This is a little off topic but I have found a ton of questions on the Vintage discussion boards about this mod. My 1980 RM 250 had small 38mm forks. I had them set up pretty well for my weight last year, but each time I would take a big jump I really could feel them flex. The only way to fix the problem was to go to bigger forks.
Since I already have the RM465 dialed in with later generation 43mm forks I decided to use the original 43mm fork setup for a 1982 RM465 for this project. I got another set of triple clamps off of eBay for 15 bucks, plus a 1982 RM250 complete front wheel for another 25 bucks.
The first thing I did was to take apart the existing front end on my 1980 RM250. Suzuki used a loose ball and race configuration. I used a long, thin punch to take out the old races and installed new generation tapered bearings on the top and bottom of the steering stem. I used my new bearing race installation tools - it was totally worth the 38 dollars for the installation tool.


The next step was to determine how close the old and new steering stem and clamps would be. The machine shop guy I have can do most anything, but I would prefer to do this swap as much on a budget as possible.
I first checked the steering stops and the centers of where the fork tubes would be. I did not want to run into problems with the tubes hitting the tank, nor did I want to have problems with my turning radius. Check out the next picture - bottom clamps are positioned so that they are bottom to bottom. The casting marks are in the same exact spots!

So, it was clear that the 43mm bottom clamp would set up just fine for steering stops and turning radius. Now, what about the stem itself? I knew it would be too good to be true if the steering stems were the same, and of course they are not. The 43mm stem is much thicker through the body and is shorter than the 38mm stem. Below shows the height difference and the thickness difference.

So you HAVE to press out both stems, and then press in the 38mm stem into the 43 mm clamp. I was worried that the area where the stem presses into the bottom clamp would be different on the 2, but in fact they are exactly the same. My machine shop guy could not believe it, because when you just eyeball it up they look different. When you mic them, they are the same! Some sort of strange optical illusion.

Now you need to look at the “lip” at the top of the steering stem you just pressed into the 43mm bottom clamp. You will find that with the new tapered bearings that there is not enough thread showing for you to properly tighten up the triple clamps. You need to machine down the lip so that all of the threads are showing when you place the tapered bearing on the stem. It should look like this:

The bottom bearing should look like this:

Then, after you lube everything up, it should look like this when assembled on the bike:

Next installment I will show how the forks, wheel, and fender bolt up!