Need Advice On Macining A Stem.

L

Lood

Active User
Registered
Joined
Dec 25, 2014
Messages
98
I'm transplanting a Honda CRF 450 front end onto my XR650L. The 450 sports thicker forks, so I'm forced to use the 450's triple clamps and stem as well. The head stock taper bearings used on the 450 are larger than those on the 650, so the best way to do this would seem to machine the 450's steering stem down, to accept the 650's smaller bearings.

I've already removed the old bearings from the stem and pressed it out from the bottom triple clamp. All indications are that the stem is made from aluminum and I have never machined aluminum. Apart from getting the outer diameter machined down to size very, very precisely, the surface where the bearing will sit also needs to have a rather smooth finish.

I would appreciate any advice or specific precautions on machining aluminum and also on how to get that smooth finish. Recommended cutting speeds, best inserts to use, etc.
In its current state, the surfaces where the bearings sit does not have a mirror or perfectly smooth-as-glass finish and the although substantially finer than the rest of the stem, the machine marks are definitely visible.

I look forward to your replies.
 
Hmmm, bigger forks, more travel and rebound to boot - sounds good! The question I have is how much smaller is the ID of the 650 bearing vs the 450? If you're taking off a few thou then okay but if you're talking about more than a few mm off the OD of the stem then I would be careful with that. Is it possible to find a tapered roller bearing with the ID of the 450 and the OD of the 650? I would rather do that than machine the steering stem.

To answer your question, most stems are aluminum. Typically, the section where the seal and bearing race sits is only about 3/4" long and bearings typically have about a 0.0002" press fit. Accordingly, tolerances are pretty tight.

Cutting speeds for aluminum would be about 200-300SFM. For a stem of about 30mm or so you should be turning it at about 800 RPM or so if using HSS. An SCLCL turning tool with a CCGT aluminum-specific AK insert would also do this job easily, probably at your lathe's max speed. If this was me I would use HSS and A-9 cutting fluid for the best finish. If you haven't done aluminum before then please practice on some scrap stock before touching your steering stem. Turning aluminum is pretty simple but when you're machining to tolerances in the tenths you need to know what you're doing. Be sure your micrometer reads accurately, too.

It would be good to know what kind of lathe you have. Chances are you will be turning toward the tailstock on this job - a left hand cut - so can your lathe pass the steering stem through the spindle? Is the spindle of a consistent diameter in the mid-shaft portion where you will be grabbing it with a lathe chuck? Do you know how to grind left hand HSS tools? If not, then what sort of lathe tooling do you have?

Sorry for all the questions. Your answers will help us help you. I hope it works out well for you, though. It sounds like a cool fork swap.
 
Last edited:
Have you checked if there is a bearing conversion kit with AllBallsRacing.com ?? They may have what you need to not have to machine the stem
I got bearings from them to put a CBR600rr front end on a CX500

And I am not affiliated with them or sell their products
 
You could also try hitting up a shop that sells bearings [generally for semi's]. They generally will be able to find a bearing with the right ID/OD/taper...

I'd rather do that than make the stem weaker...of course, this also depends on the relative size between the stem and the new frame.
 
Thanks for the replies, gents. I'm going to try and answer all of your questions in one go:

This conversion is quite common and there are indeed at least two conversion kits, which enables the conversion to be a simple bolt on exercise. However, these are only available in the US and with the current USD/ZAR exchange rate, this is not an option as it would simply make the conversion ridiculously expensive.
One company does sell a modified bearing set and I will investigate the details of this.
Many guys do follow my intended route and turn the CRF stem down to accept the XR bearings. The ID's of the the two bearing are 26mm vs 30mm, which means that the OD of the 450's stem needs to me turned down by 2mm. The stem is hollow and it has a concave shape. I didn't measure the stem at its thinnest part, but taking a guess, it isn't any thicker than 20mm in the middle part. I'm comfortable that turning the bearing surfaces down to 26mm won't jeopardize the strength or integrity of the stem, as there will be more than enough "flesh" left.

The fact that I have read nowhere about an off the shelve 30 x 47 bearing left me believing that they aren't available, but I will investigate that as well, as that would be first prize.

With regards to my equipment, I use a Graziano SAG180 lathe, which is old, but still pretty accurate. I use Iscar tool holders with Iscar carbide inserts, but I don't have the insert spec in my head, unfortunately. I have tool holders for turning in both directions. I have never worked with aluminum, but I have done a job with brass and of course lots of work on normal steel and to get the exact diameter is well within my capabilities.
The hole through the stem is tapered on the inside at each end, which left me thinking that using a live center at the tail end would enable me to turn both ends down in one go - if I'm lucky there's limited runout, of course. The top end has a flat surface between the two sets of thread where it can be held with the chuck.
 
Last edited:
Just another thought: I see that 25 x 47 x 15mm bearings are quite common and available. Maybe I should use these and just turn the XR650's original stem down by 0.5mm? This is almost nothing and I can then use the 25mm bearings.
 
So, you are looking for a 47OD, 30ID, 15Width tapered roller bearing (ideally)?
 
dave_r_1 said:
So, you are looking for a 47OD, 30ID, 15Width tapered roller bearing (ideally)?
Click to expand...

Indeed. Then there's no machining or any other modifications required and the 450's front end can basically be bolted on.
 
Lood said:
Just another thought: I see that 25 x 47 x 15mm bearings are quite common and available. Maybe I should use these and just turn the XR650's original stem down by 0.5mm? This is almost nothing and I can then use the 25mm bearings.
Click to expand...

However, I'm 99.99999% sure that the 650 stem will not be compatible with the 450 triple clamps, which rules this one out. So, it's either getting hold of the odd sized bearings or turning the 450 stem down to 26mm.
 
What about doing something crazy like making the whole stem yourself, so that it's not hollow and uses the bearings you have?
 
You must log in or register to reply here.
Back
Top