hammer. “Q” drops behind the head of the key, the slide hammer screws in to nut D, and the alternative bracket “R” welded on underneath adjusted by screw “S” steadies the tool against the shaft while the hammer is operated.


A plate-type key puller which drops over the key head and is turned a few degrees, then the bolts screwed in against the flywheel boss, jack the key out.

If a key is reluctant to come out, some PlusGas or WD40 with carefully applied gentle heat from a blow lamp often helps, but beware overheating and cracking the flywheel spokes especially with curved spoke flywheels. These penetrants are also very flammable so care must be taken if using them with heat. Having heated it up to your satisfaction, again be patient since the release often occurs in the re-cooling process. Just put everything away and leave it and come back tomorrow and try again. Never - repeat NEVER - use a pulling device over the outside diameter of a spoked flywheel – it will never come off over a taper-key but, being cast-iron and old, the flywheel will almost certainly break.

If all else fails, this puller which is custom-made from scrap to fit around the shaft, will extract a key by screwing in the jack-bolt at the end.

(Sketch: RS MacKay)

Key Head missing?

So the head of the key broke off!! What now??

In many applications of gib-head or taper keys in our works, we had to remove the head to prevent fouling of another component. When the key was almost fitted to completion, the head was sawn off, filed square and a suitable size extraction hole drilled and tapped into the length of the key. (Easy when it’s in a drill press; not so easy if it is still in the keyway/keybed!!) The key could then be finally fitted using a slide hammer to knock it both in and out again.

This effective slide hammer was easily made from parts from the scrap bin

(photo: Mike Milestone)

Don’t panic; put that hammer away! Remember to think about how to achieve it with damage limitation. It is necessary to drill and tap into the end of the broken key; the drilling process in itself may collapse the key sufficiently to enable you to drive the flywheel back off it by putting a suitable sturdy iron sleeve over the end of the shaft and hitting the flywheel back off the key. This is often not possible though, if the flywheel is fully back against the bearing. It may be necessary to apply a little heat first but again take care if doing so with a spoked flywheel.

The hole can be drilled by making up a drill guide which fits the visible keybed, and clamping it to the shaft.

The pilot block fits neatly into the key bed and is held by a G-clamp while the key is being drilled with a tapping size hole to accept a slide hammer.

(sketch: R S MacKay)

Then, having made sure the drill bit is a standard tapping size, carefully drill as deep a hole as you dare, using an extended drill - possibly one brazed onto a short length of rod.

Hopefully you never damaged the key way or key bed………! This is a possibility if attempting to drill a key of very shallow rectangular cross section.

Make up a suitable extension tap and try and tap a good length of thread into the key up its length. Again a slide hammer is necessary here but one can be easily made up from the scrap bin using a length of stud and some fairly heavy pieces of round steel.

Access to a lathe is preferred for this but not totally essential.

Sometimes there is just a broken piece of the key sticking out but with no head. For this, removal can often be effected by making a device to use with a slide hammer, after arc-welding the device to the remains of the key, taking care not to burn the shaft itself.

The heat of the welding often helps do the trick. Ensure that the earth clamp is as near the key as possible to prevent the possibility of ‘arcing’ across any ball or roller bearings. Remove the magneto first for safety.

Once welded, operate the slide hammer and, with a little squirt of PlusGas or WD 40, you may be lucky and the key will come out.

Fitting - Old and New Keys

The tools needed to fit a taper key consist of two or more decent flat files or hand files, of about an eight or ten inch if we are dealing with an average sized flywheel key, one being a smooth file, the other being a ‘second cut’ or medium.

Use of the correct tools should result in a perfectly fitted key. Nice sharp files, an extractor, some emery cloth, engineers’ blue and above all – a vice.

