Need Advice On Macining A Stem.

If the conversion is a common one then the bearings will be known and you may be able to get them locally. Perhaps one of the guys who has done this can give you the numbers on the bearings he put in? If they were not tapered roller bearings then you can have a bearing supply house convert the ball to tapered bearings and you'll be good to go.

If you must turn it, a 4mm overall reduction in diameter won't affect strength much I should think so its doable.

If I was to do this job I would chuck it somewhere in the middle to limit overhang and NOT use a live center. I would use a 4-jaw chuck and dial the part in before cutting it. I would make left hand cuts, feeding toward the tailstock. The reason for all this is to avoid cutting a taper in an area that must support a bearing with tenths tolerances.

With regard to the inserts, I suggest using a small nose radius. You are going to be taking very small finish cuts and the smaller nose radius will allow you to bury more of the nose in the cut to reduce the tangential forces you will encounter. HSS is better in this regard but if you have a insert with a nose radius of 0.007" or less then you'll probably be just fine. I further suggest you max the speed on the lathe and feed manually using lots of coolant to reduce the chance of a built up edge and to enhance the finish.

None of this is difficult to do. I could probably have done the turning job in the time it took me to write my first response. If you can turn steel with precision then this will be a simple job for you. BUT practice first!
 
I can't imagine a fork stem being made of aluminum, but i may have a limited imagination. Might be easier to modify the clamps than the stem, i.e. bore or bushing. Mike
 
I'd be very nervous about riding a dirt bike with a turned-down fork stem!
It's not just a matter of "how much meat's left", there's the stress-raiser where the section changes suddenly - to seat a taper roller bearing on the bottom yoke you're going to need a pretty square meeting of stem and surface, perfect place to start a fatigue crack - and ally's prone to those already!

I'd go with pressing out the old stem, making one fresh from a decent steel (it'll add maybe 100-150 grams at most to the bike's weight) or finding a matching bearing for the new stem / old frame.

Dave H. (the other one)
 
Dave: Many guys who did this mod actually did turn the stem down and nobody has encountered any problems, that I know of, but I do find your comment about creating a stress raiser very interesting. Getting the shoulder 100% square wouldn't be a problem. I like the idea to make a new stem. This way, the original stem would be kept standard, which would mean a lot, should the day come when I sell the 650 with its original forks. I'll then also be able to sell the 450 front end as standard.
I haven't removed the 650's stem, so I don't know what it looks like at the bottom, but overall, it is 26mm and strong enough. I'm sure it won't have a shoulder though.

Fomogo: Most motorcycle stems seem to be made of aluminum, believe it or not. Modifying the triple clamps would have been quite easy, but the fact that the 450 forks are upside down, puts the large diameter part of the fork at the top, while on the standard 650 forks, the thinner part is at the top. Unfortunately, this requires smaller size clamps, with not nearly enough meat to bore them out to accept the larger forks.

Mike: Thanks, wise comments that will certainly help me a lot, should I decide to machine the stem.

I came across a company who sell a conversion bearing kit and I think this is what I should do. This way, I won't have to machine or modify anything on either the bike or the front end.
After the transplant, I will be forced to make a new "dash" though. The 650 has mounting points on it top clamp where the ignition and speedo, etc bolt onto. The 450's top clamp has absolutely nothing of this sort.
I'm planning to replace the stock speedo with a digital version and I look forward to the challenge.
 
Hi Lood, in 40+ years of riding and wrenching I've never seen an alloy stem, probably because the manufacturers (I've so far seen Aprilia, Bimota (ooh....), BMW, BSA, Ducati, Enfield, Hardly-Ridable, Honda, Kawasaki, KTM, Laverda, Moto Guzzi, Norton, Suzuki, Triumph, Ural, Velocette and Yamaha with their clothes off for head bearings, plus a few obscure makes) are aware of the fatigue problem and that the threads are a lot weaker and less durable.

Pressing the inner bottom race onto an ally stem would be a nightmare too, it would gall up 90% of the time and you'd forever be replacing them after trying to file/scrape the burrs off to get the bearing on and winding up undersize... If I were you I'd go to some serious effort to establish whether it's ally or steel (a drop of caustic soda/lye on bare metal will tell pretty quickly), my money's on steel and reduced liability for the manufacturer.

Your comment about the shoulder and 100% square - that IS a problem, a square shoulder is a notch and seriously increases the stress level in the part under any kind of tensile flexing load! A radius is ESSENTIAL to make a durable part in that role (shock loads, fatiguing flex, SAFETY CRITICAL), particularly in ally, then add a seating washer/similar, or at least have a radiused undercut! Where I work we have BIG tensile testers for aerospace and nuclear-industry materials, some of the samples are notched to cause a stress-raiser, it's surprising how much less fatigue strength they have, and the ultimate tensile strength reduces almost as much, often more than 50% in low-modulus materials (like aluminium alloys...)

The Unusual Bearing supplier sounds your best bet ;o)

Dave H. (the other one)
 
Hey Dave, all of the modern Japanese bikes I have seen use aluminum steering stems. They hold up just fine, even in a crash. Removing and installing bearings is not a problem - I've done many of them; it is only a 0.0002" press fit and I have not seen any galling. All the stems I have seen do have a radius at the base of the stem where the lower stem seal sits; the bearing sits on top of this seal. I've done 3 fork conversions so far (all on street bikes) and have yet to see a problem with any of them. I've worked on Honda's, Suzuki's, Kawasaki's and Yamaha's and all of them have aluminum steering stems. I've even done a fork replacement for a stunter who bent his forks but the stem was fine (I did replace the triples and stem, though). Just letting you know what I've seen.

The problem with aluminum stems isn't that they're aluminum. Its that the monkey that is working on the bike is often using a Dremel tool to get his old race off so he can install a nifty tapered roller bearing. One nick and there's your stress riser. I mentioned this to one newbie on a street bike forum who had nicked his stem and he said, "Meh, I'll live with it and take my chances." It didn't help that many others on the forum agreed with him.
 
British Steel said:
I'd be very nervous about riding a dirt bike with a turned-down fork stem!
It's not just a matter of "how much meat's left", there's the stress-raiser where the section changes suddenly - to seat a taper roller bearing on the bottom yoke you're going to need a pretty square meeting of stem and surface, perfect place to start a fatigue crack - and ally's prone to those already!

I'd go with pressing out the old stem, making one fresh from a decent steel (it'll add maybe 100-150 grams at most to the bike's weight) or finding a matching bearing for the new stem / old frame.

Dave H. (the other one)
Click to expand...
The tapered roller bearing drawings that I have looked at have a radius for both the cup and cone.


Steve Shannon, P.E.
 
I must work on bikes that are considered "old" these days because i've never seen an Al stem either....
 
great white said:
I must work on bikes that are considered "old" these days because i've never seen an Al stem either....
Click to expand...

+1
Never saw an aluminum stem. And if I had to turn one down I would consider making another instead out of steel for the strength.
 
Personally, I'd bite the bullet and pay the price to get the right bearings. Especially if there is a readily available conversion kit.

No modification to either stem of either bike. Maintains oem qualities and easily returned to stock and easily esellable later on down the road (since you mentioned reselling).

Even if it's a couple hundred bucks (Canadian dollar is also very weak these days and you get rooked on all shipping outside the US so I feel your pain), I'd still buy them.

Sometimes, some things are just better off bought instead of built (yes, I actually said that).
 
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