'96 RM125 forks onto '89 KDX200

Questions and comments about converting to beefier forks..
TheRadBaron
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'96 RM125 forks onto '89 KDX200

Post by TheRadBaron »

Hey guys, sorry if this has been covered before. I did some searching on this topic and it sounds like it's a swap that's been done before, but my slow rural internet makes searching brutal. Anyway, I just brought home a nice '89 KDX200 and I have a '96 RM125 chassis in beautiful condition. It has the conventional forks (non-USD, 49mm Showa, I think) and I have the complete front end. I'd like to do the swap and I'd like to have as much information about it before I get started to minimize downtime.
It seems like a lot of the information I found was related to doing the swap on a H series KDX. It appears that the E and H series use the same steering bearings, so the swap should be similar though. Could anyone point me in the direction of a good thread that covers a swap like this? Thanks a lot.
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Re: '96 RM125 forks onto '89 KDX200

Post by Julien D »

Not a common swap, so I don't know if you will find a walkthrough. It's pretty much one of those things you just have to tear into and figure out what is needed. You may or may not need machining done on the lower triple, you may or may not need a bushing or bore on the top triple. You WILL need to get the KDX stem into the RM lower one way or another.
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TheRadBaron
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'96 RM125 forks onto '89 KDX200

Post by TheRadBaron »

You may be right about having to use the KDX stem in the RM yokes, but after doing some research it looks promising that I might be able to use the RM clamps and stem as-is.

The KDX steering bearing ID and OD are: Upper - 25 x 47
Lower - 30 x 55

The RM shaft has an OD of 30mm, top and bottom.

The head tube on the KDX and the RM frames seem to be practically identical in height, so it's possible that with a 30 x 47 upper bearing and a 30 x 55 (stock KDX) lower bearing, everything might swap right over. I don't know if it's a standard size tapered roller bearing, but All Balls Racing does sell a 30 x 47 bearing. There are of course aspects that could still make this plan fail, but I think it looks promising.
It will be impossible to tell without taking both bikes apart and comparing the parts. Hopefully I'll get a chance to work on that in the next few days. The new KDX is down for repairs while I'm waiting for a few parts to arrive, anyway.
I'll report back when I figure more out.
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'96 RM125 forks onto '89 KDX200

Post by osobad »

my 97 rm forks needed the kdx stem in the rm triples. rm front wheel and axle . had to have stem knurled as it was just a tad loose. This is on a H series
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Re: '96 RM125 forks onto '89 KDX200

Post by thebird_19 »

Measure the stems from the top of bottom triple clamp and if they're the same then you should be able to just switch out that top bearing like your thinking. I don't know why the H series would be any different unless it has a longer head tube. I'm in the process of a 02 kx hybrid right now that I mounted 07 ktm wp forks on. I think yours will be a lot easier.
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Re: '96 RM125 forks onto '89 KDX200

Post by gsa102 »

Those are essentially the the same as DRZ forks, which I put on last year and love. I did not research the bearing route, I put the KDX stem in the DRZ triples. It was 0.015" loose, so I used a permatex retaining compound which seems to be holding well. Search for a DRZ thread.
This is my rifle, this is my gun, the YZ to go fast, the KDX is for fun!

96 KDX 200, 09 YZ 250
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'96 RM125 forks onto '89 KDX200

Post by TheRadBaron »

That's cool that you did a similar swap and I'm glad to hear that you like them. I think I might have read your thread on the conversion as I was doing my research.
Is your stem really 0.015" loose? That seems like an awful lot of slack to be taken up by the Loctite compound, especially on a part that's designed to be a tight interference fit. I don't think I'd be comfortable with that, personally. I'm not one of those "safety guys" that like to tell other people that they will surely perish though. I'm glad it's working for you and I guess the design of the whole triple clamp assembly serves to hold itself together once everything is tightened up.
I took the front ends apart on my KDX and RM and my preliminary mockup is encouraging. It looks like everything might simply swap over if the correct bearings are used. I noticed a few things that could be minor problems but nothing that won't be easy to fix. It's hard to really tell until the bearings can be fitted, though.
I went ahead and ordered the bearing and seal kits from All Balls Racing. Here's what I ordered:

Bottom bearing kit, part# 99-3519-5, 30 x 55 x 17
Top bearing kit, part# 99-3540-5, 30 x 47 x 12

It ended up being $62 with shipping. Not too bad at all if it allows for a simple swap. I'll report back when I have the bearings and make some progress.
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Re: '96 RM125 forks onto '89 KDX200

Post by gsa102 »

I checked and it was 0.007" clearance. I can't detect any movement. I am curious how the bearings will work out. I need to dial in the spring rate, but I like them a lot.
This is my rifle, this is my gun, the YZ to go fast, the KDX is for fun!

