Which RAM Drill-Down Base Do I Need? B, C & D Sizes Explained
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The two things you have to get right
A drill-down base is the part that bolts to a flat surface and gives you a RAM ball to build a mount from. To pick the right one, only two things really matter: the ball size (B, C or D) has to match your arm and your device cradle, and the plate (round, square or rectangular) has to suit your surface and its hole pattern. Get those two right and the whole mount clicks together. Get either one wrong and nothing lines up, which is exactly why bases are the part people most often order twice.
RAM uses a two-part system, and once you can read it the guesswork disappears. Everything below is built around it.
Start with the ball size, not the plate
The ball is the round rubber knuckle on top of the base. Its diameter sets the "duty" of the whole mount, and it must match the socket arm and the cradle you are connecting to. A C-size arm only grips a C-size ball, so mixing sizes is the number one reason a base "doesn't fit."
- B size (1" / 25.4mm ball): medium duty. Phones, small GPS units, action cameras, light marine electronics.
- C size (1.5" / 38.1mm ball): heavy duty and by far the most common. Tablets, chartplotters, radios, agricultural and marine gear.
- D size (2.25" / 57mm ball): extra heavy duty. Large screens, tractors, forklifts, and anywhere there is serious vibration or a heavy device to hold.
Work out your size first, because it decides which bases are even on the table. (E size, the 3.38" ball, exists for industrial and military jobs, but the standalone plate range is thin, so we have left it aside here.)
Then choose the plate shape for your surface
Once the ball size is settled, the plate shape is about where you are bolting it and what holes are already there.
- Round: the smallest footprint and the do-anything default. Four-hole AMPS pattern, drops onto a dash, panel or deck with minimal fuss.
- Rectangular: a longer plate that spreads the load and gives you more bolt spacing. Handy where you already have AMPS-style holes, or want extra hold on a vibrating surface.
- Square: comes in two types. A compact 38 x 38mm utility square (available in B, C and D) for a tidy four-bolt footprint, and the larger VESA squares (C and D) made for screens with a 75mm or 100mm pattern on the back.
The bit people get wrong: hole patterns
This is where the whole order goes sideways. Not every plate shares the same hole spacing. The round bases and the small B rectangular base all use the standard four-hole AMPS pattern (30 x 38mm / 1.181" x 1.496"). The larger rectangular plates and the VESA squares use their own, wider spacings. So before you order, measure the holes on your existing cradle or surface and match them. If there are no holes yet and you are drilling fresh, you have a free choice, and round is usually the easiest.
Not sure which pattern you have? RAM publishes an official Hole Pattern Reference Guide (PDF) that maps every hole pattern to the exact base part number. Measure your holes centre to centre, find the pattern in the guide, and it points you straight at the SKU. It is the quickest way to be certain before you buy.
RAM base plate size chart: B, C and D
All plates below are powder-coated marine-grade aluminium with the ball fitted, and all carry RAM's lifetime warranty. Dimensions are the plate outline; the hole pattern is centre to centre.
