I am exploring my after market 38mm rear brakes before I install them as I am concerned about the issues that TonyK and Hussein have experienced. I would like to get mine right on the first installation.
This led to further investigation and it was found that there was .025" lash in the rear brake caliper adjusters. The lash can be found in several things in the piston adjusters. Different multi start thread and nut. Thrust bearing and bearing plate. Cir clip retainer. This lash is what is causing the increased pedal travel because the rear brake piston will not adjust out the slop between the disk and the brake pad. A temporary solution was to install shims between the pad and the caliper which has firmed up the pedal, but this is only a temporary fix. The solution was to either make new 38MM pistons and use stock X1/9 rear brake adjusters or to sleeve the stock 34MM pistons and use stock X1/9 adjuster hardware. I took the sleeve route but I think it would have been just as easy to remake the pistons.
Tony, have you gone further with this project, and did it help the issue? (That was impressive work on the sleeves for the 34mm pistons).
Checking the lash on the stock and 38mm adjusters, , it was similar between the two, in the vicinity of 20 thou on each. It is hard to measure,it the screw rotates at all, it changes the measurement.
I worry that the following description will be gibberish to people who have not taken one of these apart. Somewhere on x web there was a diagram of the adjuster, possibly gone now, The above picture may help.
In my comparison of the stock and 38mm calipers, I have found a few additional factors that may impact modifications to fix the issue. I wondered why the 38mm units used a different thread diameter and pitch than the stock ones. (8.17 mm X2.5 round cut threads versus 7.96 X2 square cut threads) I noticed that the emergency brake lever has a maximum throw of 1 inch on the stock calipers and .5 inch on the 38mm calipers (it hits the side of the caliper). This could be the reason for the difference in the thread pitch. Less throw requires a greater pitch. (I have not checked the emergency brake lever movement on the car, anyone know what that is?) Perhaps the movement on the 38mm piston is not enough to activate the adjuster?
Also the disc springs are thicker by .2mm giving 123 lbs of pressure at 25% compression as opposed to 83 lb on the stock ones. This could be to compensate for the change in thread pitch, or to compensate for the increased piston volume. Or because they were available.
What follows are are just guesses, I need some help here.
If I hone out the bore for the plunger that holds the disc springs and adjustment mechanism by 2 thou, the stock mechanism will fit into the 38mm caliper with 10 thou lash. I have doubts that this will work to eliminate the play in the system, as the stock caliper had lash as well as the 38mm caliper. Also, the reduced throw on the 38mm caliper emergency brake lever may require the steeper pitch on the adjuster. The stronger disc springs may be necessary for the larger piston volume or may impede the turning of the adjustment mechanism. There is no going back from this modification.
What might be the source of and correction for the 25 thou lash between the brake pad and disc that Tony and Hussein have experienced? All thoughts will be appreciated.
Paul Davock