43MM Fork Upgrade
Alright, I will warn you now I may have gone a bit overboard here
but maybe someone else will find this info useful
As others have mentioned, the 1980 YZ465 used a 38mm fork with double leading shoe front brake (the 1980 YZ250 used a single leading shoe brake - correction per Greg below, thanks!). For 1981, Yamaha upgraded the 465 with a 43mm fork and continued with double leading shoe front brake. The 43mm fork and DLS was also used on the YZ/IT490's and 250's until 1984. In 1985, the DLS brake was replaced by disc.
There are a few things to be aware of when considering the 43mm fork upgrade. First, ahrma post vintage rules require drum brakes. So for me that meant a pre-1985 fork. Second, there are two styles for DLS brake plates. The difference is in the location of the anchor tab. The 1981 brake plate has the anchor in the 1 o'clock position which the '82-'84 plates have it in the 3: o'clock position.
This means the '82-'84 brake plate will not fit the '81 43mm fork slider and vice versa.
If you wish to stay with a 38mm fork on your 1980 YZ250, you can upgrade to the DLB but it must be the 1980-81 YZ plate.
The following pictures compare the front and bake of single and double -leading shoe brake plates.
Left is the single leading shoe, anchor at 1 o'clock. Right is 82-84 DLS plate with 3 o'clock anchor.
The next photo shows the 1981 DLS brake with 1 o'clock anchor. You can also see the corresponding difference in the brake tab location on these two 43mm fork sliders ('81 yz465 slider on left, 83 IT490 slider on right).
A couple of more things,
- The brake levers on the '81 DLS plate are steel, the later levers are aluminum
- The DLS brake shoes are different than the SLS shoes.
- The brake springs are the same.
- Be sure to clean and lube all the pivot points, it makes a big difference!
- Adjusting the DLS takes some patience to get both shoes to hit the drum at the same time.
Now, while it is true that the fork tube diameter was the same across many years of the 490, the fork internals can vary significantly from one model year to another.
Examination of part numbers show differences in springs and damper rods through the years as well as the appeance of a new device called a compression damping blow off valve.
The compression blow off valve was introduced on the ’83 YZ/IT 490. As I had a set of '83 IT forks around, I did some searching for more information about this feature. According to Cycle Guide “The 43mm 1983 YZ/IT490 has a compression damping blow off valve that momentarily unseats and allows the fork to compress more easily when the wheel hits a big or abrupt bump. It is non-adjustable, unlike on the KX500 of the same year. The YZ wheel travel is 11.8 inches”.
As I also have a set of disc brake forks of unknow year, so I did some reading on thoses as well.
It turns out, the 1986 forks have 8-position compression damping adjustment. An alumite slider coating was used by this time “to reduce friction and oil contamination”. Fork action was give a favorable review in April ’86 issue of Dirt Rider. Another change came in ’87 with a new “travel control valve” or TVC. Unfortunately, TVC was given a poor review with Dirt Bike (1987) complaining of the “extremely harsh set of front silverware”.
What fork parts are shared across model years?
In the end, I wanted to use the best possible parts from the different fork sets I had. I compared part numbers for the various internal components and create this table to help spot which components were common between the model years.
In the table below, the model and year is labeled accross the top row and the part is labeled in the 1st column.
The letter A is used to denote the 1st version of the part for a particular row. The '81 model is the 1st fork considered so all the parts are of version A. If one looks at fork seals across all model years you can see they are all type "A" so this means they have the same part number and are interchangeable. The slide bushings and slide pistons are also shared across years.
If a part changes, say from A to B, across model years it means the part number is different and they may not be intechangeable.
Looking at the damper rod you can see they differ for every model year for the YZ. However the rod is the same for the 83 and 84 IT.
More to come I'm afraid