There's an active market in aftermarket parts for many bass guitars, as well as the parts supplied by the bass makers themselves. Broadly speaking, musicians will replace parts when they wear out or become faulty, substitute parts in order to upgrade or improve the instrument, or customise the instrument's appearance for purely cosmetic reasons.
The headstock of a bass guitar is an integral part of the neck, and it holds the geared machine heads, sometimes called tuning pegs or tuning pins, which allow the player to tune the strings. Generally speaking, they're unlikely to give problems, but if an instrument has been badly stored, corrosion may have taken hold, or in rare cases, premature wear has left the machine head unable to function.
Whilst aftermarket machine heads are available, the bassist should look at these carefully with regard to a number of factors.
The tuning post passes through the headstock, and should be of the same size as that on the machine heads being removed. The machine head is typically secured to the back of the headstock with one or more small woodscrews, and if the design of the replacement heads differs from the originals, new pilot holes will be required to fit them, and the bassist may be left with the original securing holes visible. Generally speaking, it should be clear from the seller if the heads will be a direct replacement.
Ideally, the maker's own machine heads, if still available, will be the best option, and should fit without leaving any evidence of replacement.
There may be a string tree fitted to the front of the headstock, intended to hold the strings back from the nut, such that they are aligned to the tuning posts. This is a simple element, and shouldn't need replacement unless cosmetically damaged through corrosion or similar. Secured with a single woodscrew, it's likely that either maker's own or aftermarket parts would be suitable.
If the neck is fitted with a truss rod, the adjustment nut is sometimes below a cover on the headstock. Since this is fitted for cosmetic purposes, the only valid reason to replace it is for the same cosmetic purposes. Most covers are screwed to the headstock with three or so screws, and replacement is merely a matter of unscrewing the original and putting the replacement in place.
Fitting a replacement nut, usually due to wear and tear or damage, is a specialist process, and best left to a qualified guitar technician or luthier. The owner could purchase a nut for the technician or luthier to fit, but unless the bassist has any specific requirement other than direct replacement, it's probably best to leave the selection up to the technician.
The Neck and Fingerboard
The item(s) most often replaced in the neck area are the frets – the bars of wire which allow the bassist to play defined notes. With excessive use, these can become pitted, leading to buzzing and intonation problems. In extreme cases, the fingerboard can become worn down in the area between the frets, and may require replacement. In the case of fretless basses, all the wear will be on the fingerboard itself.
Again, these are specialist tasks, best left to a guitar technician or luthier, and unless the bassist has any special requirements for the fret wire to be changed, perhaps to a thicker or thinner type, or for a specific kind of wood for the fingerboard, it's best to leave this to the experts.
Many bass guitars have bolt-on necks, especially Fenders. This was a conscious design decision in the early days of Fender guitar and bass manufacture, the reasoning being that if the neck, fingerboard or frets became damaged or worn, the bassist would simply unbolt the neck and bolt on a replacement. There's no reason not to take this approach, and many suppliers offer fully assembled necks, ready to fit to matching bodies. Given bodies and necks with similar joint patterns, there's scope for mixing and matching between necks and bodies – one notable example of this was John Entwistle of The Who, who at one stage grafted a Fender Precision neck onto the body of a Gibson Thunderbird.
Many basses are designed and built with 'through' necks, where one piece, or collection of laminated pieces, is used from the headstock right through to the butt of the guitar, and clearly there's no scope for neck replacement here.
As mentioned above, there's scope with bolt-on necks to mix and match necks and bodies. Replacement bodies are widely available, and the bassist could, if they desired, replace the body and keep all the other components of their bass. The replacement body could have different woods, or a different finish, or both. Unfortunately, it's difficult to judge the effect of a different body on the sound of the bass until fully assembled, so this level of replacement shouldn't be taken lightly.
The single part affecting the sound of an electric instrument the most is the pickup or pickups. When replacing a faulty pickup, most bassists are likely to want a replacement which retrofits easily and is a cosmetic match for the existing item, and again, the best approach here is likely to be to source a manufacturer's original replacement.
Those who are looking to upgrade and experiment, although they may well need to make physical alterations to the instrument, and may be left with marking or screw holes as a visible legacy of the original fitment, will find a range of aftermarket suppliers. Some of these aftermarket pickups may be designed to fit a particular bass, and will be a direct cosmetic and physical replacement – others may have particular tonal features, and could be marketed as generic items, not necessarily styled physically to match any particular bass. Depending on the relative sizes of pickup and body cavity, some woodworking skill may be required to fit generic aftermarket pickups, and some soldering skill will be needed to graft the replacement pickup to the electronics.
In the case of simple bass electronics, typically with one or two volume and tone controls, replacement is fairly straightforward for those with basic soldering skills, and there's no real difference between maker's own and aftermarket products.
For basses with more specialised active electronics, remedy of any faults is probably best made by sourcing the maker's own parts, unless the bassist wishes to experiment with replacements offering different tonal characteristics. It should be borne in mind that some bass guitar makers may design their electronics to fit a certain body shape, and alternative options may not physically fit the bass concerned.
General Body Hardware
On most basses, the only remaining parts are elements like scratchplates, pickup guards, output sockets, and strap buttons. Scratchplates from aftermarket suppliers are usually designed with a specific model in mind, and are often offered in radically different finishes from the maker's own, sometimes in mirrored or exotic styles. In these cases, replacement requires no specialist techniques, the aftermarket item matching the original in size, shape, and position of securing screw holes.
Pickup guards, metal plates fitted over the strings to hide the pickups, are probably best bought from the maker's own parts, or as aftermarket items designed for a specific model to achieve a good visual and cosmetic match. The most likely reason for replacement will be wear or corrosion.
Replacement strap buttons can be sourced which offer some form of locking system, which may be an upgrade on the originals fitted by the maker, or the bassist may merely want to replace a damaged or corroded strap button. Again, no specialist skill is required to unscrew one and replace with another.
Finding Bass Guitar Parts on eBay
From the eBay home page, select Buy, and from the category list, choose Musical Instruments. Some sellers may place their parts listings under the general Accessories/Equipment category, but others will place them under Guitars, Bass, and Electric Bass.
Parts which require no specialist knowledge or expertise to fit can improve both the sound and cosmetic appearance of a bass guitar, and the bassist can choose from both their maker's own, or aftermarket suppliers.