XR250 Fork Spring Thrust Bearing: Which NTN Bearing Fits (XR250 BAJA / MD30)

Hello everyone.

You often see people fitting a thrust bearing on top of the front fork spring to make the spring move more smoothly.
This post is about fitting a thrust bearing in the front forks of my Honda XR250 BAJA (the Japan-market XR250 / MD30). The fork internals are XR250 (MD30) parts, so the bearing size applies to the standard XR250 too.

There's no ride impression here. It's information on which bearing fits, so I hope it's useful.

The bearing I fitted is an NTN single-direction thrust ball bearing, 51100 series, single row, 20 mm bore. At MonotaRO (a Japanese online shop) it's this item (Japanese site).
I'll show a photo of it fitted in the fork at the end.

The bearing is the one in the photo above.

The outside diameter is 35 mm,

the inside diameter is 20 mm,

and the thickness (width) is 10 mm.

The XR has no preload adjuster pipe, so using this bearing increases the preload by the bearing's thickness.
I don't know whether 10 mm of extra preload is right, but at the very least it will differ from what the manufacturer intended.

My guess is that the XR250 springs have less preload than the BAJA's, so on a BAJA, "XR250 springs plus this bearing" might work out about right. That said, I think the effective stroke would probably drop slightly.

For the differences between the XR250 and BAJA springs, see this post.

Here's the photo with the bearing fitted.
The outside diameter is a perfect fit, and the 17 mm nut that holds the top cap fits inside the bore.