| Shape | Ball size | SKU | Plate dimensions | Hole pattern |
|---|---|---|---|---|
| Round | B (1") | RAM-B-202U | 63mm dia (2.5") | 4-hole AMPS 30 x 38mm |
| Round | C (1.5") | RAM-202U | 63mm dia (2.5") | 4-hole AMPS 30 x 38mm (plus 3-hole 120°, 46mm) |
| Round | D (2.25") | RAM-D-254U | 63.5mm dia (2.5") | 4-hole AMPS 30 x 38mm |
| Square (38mm) | B (1") | RAM-B-202U-22 | 51 x 51mm sq (2" x 2") | 4-hole 38.1 x 38.1mm (1.5" x 1.5") |
| Square (38mm) | C (1.5") | RAM-202U-22 | 51 x 51mm sq (2" x 2") | 4-hole 38.1 x 38.1mm (1.5" x 1.5") |
| Square (38mm) | D (2.25") | RAM-D-202U-22 | 51 x 51mm sq (2" x 2") | 4-hole 38.1 x 38.1mm (1.5" x 1.5") |
| Square (100mm VESA) | C (1.5") | RAM-246U | 120mm sq (4.75") | VESA 100 x 100mm & 75 x 75mm |
| Square (100mm VESA) | D (2.25") | RAM-D-246U | 120.7mm sq (4.75") | VESA 100 x 100, 75 x 75 & 100 x 50mm |
| Square (75mm VESA) | C (1.5") | RAM-2461U | 92mm sq (3.625") | VESA 75 x 75mm |
| Square (75mm VESA) | D (2.25") | RAM-D-2461U | 92mm sq (3.625") | VESA 75 x 75mm |
| Rectangular | B (1") | RAM-B-347U | 51 x 43mm (2" x 1.7") | 4-hole AMPS 30 x 38mm |
| Rectangular | C (1.5") | RAM-202U-225 | 51 x 63.5mm (2" x 2.5") | 4-hole 38.1 x 50.8mm (1.5" x 2") |
| Rectangular | D (2.25") | RAM-D-202U-225 | 51 x 63.5mm (2" x 2.5") | 4-hole 38.1 x 50.8mm (1.5" x 2") |
| Rectangular | D (2.25") | RAM-D-202U-25 | 51 x 127mm (2" x 5") | 4-hole 38.1 x 114.3mm (1.5" x 4.5") |
One extra option worth knowing: in D size there is also a larger 93.5mm (3.69") round drill-down base, the RAM-D-202U. It is a bigger, heavier footprint and does not use the AMPS pattern, so only reach for it when you specifically need that larger plate rather than a cradle connection.
A note on square bases: two different jobs
Square plates cover two different jobs, and it pays to know which you are after. The compact 38 x 38mm utility square runs across all three sizes, B (RAM-B-202U-22), C (RAM-202U-22) and D (RAM-D-202U-22), and gives you a tidy four-bolt footprint on a flat panel. The larger VESA squares are the screen-mounting plates: they come in C and D only and bolt to the 75mm or 100mm VESA pattern on the back of a monitor or display. There is no B-size VESA square, because screens sit on the sturdier C and D balls. So if you are chasing a screen mount, go C or D VESA; if you just want a small square footprint, the 38mm utility square has you covered in any size. Some of these we bring in to order, so get in touch if one shows as out of stock.
So which one should you actually buy?
If you are still not sure, this covers most of what walks through the door:
- Phone or small GPS on a flat panel: round B, the RAM-B-202U. If you would rather two bolts than four, the B-size diamond base does the same job.
- Most tablet, marine, radio and ag jobs: round C, the RAM-202U. This is the safe default and the base we reach for most.
- A monitor or screen with a VESA pattern: square C (RAM-246U) for lighter screens, square D (RAM-D-246U) for heavier ones.
- An AMPS-pattern GPS (Garmin zumo, TomTom Rider and similar): rectangular B, the RAM-B-347U, which lines straight up with the AMPS holes on the cradle.
- Big screens, tractors, forklifts or heavy vibration: go D size, round (RAM-D-254U) or square depending on the surface.
FAQ
What is the difference between B, C and D bases?
It is the ball diameter, which sets the duty. B is a 1" ball for medium-duty jobs, C is 1.5" for heavy duty and is the most common, and D is 2.25" for extra heavy-duty and high-vibration mounts.
Do all the round bases use the AMPS hole pattern?
Yes. The round B, C and D bases all carry the four-hole AMPS pattern (30 x 38mm), and the C-size round also has a three-hole 120° option. That is what makes round the most interchangeable choice.
Can I use a C-size ball with a B-size arm?
No. The ball and the socket arm have to be the same size. A B arm will not close around a C ball, and a C arm will not grip a B ball firmly. Match them.
My base and my cradle have different hole patterns. What now?
Either pick a plate whose pattern matches your cradle, or use a diamond base or an adaptor plate to bridge the two. Measure both sets of holes centre to centre before ordering so you only buy once.
How fast do these ship?
If a base shows as in stock, it is on the shelf in our Auckland warehouse and normally dispatches within one business day. If it shows as out of stock, get in touch, as we can often get it in for you.