Steering Stem Compatibility

[GetTheExtractorReady]

Hey everyone,

I'm working on my '81 XS400H, looking at doing a fork swap. I actually got a really good deal on the bike, cost me $500, and the previous owner threw in a whole bunch of extra parts, including a set of GSXR forks.

Reasons for wanting to use the GSXR forks are:
1) I like the look of the chunkier forks
2) Got em for free, so why not?
3) The XS is now ~45 years old, so I don't know how much I trust the OEM forks on it


Now, I'm not 100% certain the exact model/year the GSXR forks came from, I'm still working on that (previous owner doesn't recall, so I'll have to take some measurements and do some digging online).
However, I did try sliding the steering stem from the GSXR into the XS400 neck, but the stem diameter is too large to fit through the top hole of the neck.

Unfortunate, but I wasn't expecting the job to be simple. I don't want to modify the XS frame if I can help it, so I figured I can keep the forks themselves, and look for a stem/set of triples that will fit the (umodified) XS frame and the GSXR forks.
The forks have an upper diameter of 50mm, and a lower of 54mm.

I've seen posts on this site mentioning Aprilia's being a drop-in fit for the XS400, but I was wondering if anyone knows of a compatibility chart for steering stems between different bikes.

Thanks!

P.S. I figure it's going to come up - I know that the geometry of the front end is important to the handling of the bike, and I'm not interested in sacrificing safety for the sake of aesthetics. So, because of that, I haven't 100% committed to the fork swap yet; I still have a lot of research to do. If I can't find a setup with these GSXR forks that will have the bike being safe to ride, I'm going to scrap the idea and do something else (maybe use the OEM forks, if they're still serviceable). With all that said, this is my first build, so I know there will be things I haven't considered, and things I won't think of, so any advice, heads-up, "you-might-want-to-think-about-this"-type comments are more than welcome.

P.P.S. Does anyone have the offset/rake/trail/caster numbers for the XS400H/is there a lookup table for these measurements for different bikes?

 

[GetTheExtractorReady]

So, minor update - I didn't realize that the races are not *part* of the XS frame (as I said, first bike lol). Popped them out of the neck, and the GSXR stem is able to slide through no problem. Still need to make sure the stem is long enough.

Still very interested in hearing any info anyone has about the questions in the original post.

Thanks!

 

[GetTheExtractorReady]

Small update:
Took some more measurements, GSXR steering stem has a 30mm diameter where the bearings will go, and XS400 neck diameter is 48mm where it matters, so bearing IDxOD will be 30mmx48mm, which matches up with what I'm seeing on allballsracing.com.

The problem I'm running into now is the depth of the "pockets" in the XS neck, the bottom pocket has a depth of 16.5mm (also has a small lip at 11mm depth, haven't been able to figure out if that's a separate removable ring, or a feature of the frame itself). I'm not too concerned with the bottom pocket, but the top pocket only has a depth of 6mm - none of the bearings in the fork conversion chart on allballsracing.com have anything even approaching a width as small as 6mm. So any bearings I get from that site would protrude from the top of the neck of the frame, and I'm not sure if that's an issue. Hoping somebody has some input. Thanks!