96 KDX 200, 09 YZ 250
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'96 RM125 forks onto '89 KDX200

Post by TheRadBaron »

Well, I did some more fitting last night and it's going to be a bit more involved than I had hoped. When I compare the two stems side by side it's clear that the KDX stem is a good deal longer than the RM stem. I don't know why I didn't compare the stems from the get go. Duh... I guess I got wrapped up in other aspects of the swap and it slipped my mind. I've been doing this stuff a long time but I still make knuckleheaded mistakes.
The good news is that I still think the swap is a winner and I have a plan. I thought about trying to swap the KDX stem into the RM clamps but I don't want to go this route for a few reasons. First and foremost, I don't like the idea of the KDX stem not being a tight press fit into the RM clamps. Plus, then I'd have to shim the RM upper clamp to the stem. Also, I already bought the bearings that I'll need for the RM stem. Finally, the RM stem is aluminum so it's lighter than the KDX stem and I'd like to save weight anywhere I can.
My plan involves first pressing the stem out of the RM lower clamp. The lower clamp has a raised boss that the lower bearing seats onto. The boss is raised about 1/4" from the surface of the clamp. I'll have this boss milled down to the minimum height necessary for the bearing to install properly. I can probably take about 3/16" or so off. Then, when I press the stem back into the lower clamp, I'll press it in about 5/16" less than it was. Even with the material that I'll remove with the mill there's still about 1 1/4" of thickness in the lower clamp for the stem to press into. I'm confident that the integrity of the assembly won't be compromised by slightly less engagement between the stem and lower clamp. Between these two adjustments the stem will effectively be 1/2" longer and I think that's about what I need to make the RM stem work.
Like I said, it's a bit more involved than I hoped it would be but it still shouldn't be too tough. Once I get the whole thing engineered I'll post up some pictures of everything.
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Re: '96 RM125 forks onto '89 KDX200

Post by Julien D »

If you're going to press the stem in less, you'll also have to machine the pocket deeper on bottom of the lower where the snap ring seats. Seems to me that you are going to be causing more trouble than you are preventing. Let us know how it works out, though.
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TheRadBaron
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Re: '96 RM125 forks onto '89 KDX200

Post by TheRadBaron »

Hmmm. You maybe be right. It's been a while since I had a steering stem apart and I forgot about the snap ring. I think I'm going to press the stem out today so I'll see what it looks like when I have it apart.
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Re: '96 RM125 forks onto '89 KDX200

Post by KarlP »

If you have a complete RM chassis, how about doing a hybrid? I bet an RM with that E 200 motor would be great!
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TheRadBaron
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'96 RM125 forks onto '89 KDX200

Post by TheRadBaron »

That was my original plan but I decided against it for a few reasons. I had planned on getting a basket case or broken down KDX and just using the engine and a few other parts to make the KDX/RM hybrid but I ended up getting a complete KDX in good condition. This kind of shifted my plan over to keeping the KDX a KDX...except with an RM front end.
There were a few reasons I decided to forgo the hybrid. First and foremost is that I have a lot of other projects going right now and I don't have much time to devote to this one. I want to get the KDX up and going soon since spring is coming and I'd like to get in as much riding as possible. I don't really want to spend all the time engineering the swap into the RM. Plus, I just really like the look of the E-series KDX. I think it's a much better looking machine than my RM. I know, that shouldn't really matter but it is a slight consideration. I also think that I'll be able to part out a lot of the RM and recoup some of my money that I'm spending on the KDX (new top end, DG pipe, etc.).
Back to the fork swap. The RM stem doesn't use a retaining ring but it does have a shoulder that bottoms out on a corresponding shoulder in the bore of the lower clamp. I have a friend of a friend who owns a machining business on the side and does a lot of custom motorcycle work. I'm going to take the lower clamp to him this evening an talk about it. If I have him take the 3/16" off the top of the clamp like I talked about earlier and have him machine the shoulder in the lower clamp 1/4" deeper into the bore, this should make everything work. I don't know a lot about machining but it doesn't seem like either of these operations are complex or would involve a lot of setup. Plus, none of the dimensions that are being machined involve fine measurements. Hopefully it's not too expensive.
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'96 RM125 forks onto '89 KDX200

