pro's and cons of a 17mm or a 19mm mc. I know there was talk about it but dont remmber the out come

OK. I don't have a 914 yet so I don't even know what the stock M/C is, but I can tell you that the 19mm will take about 25% more pedal effort for the same stopping performance as the 17mm. The 17mm will result in more pedal travel, but less pedal effort.

How is that for some useless information?

Andrew

19mm with stock brakes feels kinda like you are pressing a 4x4 into a brick wall. Unless you are used to it or someone tells you before hand, the first stop can make you take up religian.

Either one works fine. The difference is mostly in what you think feels better. Very few people can't lock up the brakes using the 19mm one; you'd have to be very weak indeed for that. But it can be nicer to not have to press as hard.

--DD

what kinda car you got? what kinda brakes? what are you going to do with it ?

stock /4, even a 2,0, with stock brakes, and stock-ish wheel and tire combinations, in street (not necessarily mountain) driving applications, 17mm will work fine. for track use, you -may- find yourself at the hydraulic volume limit, if you use the brakes a lot (so don't do that) especially with two drivers. depends on the track, too ...

last time i checked, the 19mm MC was cheaper, but not by a lot.

19mm was standard on the 914.6, which ran bigger piston calipers front and back. when i increased front caliper size even beyond that, in went from the OEM 19mm MC to a 23. in my 911, the OEM MC was 20,5mm and i'll be replacing that one with a 23mm too.

i like a firm pedal and i accept that it may be trickier to modulate than a smaller MC.

IMHO - it's not worth agonizing over. a MC will cost you $100 and a weekend. make your best guess on what you think you'll like, and if it turns out you don't like it, yank it and put in the other one.

but - lots of MC's have been replaced at least once by now - so you may or may not know what you're running in the car unless you've had it since new or replaced it once yourself already. the size is cast on TOP of the MC, so it may take some clever work with dental inspection mirrors to know what you've got now before you pull it out. more than a few people thought they'd be 'upgrading' to a 19mm MC only to discover some long-ago prior-owner thought that too ...

i think the 19 mm one has only one electrical connection on it, and the 17 has 2....

That's the switch installed in the MC, not the MC itself. I think the new ones are mostly or all one-connector switches, 17mm and 19mm both. Just plug in the wire that is not brown, and tape up the brown one out of the way. The older switches ground through a separate wire (the brown one), and the newer ones ground through their threads into the MC.

--DD

The 411 with the same front brakes as the 914 and drums on the rear got a 19mm.

The 911S with the same size pistons in the brake calipers as the 914/6 got the 20mm.

I have happily used the 19mm since the late 70's in several 914's.

DaveP

true enough on the 411 but drum brakes need to displace a lot more fluid...

do aluminum 'S' calipers really use the same size pistons as the iron 'M' calipers ? i did not know that. i just got a set of SS pistons to rebuild my 'S' calipers but i'm not going to pull one from an 'M' caliper to compare... i know the 'S' pads are lots bigger ...

i had to replace a 914.4 MC once - it got a 19mm...

I just went through this last month, I did get the 19mm and installed it with more or less no problems..

-the 19mm does reduce the pedal travel a bit, but not a huge difference.

-the pedal feels firmer, which I like

-you can notice that you need to step a bit harder on the pedal to get the brakes to really bite, but also not a big difference, and once again, the pedal does feel more firm.

-I just unscrewed the old sensor off the old MC and put onto the new one...no problems so far, but maybe DD is right and so I'll go back and put the sensor that was on the new one back in.

Yep, "S" , "A" , and "M" calipers all use 48 mm pistons. "A" and "S" just use bigger pads.

Early 911 and 914/6 use 38mm pistons on the rear. Later 911 rears and 914/4 fronts use 42mm. All these use the same size pads; the outer caliper half is basically the same casting on all four calipers, just machined differently.

DaveP

25% less travel.



25% firmer? If not, then there was air in the system which you got out when you did the replacement...



25% harder to get the same braking effect. (See a pattern here?) You reduce your "hydraulic advantage" over the calipers pistons by doing the swap. By 25%, the difference in area between 17mm and 19mm. It has the same effect as moving the pivot on the brake pedal upward, shortening the pedal arm by 25%.



If the switch works, and it ain't leaking, leave it alone. Why mess with it and take the chance of introducing air back into the system again?

--DD