To do the job correctly, a tin of engineers’ blue is useful, though if fitting just one key occasionally this can be dispensed with and a felt-tipped blue marker substituted. A decent brass drift and a hammer are also necessary. You will also need some sort of simple extraction tool as described above, especially for a new key, as the key will have to be put in and removed many times as the fitting process proceeds. If the tool described earlier is not available, a morse-taper drift from a drilling machine is useful for this, as long as the radius is ground off the back of it. Some strip emery cloth of a medium grade (120grit) is also necessary.

If attempting to refit a used key which otherwise looks OK, a straightedge (a combination- square rule) is required to ensure the bottom of the key is flat and did not get at all bent in the previous struggle to extract it.

It is important that the bottom of the key is kept flat at all times, check a used key for flatness before considering its re-use.

A bench vice is essential to hold the key while filing; our new fitting-shop vices always went to the tool room to have the jaws ground smooth before use, to avoid any marks being made on the key or indeed on any work-piece while being held and worked on.

Taking a ‘Used’ Key first:

Certain high quality engines will have the engine serial number stamped on the original keys (Ruston Hornsby & Petter being a good examples); on a twin flywheel engine there may also be a single digit number stamped, signifying which end of the crankshaft the flywheel key came from if it is not obvious, with a corresponding stamp on the end of the shaft.

These keys from the flywheels of a Petter Atomic have the engine’s serial number and the flywheel number clearly stamped in.

(photo:Philip Thornton-Evison)

Any other digits could be the fitter’s personal mark, or as previously suggested, that of a build inspector. One feature we used to employ, especially on larger taper keys, was to cut a groove in the top surface about 10 to 15 thou deep to hold a smear of grease on final assembly. I was delighted recently to find this feature upon removal of the flywheel key of my wartime Lister D, it still held grease, having been removed with very little difficulty for the first time in nearly seventy years.

The key is part way out, revealing that it was grooved before final assembly and retains a good amount of its original grease in the groove making extraction seventy years later easier.

Remove any burrs on the gib-head occurring from the extraction process using the second-cut file, reshape the head and give it a polish with emery. Check for flatness by holding a rule up to the bottom of the key; if it is at all bent, discard it and seek a replacement new key from a stockist. If it is considered to be OK, remember it is a vital component and has to do sterling service locating the flywheel as it revolves, with possible additional dynamic forces from misfires or kick-backs, and may need also to overcome certain axial forces from any governor mechanism used by the engine.

Turning to the crankshaft, and this applies equally to fitting a new key, it is essential that the end of the shaft is clean and free from burrs and rust. Gently run the edge of the file up the keybed in the shaft to clean the sides and bottom without removing metal. Do the same with the keyway in the flywheel but apart from this, NEVER file the two components, only the key. Fit the flywheel to the shaft making sure it is the correct way round. The keyway in the flywheel is tapered in manufacture so it must be fitted with the large end outwards. (An enquiry in Stationary Engine recently asked which way around the flywheels of a particular engine should be fitted? - it is obvious from looking at the tapered keyway, accurately machined in the boss during manufacture.) With the flywheel in its correct alignment along the shaft, which can be seen usually by the stains and markings from over the years, enter the greased key in the slot and gently tap it home with a hammer and the brass drift. Do not bash it in hard, this could result in a cracked flywheel boss; there should still be a gap behind the head, generally speaking about the width of the key, sufficient to get an extractor behind it when the key is properly ‘home’.


Shows a key fitted to a Ruston Hornsby engine with sufficient space behind the head for extraction.

It is held in on all four sides and by the skill of the fitting of the taper so may have to be adjusted slightly by filing, using the blue marker as a guide and then polishing off the shiny high spots. 

Now for a New Key:

So the key was badly damaged during the extraction process? Oh Dear!! We will assume a suitable new square or rectangular section key has been obtained from the local stockist or has been machined to specification from a piece of key-steel EN6 or EN8 to BS 46 by a friendly machine shop. For the purpose here of description, imagine an arbitrary size of ½ inch wide under the head. With the flywheel in place and the keybed and keyway suitably cleaned and aligned, the first thing is to check the fit in the keybed in the shaft. Standard new keys and stock key steel are a few thou oversize on width to allow for fitting.

