43 mm Fork Upgrade Continued...
So to re-cap, I have spare '82YZ490, '83 IT490 & unknown year YZ490 disc brake forks to choose from in making the switch from the 38mm to 43mm.
In the end, I decided to go with the '83 IT490 forks. In addition to the 465, I also have an '83 IT490 and like the way the forks feel on that bike. Again, these forks have the blow off device that the '82 forks do not have.
Another factor in my choice is that I may try the Race Tech emulators in the future. The emulators would make the blow off valve in the '83 forks irrelevent. Because of this, I thought it best to hold back the '82 forks for the Race Tech modification. And I did not pursue the possibility of using the newer internals on the disc fork as I thought I might because of time constraints.
For kicks, I have listed damper rod part numbers across the model years. You can see they are all different.
Damper Rod Part Numbers
’83 23X-23170-L0-00/ 26A-23170-L0-00
’84 39X-23170-20-00/ 26A-23170-L0-00
Now, we all know that sometimes parts will interchange even though the part number is different. I did not look to closely at this but did notice the damper bolt holes in the different year sliders were dissimilar enough to make interchange unlikely without doing some real work...
Moving on, here the brake side '82 yz490 slider is compared with the '83 IT490 slider. Interestingly, the bottom of the YZ slider is welded on whereas the IT slide is cast as one piece (like the '81 YZ slider). Another difference involves the axle. The YZ axle (top in photo) threads into the slider. The IT axle (bottom) slides through an unthreaded hole and is tightened with a separate nut. The IT forks requires use of the IT axle or machining of the YZ axle shoulder (on left) to match the IT design. Shortening this shoulder allows the axle to move further into the hub thereby exposing the threads per the IT arrangement.
Here are a few photos of the IT fork rebuild for the curious. The replacement seals are new from Yamaha.
I did not have a tool to keep the damper rod from spinning when removing the damper bolt. The air wrench was essential for overcoming this problem. I found it best to use repeated short bursts to get the bolt to back out of the slider without spinning the rod too much. Make sure you still have the fork spring installed as that is the only thing providing resistance to rod spinning.
Note: While the forks were apart I was able to fine a nut that fit into the hex on top of the damper rod.
This nut will be used to make a tool to keep the rod from spinning when doing fork rebuilds in the future.
Fork caps and springs.
Here is a comparison of three different fork caps and 82 YZ490 and '83IT490 fork springs.
Even though the parts are different in some way and have different part numbers, they are still interchangeable in the fork tubes.
Keep in mind though, the fork caps with the staight valves require use of the offset top triple clamp to keep the bars from interfering with the valves (my understanding is there are 3 versions of the top triple clamp for the 43mm forks).
See how overall length is the same for these spacer/spring combinations.