38mm Piston Rear brake Caliper install

Discussion in 'Workshop Forum' started by Rupunzell, Oct 29, 2016.

Tags:
  1. lookforjoe

    lookforjoe True Classic

    Takes regular X1/9 pads. I had a brain fart & thought I was using calipers that accomodated the wider front pads....

    EDIT - I didn't check to see if there is enough meat on the ears to machine for wider pad fitment
     
  2. TonyK

    TonyK True Classic

    Location:
    Grimsby Ont Canada
    I may have made a mistake sending these to Bob Martin's place. They look so good he may not ship them to me in Canada.

    Ooops

    TonyK

    Grimsby Ontario Canada.
     
  3. fiatfactory

    fiatfactory Steve Cecchele

    Location:
    Western Australia
    They look like new calipers Hussein... not original by the look of it, so pattern parts but new... every Fiat or Bendix made caliper has always had Fiat 125 markings on it like the pics Bernice posted.. probably off a lada or similar.

    SteveC
     
  4. lookforjoe

    lookforjoe True Classic

    EDIT: Tony, when you recieve yours, can you measure the shoulder to see if 14mm could be removed without cutting back the entire flat. That would allow the use of front pads in the rear (7mm additional per pad)

    Be prepared for a significant increase in pedal travel with these. I was sure I still had air in the lines at first, but that is not the case. Quite disconcerting under gradual braking, less so when driving more spiritedly...

    EDIT: I do also have the larger brakes from Vicks up front. Perhaps I need to look into an alternate master cylinder at this point...
     
    Last edited: Nov 22, 2016
  5. lookforjoe

    lookforjoe True Classic

    Bernice - what would be required to counter the excess pedal travel - a larger or smaller bore MC? I still have my original, so I could look into sleeving to increase or decrease the bore as appropriate....
     
  6. Rupunzell

    Rupunzell Bernice Loui

    Location:
    California
    The increase in pedal travel is not much at all. IME, not enough pedal travel increase to be concerned about. Do check the brake pedal stop (brake stop light switch it is threaded and serves at the pedal limit) to optimize pedal -vs- throttle pedal position.

    Bernice
     
  7. lookforjoe

    lookforjoe True Classic

    Hmmm... mine seems quite noticeable. All components are new at this point. I do have the larger Vic's brakes up front, so I'm wondering if that contributes to additional travel. When I'm actually driving around, I don't notice it so much except for the initial switch in feel (from one car to another)
     
  8. Hey Hussein,

    I'm using 4 piston Willwood calipers and vented discs on the front from Mark Allison, and front calipers on the back (hence the need to incorporate a hydraulic handbrake) and have just a tad more brake pedal travel with the standard master cylinder. I like that I have MUCH better modulation of the brake pedal pressure and have adjusted it's height to suit heel and toe downshifts.

    The brake balance I now have (in conjunction with my tyre/suspension setup) is spot on, in that it suits my driving style. :D:D:D

    Cheers,

    Rob
     
  9. lookforjoe

    lookforjoe True Classic

    hmmm.. OK, I'll try moving the pedal stop, since it doesn't need to be as far out as it is anymore....
     
  10. Rupunzell

    Rupunzell Bernice Loui

    Location:
    California
    54mm diameter front caliper piston diameter. If yes, those WILL affect pedal travel and front to rear brake balance. Due to the front weight bias of FWD chassis, they have typically have much larger front brake caliper piston diameter to get the front to rear brake bias proper for the FWD chassis. If the brake rotors are larger than stock of 215mm_ish diameter, this will further aggravate the front brake lock up problem with a stock 3/4" diameter brake master cylinder. Simply transplanting a front brake system designed for a FWD chassis with a 7/8" diameter brake mater cylinder is not going to work properly, the front to rear brake bias ratio is going to be all out a whack.

    Getting the front to rear brake bias ratios and pedal travel proper is likely going to require to different diameter brake master cylinders. a 7/8" diameter for the front and a 3/4" diameter for the rear (with 38mm diameter caliper piston) on a brake bias bar. We tried a similar set up on the LeMons racer and it was a disaster.

    Improving the brakes on the exxe involves far more than simply larger brakes, the front to rear brake bias ratio must be correct to a myriad of problems from pedal travel, front brake lock up too soon before the rear or rear brake lock up before the front and more. The really good drivers use brake bias to shift weight entering a corner aiding in corner entry. Add to this tire wear, fuel load and track conditions is why any serious race car will have driver adjustable brake bias.


    Bernice
     
  11. fiatfactory

    fiatfactory Steve Cecchele

    Location:
    Western Australia
    A larger master cylinder bore will REDUCE line pressure, and require more pedal effort to produce the same line pressure... simple hydraulics...

    100lbs of pedal force onto a 1" area piston will produce 100lb/sq inch of line pressure... transfer this to a 2" area caliper piston and you have 200lbs force pushing on the back of the pads.