Holding the key in the vice, carefully file and emery cloth the sides flat removing just enough metal to make a sliding fit in the keybed. The bottom edges of the key will need a small chamfer, say 0.5mm wide, up their length as the key-bed will no doubt not be perfectly square in the bottom corners. A taper gib-head key must be a tight sliding fit in the keyway/keybed and fit all over, unlike a parallel key which does not have any top-fit at all.

See if the small end of the key will enter the ½inch wide rectangular hole formed by the keybed and keyway. If it does you are well on the way, so now file a similar but wider chamfer along the top sloping edges of the length of the key. It cannot be stressed highly enough that the process of key fitting is one of trial and error and above all - patience. One respected engine restorer told me “I think of it as the most repetitive thing one has to do accurately, requiring oodles of patience” 

Let’s say it will enter by an inch, so remove the key and, turning it upside down in the vice and using the second-cut file, remove metal by filing the bottom of the key flat.

Filing the bottom of the key flat while maintaining a slight chamfer down each edge. Note the file is fitted with a proper handle as a sharp file tang can inflict a nasty wound to the palm.

Maintain the chamfers, this is essential.

Flat filing is a special skill. I have tried to teach it to countless apprentices, and later on, to engineering students with some success, but it takes time and practise to acquire the correct procedure. I cannot show you how to do it on paper, it is a skill that you ‘acquire’ with practice……… or not!

Keep the file clean by dressing it with a wire brush from time to time and keep on entering the key in the hole and re-filing, re-chamfering, until you have removed sufficient metal from the bottom for the back of the head to get to within about an inch from the boss when tapped home. Now reach for the engineers’ blue or your felt-tipped pen. Carefully coat a thin smear of blue or ink on the bottom and top of the key; a very thin smear, just sufficient to be visible. Gently tap the key in again until tight, remove it and have a look at the blue or ink. Hopefully you should see that the blue will have been rubbed off leaving a shiny high spot all down the length of the top and bottom of the key and all over. This shiny area is now the only area to file from now on. Start with the bottom of the key first and remove the shiny high spot from that by filing flat with the smooth file. Then turn to the top, which will almost certainly need some high spot metal removed. (Image: Drawfiling.jpg) Again, maintain the chamfers down the edges, top and bottom. Re-coat the surfaces with another very thin smear of blue or ink and go in again. Eventually you are aiming to get the bottom of the key shiny all up its length first, then having done so and when satisfied, keep removing shiny spots from the top of the key until it is shiny all over when fully home, to within just over the width of the head away from the boss of the flywheel, in this example ½ inch. Check constantly for a flat bottom. If all is correct, the small end of the key should be just visible at the back of the flywheel.

If you have the facility of a milling machine, you could at this stage, machine a shallow (say on a ½inch wide key, 0.5mm deep by 5mm wide) groove centrally down the length of the top of the key to retain grease. (You could have also cheated earlier by removing a few thou at a time from the bottom of the key using the mill to save filing until the final fitting!!)

Replace the key tightly, using blue, and remove again before finally finishing off with a smooth file and a polish with emery cloth. It is advisable to grease (Coppaslip or Molygrease are excellent) the key before finally gently but firmly banging it home, using the brass drift onto the sloping face of the gib-head. As I was told many times, “Remember it might be YOU who have to take it apart again – and sooner than you think!” How correct that often proved to be when “on the road”! Eventually, after first running the engine, try and remember to check the key for tightness, giving it that final tap if thought necessary.

