2011 XKR. TRW pads. Changing rear pads and noticed each pad has a pair of locate pins on the back side. The spread on one pad is 20 mm which fit into the dimple holes of the piston top.

I noticed the OEM pads which I removed (24K miles) used the much wider spread pin pad on the piston rotor. e.g., the pins were NOT utilized in locating the pads to either the pad carrier on the outside, or the piston end on the inside. It is possible someone replaced pads prior to this, however, everything looked stock OEM as it was being dismantled.

Are these pins necessary to locate the pads, or should I follow the original, and install pads w/o locating pins into the piston end holes. My fear is that if I locate the pin into the piston, it cannot turn. And I am assuming the piston turns counterclockwise as lengthens travel as the pads wear.

 

Those little pins, about 3/16" on the back steel? Those don't do anything but represent the hole on the opposite side of the steel where the brake material locates. When the plates are stamped, the pins pop out on the other side merely by the operation.
It's a manufacturing process, and those are just the residuals of that.

 

I found a good video:

 

Thanks Cee Jay

 

Rear calipers that also have a parking brake action incorporate a ratchet mechanism to take up the pad wear, hence the need to rotate them back in when replacing worn pads with thicker new pads. For this ratchet mechanism to rotate within the piston, the piston must not also rotate, hence the pins to lock the piston. They are required and must match the indents in the piston face - slightly rotate the piston to mach at the point of fitting and don't let them rest on the piston.

IIRC, the pad with the close-spaced pins should be the inner pad and the one with the wide-spaced ones, which would straddle the piston is the outer pad that takes the anti-rattle spring. Maybe they were incorrectly switched before?

I'm guessing you are looking at one of these: TPC1449 / TXM1449 / GDB1877 replacing Jaguar C2D3792.

 

Yes, I have the TPC1449 kit.

 

There is a close-up picture here: https://partlimit.com/products/trw-t...-brake-pad-set

The pad with the spring and the wide-set pins is the outer pad. The inner pad has the closer set, "vertical" pins. These are the ones that must go into the piston recesses.

 

Your explanation makes sense. Plus, it's too much of a coincidence for the pad locating pins and the piston recess holes to directly line up as installed, to not have a functional purpose.

My electronic parking brake worked just fine without locating the set of pins to the piston cap hole. I think hydraulic pressure held the piston cap tight against the caliper... so it did not rotate and could be adjusted outward taking up slack of the used pads.

 

Most rear pads have these pins and I know that when the piston sits on the pins, the brakes don't work well and this is something to definitely avoid

If there are no pins, I expect it can still work until the mechanism gets stiff and then it won't move anymore, just turning the piston. Might accelerate the time to a seized mechanism. Anyway, once you have done it wrong, you always remember to check

 

I don't recall any slots on my rear pistons at all. Course, it's been several years since I did the job.

 

I know now why the pads were placed as they were when I got the car.

The vertical pins on the backside of the TRW FC1499 pads are spaced just a bit wider than the piston detents. Like one mm or so. Just enough to prevent the backside of the pad to fully seat on the piston crown outer ring. Hence it rocks slightly. I measured with a caliper and dry fit. Both ways reveal a mismatch between pins and holes, and I don't want to force fit the two.

I also noticed the spacing on the wide pins is too wide to fit inside of the caliper bracket tongues. One pin would always be located under the caliper bracket tongue and I had to grind down one pin flush to get it seated properly within the caliper bracket.

I'm going to install without locating pins, as it was when I dismantled, and keep an eye on the EPB adjustment.

 

Your measurements worried me as TRW are a major OE and they don't usually get basic dimensions wrong. You must be on to something! I had a look at mine and the workshop manual. I was wrong in this instance, and like you, I also refitted pads as I found them (opposite to how I would expect rear pads to be fitted). These calipers don't use the pins to lock the piston. The inner pad takes the spring (and the wear sensor, if required) and has the wide-spaced pins. The outer pad has the closer-spaced pins which don't seem to serve any mechanical purpose.

I should edit my earlier post so as not to mislead others who stumble across this thread in the future but it seems to be too late

 

Yes I agree. It was the wear sensor notch that caused me to think the pin separation was not related to inside/outside fitment.
The wears sensors usually mount on the inside caliper, and the only pad with a provision for the sensor was the wide-spread pin.

I have since driven the car, and the electronic parking brake re-set was according to the instructions and went without a problem.

Thanks for getting back to help clarify this issue. As you note some others might be confused as I was and confirmation as to fitment will help them greatly.

As to re-editing your posts, I am fairly certain if anyone who reads the short thread will come to the correct conclusion relating to the pins and the piston.

 

These little pins damaged my piston dust covers on both sides. They were touching the covers and contributed to rubber drying out and cracking when running really really hot. Happened on both side. I replaced covers on both side and switched to different rear pads on my XKR-S.