    If you increased the master cylinder bore to 2" square and kept the same 100lbs of pedal force, the line pressure would be just 50lb/sq inch, the same 2" square caliper piston will then only produce 50lb/sq inch of force, so 100lbs pushing on the back of the brake pad.

    Sure the 2" area master cylinder will only move half the distance to transfer the same VOLUME of fluid, but the pressure at the caliper will be halved if the master cylinder area is doubled.

    The change from a 34mm piston to a 38mm piston will not significantly increase the amount of fluid VOLUME required to move the piston the 0.1mm or so before pad to rotor contact occurs...

    pi x r square x distance
    3.14285 x (17 x 17) x 0.1 (fluid displacement to move a 34mm piston 0.1mm)
    3.14285 x 289 x 0.1
    90.82 cubic mm (which is .9082cc)

    3.14285 x (19 x 19) x 0.1 (fluid displacement to move a 38mm piston 0.1mm)
    3.14285 x 361 x 0.1
    113.45 cubic mm (which is 1.1345cc)

    extra displacement required by 38mm piston over a 34mm piston
    113.45 - 90.82
    22.63 (which is 0.2263cc)

    3.14285 x (9.525 x 9.525) x 1mm (volume of fluid displaced by the master cylinder in 1mm of pedal stroke)
    3.14285 x 90.76525 x 1
    285.13 cubic mm (which is 2.8513cc per mm of stroke)

    extra stroke required on the rear brake circuit by increasing piston size by 4mm
    1 /(285.13 / 22.639)
    0.08mm (approx 1/12 of a mm)

    the simple fact is the 38mm piston rear calipers were used on other Fiat models and used the same 19.05mm master cylinder bore... so the increase in pedal stroke for that particular change was not considered significant by Fiat engineers.

    SteveC
     
  12. TonyK

    TonyK True Classic

    Location:
    Grimsby Ont Canada
     
  13. Deane

    Deane True Classic

    Location:
    Abbotsford, BC
    Ok so both Haynes and Clymer manuals state that mark on the piston is to be on bleeder screw side. ?
     
  14. TonyK

    TonyK True Classic

    Location:
    Grimsby Ont Canada
    IMG_3457.JPG IMG_3456.JPG Hussein, there is only 21MM of material on the caliper and removing 14 MM is pushing it in my opinion. Calipers came in today from Canada Post (Sunday) They are new and 38MM as I removed a dust seal to check. Also I will dismantle the caliper, sandblast and use the Eastwood powder coating system to powder coat each caliper. I will post pictures and a short video.

    See pictures.


    TonyK.
     
  15. lookforjoe

    lookforjoe True Classic

    Thank you, Tony.

    Yup, leaving 7mm for clamping force seems iffy, for sure...
     
  16. ng_randolph

    ng_randolph Bjorn H

    Location:
    SF Bay area
    As does the factory service manual. There is a bit of a relief ground into the inside of the piston on the side of that mark to help air move up and out. Turning the piston 180° from where it should be just makes it a bit more difficult to bleed all the air out.
     
  17. PaulD

    PaulD Paul Davock

    Location:
    Kitchener, Ontario
    Bernice:

    Now that you are driving your 74 with the 38mm caliper rears and Uno Turbo front calipers, how is the balance overall? Do you use the Wilwood brake pressure regulator that you installed on the front brakes? Do you wish it was on the rears when you are driving hard in the rain?

    I will be doing this installation this winter and your input will be helpful for me.

    Paul
     
  18. TonyK

    TonyK True Classic

    Location:
    Grimsby Ont Canada
    I have powder coated these at home with Eastwood's home powder coating system.

    On the down side after changing these I realized how black and dirty my other calipers were. I am going to have to pay attention to keep these clean.

    https://youtu.be/K5XhnNPzbjQ

    TonyK.

    Grimsby Ontario Canada.
     
    lookforjoe likes this.
  19. kmead

    kmead Old enough to know better

    Location:
    Michigan
    Shame! Shame I say, dirty calipers indeed. Before you know it you will have powdercoated the front ones and chromed the under carriage, show up with it on a trailer and have mirrors to show off the toothbrush clean underbody with a collection of trophies and such festooned alongside...
     
  20. TonyK

    TonyK True Classic

    Location:
    Grimsby Ont Canada
    I installed the calipers and bleed the system and let it sit over night. Next day still more air. I had to move the car out of the shop yesterday as I am painting another front hood for the car. The pedal really seemed mushy, then I remembered a few months ago I rebuilt all of the calipers on another 79 X and bled it then took it down the road. Really mushy pedal, I then bled again and there was air, it firmed up nicely afterward. I think air gets trapped in the caliper and it needs to be road run before it all goes to the top of the caliper. When I was braking 2 pumps of the pedal and it came up nicely and was firm. When I get it back into the shop I will bleed the rears again.

    TonyK.

    Grimsby Ontario Canada.
     

Share This Page

  1. This site uses cookies to help personalise content, tailor your experience and to keep you logged in if you register.
    By continuing to use this site, you are consenting to our use of cookies.
    Dismiss Notice