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.



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

The correct kit is difficult to come by. You're dealing with the parking brake mechanism inside the piston. You need a kit with the appropriate seals for the parking brake portion of the caliper and the o-rings between the caliper halves. I've not heard of replacing the bolts. Eric Shea at PMB Performance rebuilds these calipers and does a great job at a fair price. He probably would be a good source for the parts.

The Cap'n

Unless I'm grossly mistaken, the mechanism has nothing to do with the parking brake. I'm talking about 911 rear "M" calipers.

I knew about the o-rings (but didn't say anything). I think it's funny that they are not included in any of the re-build kits I've seen. As if you could replace the piston seals without taking the caliper apart. Not replacing the o-rings would just be too stupid.

Wes

Put them in a box. Send them to PMB with a check for @$250.00 Wait 2-3 weeks, enjoy the artwork that comes back. Eric keeps his many customers (and wives) happy with superior skills!

Doing it yourself will likely cost you more and take you longer...

OOPS! I read "rear calipers for my 914" and figured you had 914 rears. You ARE aware that running a car in the street without a parking brake system, separate from the service brakes is illegal, right? The Cap'n

Cone; I agree that Eric does a great job and his prices are good for the quality, but I just like doing stuff my self.

Cap; I'm running the 911 parking brake.


But that leaves all the original questions un-answered!

Wes

You ask a lot of questions...

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).

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!

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

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

At twice the price I'll buy the Ribe Bit set and the 3-stone hone and the seal kits and all the plating... hell, I'll even put them together with the pistons in the proper orientation and with the spacers done up mighty right!

One thing I've noticed when doing searches on brake questions -- the topic is normally finished after Eric states what the reality is. Just look at the name of the last entry. Normally it's Eric.

(does my "question count" re-set if I start another topic?)

(I'd really like to know if it's worth buying factory parts through PMB or just go to the dealer, but I'm afraid to ask. I do need to buy new Ribe bolts due to the prior owner messing them up)

Wes

PM your credit card info for the answer...

Just dive in, if the bolts are factrory they are sufficent , if you think your an engineer great, or if you fuched up the ribe bolts,if you dont get it send em to eric.
I rebuilt my buddy freds in an hour no fancy zinc plate though.
sometimes doing answer alot of questions how.

Bem feito, acertou.

Alright... I feel sorry for ya. Here's my 5 minute (or $10.41666) answer:

I doubt if you can buy the bolts from the factory but you could try (901.352.978.12 and 901.352.978.13). Depending on which ones are damaged I may have some or... again, depending on which ones (the short ones for example) you may be able to replace them with a pair of long fasteners out of a core set of early 914 front calipers.

Show pictures of the damaged heads and no... it's probably not great idea to reuse them unless you can be assured you can get a proper torque on them.

Seriously, cap head (of any kind) M7 fasteners do not exist. I've found standard M7 hex (won't work, trust me) but not cap heads.

If you find a source I'll take the $10.41666 off your account.

Just like Eric, I've been rebuilding calipers for about 25 years. You do need to have the proper tools, like the Ribe bits (and thankfully I have a great source for them as the bits do break occasionally). If you use the wrong bit, the likelihood of damaging the unobtainable bolts is quite high. I have searched the world looking for the M7 hardware, and even the OEM supplier does not offer the bolts on the market. I can get the bolts custom made, but The cost per bolt is VERY high, as is the minimum order. I make many of the custom tools, for the various procedures, myself. Sure I can slap together a pair of calipers in a couple of hours, but not if I want to do it correctly. The liability if something was to go wrong is just too high.

The knock-back pins and the orientation of the pistons was developed with a lot of engineering effort over many years. I believe that if the parts are there, then keep them, and use them correctly. Trying to second guess years of research by experts when you are an amateur is not a good thing.

OK, bolts are an issue.

901.352.978.12 is NLA.

So, here is what I did;

I went by ARP and picked up 1/4"x28 bolts and nuts. 1/4" is slightly larger than the original 7mm.