Post by TheRadBaron »

I discovered one more machining operation that will need to happen to make this work. The portion of the stem that actually presses into the lower clamp is of a slightly larger diameter than the portion that the lower bearing installs onto. I guess this is why the KDX stem would be slightly loose if installed into a RM or DRZ clamp. After some more measuring I determined that 3/8" should be the optimal amount of increase in effective length of the stem. So 3/16" off the raised boss and 3/16" increase in depth of the bore shoulder should do it. Then I'll need to have 3/8" of the slightly larger OD of the stem machined down to the same diameter as the bearing surface. I talked to the machinist and he said that what I needed would be a piece of cake and the whole operation would run me $20. Not too bad at all. I realize that a lot of folks might not have access to such a cooperative machinist, so this swap could be a bit more expensive. I'm still feeling good about it, though. Here's a drawing I made to illustrate the machining that I'm having done.
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Re: '96 RM125 forks onto '89 KDX200

Post by osobad »

just have the kdx stem knurled
TheRadBaron
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'96 RM125 forks onto '89 KDX200

Post by TheRadBaron »

I thought about doing that but ultimately I like this plan a bit more. I like the reduced weight of the aluminum RM stem and if I use it I won't have to shim the 30mm bore upper clamp onto the 25mm bore KDX stem. Plus, I already have the shiny new upper bearing that I need to use the RM stem that I bought when I went off half-cocked on this plan in the first place. :grin:
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'96 RM125 forks onto '89 KDX200

Post by TheRadBaron »

I picked up the parts from my machinist last night and everything looked great. I pressed the stem back into the clamp, pressed the new lower bearing on, greased everything up an installed it. Man, it went together perfectly. The steering stops will need to be extended a bit to avoid the forks hitting the tank at full lock, but I understand that this also needs to be done for the KX fork swap.
All in all, I'm calling this swap a winner. Of course, I haven't ridden the bike with these forks yet because I have the engine completely torn down but I'm betting that it will ride great. Gsa102 reports that he likes his and they're supposed to be basically the same forks.
As long as you can have some simple machining done it's a straightforward and inexpensive swap.
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Re: '96 RM125 forks onto '89 KDX200

Post by diymirage »

when I put my KX forks on I just drilled some holes in the steering stops and tapped treads into them
put a little machine screw in there and space it up with washers
newbbewb wrote:DIYmirage has it right.


-1996 KDX 200 woods weapon (converted to 99 green body)
-1996 KDX 200 plated street toy (barney edition)
-2003 Yamaha TTR125-L (wifeys bike)
-1997 KDX 220 project bike
TheRadBaron
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Re: '96 RM125 forks onto '89 KDX200

Post by TheRadBaron »

Right on. That was my plan, too.
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'96 RM125 forks onto '89 KDX200

Post by Gotanubike »

I have a chance to purchase a '97 rm125 front end for my 1990 kdx200...is it worth it?

He is parting out the bike without a frame. Waiting on his asking price for the forks, triples, wheel, brake caliper...anything else I need?
1990 KDX200
Bike Profile -> http://www.kdxrider.net/forums/viewtopi ... 61#p136315
Suspension Overhaul(Shock+89-92 conventional forks) -> http://www.kdxrider.net/forums/viewtopi ... 15&t=15255
96'-98' RM125 Showa 49mm fork swap -> http://www.kdxrider.net/forums/viewtopi ... 04&t=16994
KDXrider world map! -> https://www.zeemaps.com/map?group=186158
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