Damaged keyways

Often, on a newly acquired engine, the keybed in the crank is seen to be damaged or burred up, Sometimes, just a polish with emery cloth and deft work with a decent file will remove the burrs but should the sides of the key-bed be severely damaged, what then? If the engine has been run for any time with the flywheel loose, then the key-bed will be enlarged or even worse, worn into a bucket shape. The shaft in this case will have to be removed and re-machined. A stepped key will also have to be manufactured to suit the step, not an easy fitting job unless you are skilled. If the wear is bad, the keyway can be welded up and re-cut but this will require a professional engineering shop and it is possible the welding process will cause the shaft to bend unless MIG welding is used. In a ‘worse case scenario’ the flywheel too may have suffered. [2]


Some dreadful bodges have been seen on the rally field involving flywheel keys. Horrific tales are told for example, of engines seen running with the flywheel held by two wood screws hammered in as wedges.

At a steam rally a few years ago, someone was sited next to a Lister engine. After about ten minutes of running, the Lister’s owner came and asked if it was thought possible he had a bent crank. On close inspection he was advised that the flywheel was moving about an inch up and down the crank shaft, and that he needed to stop the engine; it was apparent the key was loose in the keyway. The owner proceeded to wedge a screwdriver into the gap behind the pulley wheel and hammer the key back in. Twenty minutes later the engine was running and the flywheel again moving. He was persuaded to stop the engine. He again hammered the key back in and then assured everyone that the flywheel couldn’t possibly come off, as the pulley wheel would stop this. On grabbing hold of the pulley wheel and pulling on it to prove his point, all he managed to do was remove the pulley wheel from the crank as it didn’t even have a key fitted…...! Needless to say he didn’t run the engine for the rest of the day.

Similarly, at a local club rally, a Wolseley WD2 had been fitted with an unoriginal pulley wheel which had a ‘roofing bolt’ hammered in the keyway instead of a key. This engine was not allowed to be run.

Engines have been seen running with the key “secured” with a Jubilee-type clip tightened against the head of the key and around the shaft. A certain recipe for disaster.

Rally Safety Officers would gain more credulence by asking to check the tightness of flywheels before allowing any engine to run, rather than worrying how far from the public the engine is (to the nearest centimetre) or whether the flywheels rotate “toward” the crowd.

Intending purchasers of an engine, ‘restored’ or otherwise, be warned. Check first the tightness of the flywheels, carefully examine the flywheel keys as far as possible for a good indication that the key beds in the crank are not damaged to avoid some heavy expense at a later date.


Stationary Engine Parts have keys for most Lister Petter engines in the relevant sections of the website. However if you need a key that isnt listed please contact us and we will do our best to assist, failing that the following may be of assistance.

Any good engineers’ supplier should be able to supply a gib-head key blank to suit the dimensions given. Here are a few quoted examples, used and recommended by various experienced engine restorers but Google should produce one for you in your area:-

  1. Tasman Industries Ltd www.keysandpins.com
  2. Mallard Metals supply all metals in the size you want; you will find them very helpful.
  3. Noggin End www.nogginend.com for keysteel
  4. College Engineering Supply www.collegeengineering.co.uk who are cheaper on specific things, but can be dearer in total including shipping

If difficulty in obtaining ready-made key blanks, then ask a competent engineering shop to make you up one (or a pair) to your requirements. Make sure they know they are made in the correct specification of steel, EN6 or EN8 and to BS46 or later standards.

Further reading:

  • Sketches for this feature were borrowed from an article in May 1992 issue of Stationary Engine magazine issue 219, entitled “Key Lore for Beginners” by the late Richard MacKay to whom the author is most indebted - I could not better them!!
  • [1] See Nigel Mc Burney’s article in Stationary Engine magazine issue 297, November 1998 “Fitting a stepped flywheel key”
  • [2] See Nigel Mc Burney’s article in Stationary Engine magazine issue 281, July 1997 “Removing a tight key”

Both these last two articles were published as part of an excellent series “Restoring a Ruston Hornsby Open Crank engine”, also still available in book form from Kelsey Publishing.

My thanks to all concerned who supplied photos, diagrams, etc to enable me to compile this feature: Mike Milestone, Patrick Knight, Philip Thornton-Evison, Lefteris Hadjizacharias and to the late Richard MacKay for his article in Stationary Engine magazine nearly twenty years ago.

Eric Brain.