They are installed with the head of the bolt on the opposite side from the original configuration due to the the shoulder on the bolt head being larger than the shoulder on the nut. The shoulder on the nut just barely fits within the cavity on the caliper body. The shoulder on the bolt head doesn't fit.

No special tools. Just a 5/16" 12 point deep socket.

Total cost for both calipers was $22.35

Now I expect to hear about them not being strong enough!

Wes

It's your life.

I would NEVER do that as 1/4" is "SMALLER" at 6.35mm

Now I expect to hear about some new calculator you have.

You can lead a horse to water...

I measured the shank on the original Ribe bolt and it has a diameter of 0.248". It steps down from the 7mm threads.

The shank on the bolt from ARP is 0.249". It's pretty much the same diameter as the threads.

My wording of 1/4" being larger than 7mm was wrong.

Wes

Wes,

I like your ARP solution. Nothing quite like good old fashioned empirical data!! I suspect the original bolt shanks were sized such that thread rolling would produce the correct thread form (7mm). The minor diameter of a 1/4-28 3A thread is .2110-.2190, so the relationship to the original Porsche bolt is nearly the same as the 7mm thread form . I suspect that ARP bolt has a high tensile rating and is very likely quite adequate for the application.

What are the ARP P/N's and length of the bolts?

BTW, while doing a quick search, I ran across this site of Ti bicycle hardware that you might enjoy:

http://www.bikepro.com/products/stems/stem_hard.html

Andys

This whole thing got started when I was just putting out a couple simple questions, hoping to get what I thought would informative to others, as well as myself.

It quickly turned into a "there is only one way to have it done correctly" thing. I've never subscribed to that concept. It's my opinion that even when a person has done this type of work for years, and over those years has gathered a major amount of information, there are "givens" that are never re-thought. I feel it's healthy to re-address some of these things. Maybe it just ends up being a confirmation that what was thought was correct. Maybe a new solution is brought to light.

I posted up the comment on the ARP bolts thinking that they may be a valid solution. Depending on the type of failure that the bolt could see, they are not as strong as what the original factory bolts were when new (prior to 30 years of heat cycling).

Trying to get valid information on what loads the bolts would see is difficult, should you want to validate their ability to handle the load. As an example, what would be the maximum line pressure in the brake system? What value did Porsche use?

As a result, people fall back on the original factory design, which may not in reality meet what would be current excepted design parameters.

All of this would be a moot point if the bolts were still available from the factory! (it would seem that the nuts listed in the factory PET are also not available any longer)

I don't have a source of core calipers to find re-usable bolts and looked for an alternative. I put out an option and got slammed for it.

As a result of not knowing what the design load should be for the bolts, I can't recommend their usage.

Wes Vann

You asked questions. I gave you the answers.

I would have gotten you some factory fasteners but, you never asked and never came back with your requirements (long, short??)

I hope you don't think I was "slamming" you although; after answering "all" of your questions on how to rebuild calipers yourself to the best of my ability, joking around a bit and finally offering the correct fasteners to you, I somehow felt you were going to do your own thing anyway, hence... Lead a horse to water. Sorry if that was harsh.

I simply would not and will never recommend 1/4" fasteners in place of a proper M7. I don't care who makes them. Frankly, I think ARP bolts are the cats-ass... again, I simply would not recommend 1/4" fasteners in place of an M7. When they break, these (Ribe and Verbus) fasteners break in the threaded section. I've seen many of them and I've never seen a shank brake. I believe the 1/4 threaded bolt is 10% "smaller" in the threaded portion where it counts, where they have been know to break (sorry to the empirical data guy but, I'm not following). I have no comment on the ARP fasteners other than to say they look really nice and I know them to be of excellent quality.

If you want data points and values that the Porsche engineeres used, try Porsche. This is a public forum where you'll get all sorts of goofy answers including mine. I am not here to second guess the engineers at Porsche or ATE. I never will. They are more knowledgable than me on this stuff. Period. You are free to do so however.

Bottom line, if you know all the answers, I fail to see what my experience can offer you here other than pure entertainment as you attempt to prove me wrong. I'm sorry I got involved at this point.

I'll refrain from further comments herein. Sorry if I offended in my attempt to help.