520, 620 or Hex-Hole? A Mini 4WD Bearing Buying Guide
Choose a bearing by its mounting location, dimensions and shaft interface before comparing its price or material. “520”, “620” and “hex-hole” are not three equivalent alternatives. The first two identify size categories in Mini 4WD usage; the last describes the hole shape. A bearing can be the right outer diameter and still be the wrong thickness or interface for the assembly you are rebuilding.
Identify the location before opening a packet
Start with the kit or accessory instructions. Is the part supporting a wheel axle, sitting inside a counter gear, or installed in a roller? Write down that location and the original part reference. A roller's outside running diameter is not the diameter of the bearing inside it. Buying a “13mm roller bearing” by the roller's visible size can lead to the wrong part.
Do not assume every gear ratio uses the same counter-gear bearing. Follow the diagram for that particular gear set and chassis. Keep removed plastic bushes beside their replacements until you have checked fit and movement. If the manual does not identify a ball-bearing substitute, ask about the exact assembly rather than ordering by a familiar number.
Separate size, interface and construction
Tamiya's 15523 plastic set provides a useful size reference: 620 components have a 6mm outside and 2mm inside diameter, while 520 components have a 5mm outside and 2mm inside diameter. Those descriptions alone do not specify every commercial bearing's thickness. Measure or read the exact item page rather than treating a nickname as a complete engineering drawing.
Construction is a separate choice. A low-friction plastic bearing is not a ball bearing just because both fit the same location. Likewise, “hex-hole” and “round-hole” describe the interface. Match all these details together. Replacing a part with something that merely looks smoother can change axle clearance or prevent a cover from seating.
Compare the actual 15287 and 15519 products
Tamiya describes hex-hole set 15287 for car axles and gives a 6mm outside diameter with 2.5mm thickness. HG round-hole set 15519 is listed as 6mm outside, 2mm inside and 2mm thick. That half-millimetre thickness difference is a concrete reason to inspect the whole stack. They should not be presented as dimensionally identical just because the outer diameter matches.
For a 520 ball-bearing purchase, AO-1017 94752 is a specific product to check against the relevant instructions. It is not a universal replacement for every plastic part carrying a 520-style label. A PRO counter gear and a roller can require different supporting hardware even when a bearing size looks familiar.
Install without adding a new source of friction
Clean loose dirt from the mounting location and inspect the shaft for bending or damage. Seat the bearing squarely without forcing it into the wrong housing. Reassemble in the documented order, then turn the assembly gently with power off. Check the wheel or roller against nearby plastic, spacers and the body.
If tightening makes a roller bind, stop and inspect the clamping surfaces and the accessory's specified washers or collars. Do not solve it by leaving a critical fastener loose. If an axle now has excessive play, compare the new bearing dimensions and surrounding parts with the original stack. Keep a clear fault separate from an unmeasured claim about higher speed.
Read the label in the right category
| Label or exact item | What it tells you | What still needs checking |
|---|---|---|
| 520 size reference | 5mm outer / 2mm inner in 15523 | Exact thickness,construction and location |
| 620 size reference | 6mm outer / 2mm inner in 15523 | Exact thickness,construction and location |
| Hex-hole15287 | Axle product; 6mm outer, 2.5mm thick | Shaft interface and complete stack |
| Round-hole15519 | 6mm outer, 2mm inner, 2mm thick | Instructions and clearances |
| Plastic15523 | Five 620 and two 520 plastic parts | Not equivalent to a ball-bearing pack |
Product checklist
Buy for the exact location and quantity required. The three product links below identify distinct packs, not a universal ranking. Keep the original parts and record which bearing went into each position. Recheck the kit manual and current listing before ordering; no lap-time improvement is measured here.
Frequently asked questions
Are 520 and hex-hole competing bearing types?
No. 520 is a size label in this context; hex-hole describes an interface. You must check both dimensions and application.
Is every 620 bearing 2.5mm thick?
No. Tamiya 15519 is specified as 2mm thick. Read the exact product dimensions.
Does a plastic bearing contain balls?
Not necessarily. Tamiya 15523 contains low-friction plastic bearings, not ball bearings.
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Sources and specification checks
Checked 19 October 2026. Dimensions above are attributed to the named Tamiya items. Do not generalize a product-specific thickness or fit to every bearing sharing a nickname.