In a perfect world, everything would be universal. There would only be one measuring system worldwide that everyone understands and all manufacturers use as a basis for their products.
But, unfortunately, this isn’t the case, and it even applies to the knobs and pots on your guitar. Therefore, you need to know what type of control shaft you have on your guitar in order to buy knobs that will fit it perfectly.
What is a Control Shaft on a Guitar?
If you look at a potentiometer (pot for short), you will notice a shaft coming out of it which is where the manufacturer installs a knob to control the pot. As shown in the picture below.
However, you can't normally see a pot unless you take a backplate or scratch plate off a guitar depending on the design.
All about Splines
A control shaft usually has a certain number of splines (or ridges) around its circumference. This number will dictate what knob will perfectly fit on it.
There are three common types, coarse, fine, and solid shaft.
Coarse Splined Control Shafts and Knobs
These are metric in size, which is why they are also known as Metric, or Imported. They are the most popular type of control shafts and are featured on a wide range of popular imported guitars.
They feature a 6mm split shaft and have 8 splines on each side of the split, making a total of 16 on the pot. However, to account for the split in the control shaft, the guitar knobs have an additional 2 splines, therefore totaling 18. This is how they are labeled, so if you have an imported guitar, you will probably need an 18-spline knob.
Quick Tip - If you’re not sure what type of knob you need, all you have to do is to take one of the knobs off your guitar. If you’re not sure how to do that, check out How Do I Remove the Old Knobs from My Guitar. After you have removed the knob, carefully count how many splines are on the control shaft - 16, 20, or none.
Fine Splined Control Shafts and Knobs
These are imperial in size, and include popular US-based brands such as Bourns and CTS. They are 0.235" of an inch in diameter, which is 5.95mm, making them ever so slightly thinner than the metric version, with finer splines.
Being smaller in size, they have 10 splines on each side of the control shaft split, making a total of 20. But, once again, the knob that perfectly fits onto the shaft will have additional splines (to make up for the splits in the control shaft), this time making a total of 24.
The thinner spline design means that a knob designed for a fine spline pot will not fit onto a course spline control shaft. So make sure you buy the correct knob that will fit perfectly onto the control shaft of your guitar’s potentiometers.
This type of pot is only usually found on higher-end guitars and the potentiometer brand will often be listed in the product description, guaranteeing quality.
The above-mentioned 2 shafts are split shaft control. There’s still solid shaft control.
Solid Shaft Control Shafts and Knobs
And finally, we have the solid shaft, which has no splines and features a smooth, ridgeless control shaft. It therefore requires a knob with a set screw, which is screwed directly into the control shaft, as opposed to the push-on design of the other two designs.
It is an imperial size of a ¼ of an inch in diameter or 6.35mm, making it the largest of the three sizes. Therefore you can only comfortably install a ¼” diameter knob to it because knobs of the first two splined designs will be too small.
Can I fit a Coarse Splined Knob on a Fine Splined Control Shaft?
As with everything DIY, there is always a way to get something into the wrong-sized space. Yes, you could force either a coarsecourse or fine splined knob onto the wrong control shaft and if you apply enough pressure it will go on. But it will never be a perfect fit and be completely true. Therefore any super fine volume or tone adjustments will be a little harder. It will also be a lot harder to remove in future.
However, if you have a set-screw knob, you can install it onto a coarse or fine split-shaft potentiometer but you might need a very thin sleeve to place around the control shaft. The side-screw design should secure the knob in place, but make sure that you screw into the side of one of the two shafts, not where they meet.
If you have bought the wrong-sized knobs and they don’t easily fit on your guitar, then the best advice is to put it down to experience and buy some that do. They can then be quickly and easily fitted to your guitar perfectly, allowing you precise control over your volume and tone knobs to get you the sounds you want.
Wrapping it Up
There you go, everything you need to know about the different sizes of Potentiometers and knobs on any guitar. So, now that you know what you need, it’s time to choose the perfect set of knobs that will make your guitar look amazing. If you need some advice on that, check out What Control Knobs are Right for My Guitar?.
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