The short answer is: usually not, and getting it wrong has consequences. A loose cap causes vibration, accelerated wear, and in a worst case, driveline failure at speed. But the full answer is more nuanced than a flat no, because there are specific conditions under which caps can be swapped without issue, and understanding those conditions is what separates a confident repair from a costly mistake.
U-Joint
Why People Ask This Question
Most people searching this question fall into one of two situations. Either they have a U-joint with a damaged or missing cap and are wondering whether they can salvage the joint by pulling a cap from a spare, or they are doing a drivetrain upgrade and have ended up with mismatched yokes and driveshaft ends that do not share the same U-joint series. Both scenarios are real, both come up regularly in workshops and on the trail, and both deserve a straight answer.
What a U-Joint Cap Actually Does
A U-joint cap, also called a bearing cup, is a hardened steel cylinder that fits over one of the four trunnions on the cross body. Inside each cap sits a ring of needle bearings that allow the trunnion to rotate smoothly under load. The cap is retained in the yoke ear either by an external snap ring sitting in a groove on the outside of the cap, or by an internal snap ring sitting in a groove inside the yoke bore. These two retention methods are called outside lock and inside lock, and this distinction matters more than most people realize.
Outside Lock vs Inside Lock: The First Incompatibility
Outside lock and inside lock caps are not interchangeable, full stop. The groove location is different, the cap geometry is different, and attempting to fit one type into a yoke designed for the other will result in either a cap that cannot be properly retained or one that sits at the wrong depth. This is one of the most common mistakes made when sourcing replacement U-joints, particularly when the original joint has been removed and the lock type was not noted before disassembly. Before buying any replacement, identify whether your yoke uses inside or outside retention.
Series and Cap Diameter: The Second Incompatibility
U-joints are classified by series, and each series corresponds to a specific cap diameter and overall joint width. The most common passenger and light truck series in North America include the 1310, 1330, 1350, and 1410, among others. Each series has a defined cap diameter, and the yoke ears are machined to that diameter. A 1310-series cap measures approximately 1.062 inches in diameter. A 1350-series cap measures approximately 1.188 inches. Those differences sound small, but a loose cap in a yoke bore will rock under torque, creating the kind of play that shows up as driveline vibration, shiny rub marks inside the yoke, and premature failure. A cap that is even marginally too large simply will not press in.
This means that even if two U-joints look nearly identical, caps from different series are not interchangeable. The GM 3RL and the Spicer 7290, for example, are dimensionally close enough to cause confusion but will not properly interchange. When in doubt, measure rather than assume.
When Can You Swap Caps?
There is one scenario where swapping caps is technically feasible: when both U-joints are the same series, same lock type, and same manufacturer or equivalent spec. In a pinch, such as getting a vehicle off a trail after a U-joint partially failed, pulling a cap from an identical spare joint to replace a damaged one is a legitimate temporary fix. The reason it is considered temporary rather than permanent is that needle bearings and their mating trunnion surfaces develop a wear relationship over time. Mixing caps and trunnions from different joints resets that relationship and can create slight stress concentrations that shorten service life. It is not catastrophic in a single use, but it is not best practice for a long-term repair.
What About Combination Joints?
For the drivetrain upgrade scenario, where one end of a driveshaft uses a different series yoke than the other end, there is a proper solution: combination or conversion U-joints. These joints are purpose-built with two different cap sizes on the same cross body, allowing two mismatched yokes to be connected. The tradeoff is that the joint is only as strong as its smaller end, so combination joints are not suitable for high-torque applications. They are a practical solution for mild street builds where the series mismatch is one step, such as bridging a 1310 to a 1350.
How to Measure Your Caps Correctly
If you are sourcing replacement caps or a complete joint and do not have the part number, measuring is the most reliable approach. Use a digital caliper rather than a tape measure. For outside lock joints, measure cap diameter and overall width from cap end to cap end. For inside lock joints, measure from the inner face of one lock ring groove to the same point on the opposite side, then add the thickness of both lock rings to get the effective width. Write down both dimensions and cross-reference them against a series chart from a reputable supplier.
A Note on Linear Bearings and Precision Components
Interchangeability questions do not only come up in driveline work. In precision machinery, CNC equipment, and 3D printers, the same principle applies: dimensional compatibility is everything. A standard linear ball bearing is a good example of a component where dimensional standardization across manufacturers makes interchangeability generally reliable, provided the tolerance class is matched. When the ISO standard defines dimensions tightly enough, cross-brand substitution works in practice, and the LM20UU size specification is a textbook case of this: bore, outer diameter, and length are consistent enough across reputable manufacturers that substitution is straightforward.
For sourcing these bearings and related linear motion components, Zhejiang Siqiang Bearing Manufacturing Co., Ltd. is a China-based manufacturer worth considering. Located in Lishui City, Zhejiang Province, Siqiang produces a full range of linear bearings including standard, open, adjustable clearance, stainless steel, and flange variants in the LM20UU series. Their products are manufactured to consistent dimensional tolerances and are available for direct inquiry. Visit zjsqlinear.com to browse the full range.
SQ LM20UUAJ
The Bottom Line
U-joint caps are not generally interchangeable across series or between inside and outside lock types. Within the same series and lock type, temporary cap swaps from identical joints are feasible but not recommended as permanent repairs. For mismatched yoke applications, combination joints exist specifically to solve the problem. In all cases, measure first, identify the series and lock type, and do not assume visual similarity means dimensional compatibility. A proper fit is not approximate in driveline work.
