questions about "M" calipers Options

[Wes V]

I'm rebuilding a set of rear "M" calipers for my 914 and have a bunch of questions.

1. Inside the bore of the caliper there is a pin that goes into a mechanism within the piston. What the heck is it for? How do I remove it? Is it screwed into the caliper body or just press fitted into place. It needs to be removed in order to hone the bore.

2. If I can get rid of the "pin" within the bore of the caliper, what pistons can I use. I know it's easy to remove the mechanism within the piston (simple snap ring), but I have to get new pistons anyway. An actual porsche part number would be nice!! (Stoddard lists some aftermarket 38mm stainless steel pistons, but I'm not sure if they are applicable or have the stepped to surface talked about in the next point)

3. The pistons have to be installed (rotated) as shown in the following diagram.

(IMG:http://www.performanceforum.com/wesvann/914a/piston-angle.jpg)

This places the lower "stepped" surface of the piston in the upper position.

It's not due to the pad having an "index'd" matching surface (the pad just has a flat surface), and as such I've got to ask why it matters? (I have to assume it's so that the initial "bite" of the pad is lessened on the leading edge)

4. Are any of the "rebuild" kits better than others. (the kit normally is just the piston o-ring, dust seal, and retainer clip) Are there any to avoid?

5. The four bolts that hold the calipers together are 7mm (from the factory) and when loose, the parts of the caliper sure sure do flop around a lot!! Has anybody used 8mm bolts to help alignment and also be stronger. Where would you get 8mm bolts in the correct grade (12.9?). If you just use the factory 7mm bolts, are there any secrets to keeping everything aligned while tightening. (since I'm talking about the bolts, what should they be torqued too?) Other than the factory, is there a good source for the bolts?

6. Any idea on cost of zinc plating? (the parts have been blasted and clean)

Wes Vann

[Eric_Shea]

You ask a lot of questions... (IMG:style_emoticons/default/biggrin.gif)

1. Knock-Back mechanism. It was used by ATE from say 64 through 75 to help ensure a high pedal. It helps keep the pad close to the rotor by knocking the piston back out toward the rotor. It most certainly does "not" need to be removed to hone the bore. Use a proper 3 stone brake cylinder hone and there's no issue whatsoever. Those cost money though (a whopping $16.00 at your FLAPS). (IMG:style_emoticons/default/wink.gif)

2. You can get rid of it if you want. ATE did in 76 by using the later model cup style piston. You can get these (cup or regular) in Stainless from us for $32.00 each. They offer much more cooling surface area than the standard pistons. Simply break the pin off. You can leave them in and take out the mechanism inside your pistons but, there's no real advantage. Again, these do not have to come out to hone the calipers. However, you should remove and clean the internals of the piston if you can.

3. You are correct. By lowering the face of the pad slightly into the oncoming rotor this helps stop squealing and bed the pads properly. Others recommend chamfering the pads when re-padding the brakes, but with the step piston ATE made sure this is not needed. Put them back in properly. If you don't have the clips (which no longer come in the kits) then look at your drawing above and place the lower edge of that notch on the centerline. Clips are not necessary.

4. No. You'll probably only find FTE kits (part of the FAG group). They are of excellent quality and I see no advantage over ATE or visa-versa. I can supply those and they would come "with" the caliper 1/2 seals.

5. There is no source for the proper M7 hardware. I have not found any issue with loose caliper spacers etc. M8 would require a costly machining of the caliper and I highly recommend against it. Lining them up is very simple:

Install an 11mm socket on a 3" extension. Have it close by on the bench. Hold the nose section in your left hand with the outer left bolt installed when the piston is positioned up and is facing you. Hold the bolt in with the forefinger of your left hand. Install the two seals. Balance the 10mm spacers over the seals in position. Install the other two seals. Now drop the back 1/2 of the caliper over top of the nose section using the one fastener as your guide. Make sure the right 1/2 rests precisely on the spacer and seals. Install the 11mm nut on the single M7 fastener until it's finger tight. Lift your forefinger off and install the right outer fastener in the same fashion. Keep the spacers basically inline. Install the 11mm and hand tighten. Now (while still holding the caliper upright by the nose section) install the two long fasteners in the middle and hand tighten the 11mm nuts. Next, get the 11mm and the extension and start to hand tighten further while using your thumbs to position the spacers. The 3" extension and 11mm socket should allow enough force to get the spacers positioned properly so you can turn it up and put it in the vise.

Next torque as follows:

Number the fasteners (looking at the 11mm nuts not the Ribe bit side... you do have Ribe bits right?) 1-2-3-4.

With an R6 Ribe bit on the fastener torque to 7ftlbs in this order - 2-3-1-4 using the Ribe bit to hold it and torquing with the 11mm socket. Next,
Torque to a final value of 17ftlbs in this order - 2-3-1-4

6. Prices vary wildly. Find one that will zinc the calipers and black oxide the fasteners. Many have a $50.00 minimum...

Any other answers are $125.00 per hour! (IMG:style_emoticons/default/biggrin.gif)

[Wes V]

Eric; (without a "K")

Thanks!

(Ya, I tend to ask a whole bunch of questions, but I think most of them are worth while.)

Wes

[DBCooper]

(Ya, I tend to ask a whole bunch of questions, but I think most of them are worth while.)

At $125 an hour they better be, but that's good advice at twice the price.