Introduction: A 38 mm bore is not a random figure—it comes from shaft strength, gear spacing, and housing limits that standard bearing sizes cannot always satisfy.
Anyone who looks up a 62/38 ball bearing runs into the same small puzzle: 38 mm is not one of the bore sizes that appears in a standard ball bearing catalogue. The usual steps near that size are 30 mm, 35 mm, and 40 mm, so a 38 mm bore raises an obvious question about where the number came from. The answer starts with the transmission, not with the bearing catalogue. this guide follows that chain of decisions: what the standard metric bore pattern looks like, why a shaft journal can land on 38 mm, how gear center distance and housing space close off the other options, and what the 62/38-2RS/C3 code tells a technician about the bore inside it.
Metric ball bearings do not come in every size. Above a 20 mm bore, catalogues follow a step pattern of 5 mm: 20, 25, 30, 35, 40, 45, and upward. The two-digit bore code that most technicians already know works because of that pattern, where code 06 means 30 mm, code 07 means 35 mm, and code 08 means 40 mm. A 38 mm bore sits on none of those steps. Divide 38 by 5 and the result is 7.6, which is not a number a bore code can carry, so the bore is printed as a plain measurement instead. That detail alone is why 62/38 reads differently from a 6207 or a 6208. The second half of the puzzle is the outer envelope. In a standard ball bearing, choosing a bore also chooses a dimension series, and the dimension series sets the outer diameter and the width. The 62 family uses light-to-medium deep groove proportions, and in that family a 40 mm bore normally arrives with an 80 mm outer diameter and an 18 mm width. The 62/38-2RS/C3 that Smart Bearing lists keeps the 18 mm width but pairs the 38 mm bore with an 85 mm outer diameter and a 0.40 kg unit weight. Those numbers do not fall out of a catalogue table. They are chosen around the space a transmission can give the bearing, which is why the bore reads as a design decision rather than an off-the-shelf pick.
A transmission layout usually starts with the gears, not the bearings. The designer selects a gear pair to reach a ratio and a torque capacity, and that selection fixes how far apart the two shafts must sit. Shaft diameters then follow from the torque carried and the bending loads applied at the gear mesh, and the bearing bore follows the shaft. Working in that direction is what produces a bore size like 38 mm. Two constraints do most of the work: the journal diameter needed to keep the shaft stiff under load, and the housing room left once gear center distance is fixed.
Gear teeth mesh correctly only if the shafts stay where the designer put them. A transmission shaft carries torque in torsion and gear separating forces in bending, and the journal under the bearing has to be thick enough to resist both without tilting. When a shaft has a long span between supports, or carries a heavy gear at the end of it, the required journal diameter climbs past the 35 mm step without needing to reach 40 mm. Turning that journal down to 35 mm to match a standard bore would make the shaft more flexible and shift the deflection at the gear mesh. Machining it up to 40 mm would force changes to gear hub fits and to the housing bore. A 38 mm journal is often the size that satisfies shaft stiffness without disturbing anything else, and the bearing bore simply follows it.
Bearings never sit in open space. The outer diameter has to clear the neighboring shaft, the gear rim, and the housing wall, while the width has to fit between the shoulders that locate the inner and outer rings. Center distance between two transmission shafts is fixed by gear geometry, so a larger bearing on one shaft eats into the gap between them. That leaves a narrow window for the bearing designer: enough outer diameter to carry radial load and enough ring wall to stay round under press fit, but not so much that the ring collides with the housing. A standard dimension series does not offer a 38 mm bore at all, so the ring is built to the housing envelope instead, with C3 clearance and 2RS rubber seals covering the thermal and contamination side of the same space.
A technician reading 62/38-2RS/C3 can pull four separate facts out of the string. The 62 sets the dimensional family: a single row deep groove ball bearing with light-to-medium proportions, which is the common layout for a shaft that sees both radial and axial load. The 38 states the bore in millimeters, written as a plain number because it does not fit the two-digit code system. The 2RS marks two rubber seals, one per side, keeping grease in and dust out. The C3 marks internal clearance wider than the normal group, which gives the bearing room to keep running as the transmission warms up and the rings grow. What the code does not carry is a vehicle list. Two transmissions can arrive at the same 38 mm bore for different reasons: one because of shaft stiffness, another because gear center distance left no room for a standard ring. That is why a technician checks the measured shaft journal against the bore and the housing bore against the 85 mm outer diameter before treating any bearing as the right one. The 18 mm width decides whether the ring sits correctly between its shoulders, and the 0.40 kg unit weight is a handy cross-check when a part has to be identified by hand on a bench. Read that way, the code becomes a small fingerprint of the transmission the bearing came out of. A 38 mm bore says the shaft was sized by load and stiffness rather than by catalogue convenience, and the 85 mm outer diameter says the housing was built around that shaft rather than the other way round. When a replacement is needed, those two figures plus the 18 mm width do more useful work than any brand name, because they are the numbers that can be confirmed against a shaft journal measurement, a housing bore gauge, and a stock listing.
The 38 mm bore in a 62/38-2RS/C3 bearing is a record of a design decision rather than an arbitrary figure. Metric bore steps move in 5 mm increments, and 38 mm falls between them, so the size exists because a transmission shaft needed a specific journal diameter and the housing around that shaft left a specific amount of room. Shaft stiffness sets the lower limit, gear center distance and housing walls set the upper limit, and the bearing that emerges from that window carries a non-standard bore. Reading the code that way explains the size and keeps attention on the two measurements that matter most: the shaft journal and the housing bore.
A：Standard metric bore codes step in 5 mm increments once the bore passes 20 mm, which is why the sequence runs 20, 25, 30, 35, and 40 mm. A 38 mm bore cannot be expressed in that two-digit system because 38 divided by 5 is not a whole number, so the size sits outside the normal catalogue sequence. Dimension series tables also have no 38 mm bore row, which means the outer diameter and width are set by the application rather than chosen from a standard family. That combination is what makes 38 mm a non-standard bore rather than an unusual catalogue option.
A：The 62 in 62/38 describes the dimensional family: a single row deep groove ball bearing with light-to-medium proportions, suited to a shaft that carries both radial and axial load. The 38 is the actual bore measurement in millimeters, written as a plain number because it does not fit the two-digit bore code used for 20, 25, 30, 35, and 40 mm sizes. Read together with the rest of the designation, 62/38-2RS/C3 identifies a 38 mm bore bearing with rubber seals on both sides and C3 internal clearance.
A：Two different transmissions can land on the same bearing envelope for different reasons. One layout may need a 38 mm journal for shaft stiffness, while another arrives at the same size because gear center distance and housing walls left no room for a standard ring. The full envelope—38 mm bore, 85 mm outer diameter, 18 mm width, single row deep groove structure, C3 clearance, and 2RS seals—is what ties a bearing to a particular layout, so the shaft journal and the housing bore are the figures worth checking.
NIST SP 960-17: Rolling Element Bearings Technology and Applications
62/38-2RS/C3 Transmission Bearing 62/38 Ball Bearing 62/38-2RS - Stock for Sale
