Frustrating day with calipers

Discussion in 'Rear-Engine Fiats' started by kmead, Aug 24, 2019.

  1. kmead

    kmead Old enough to know better

    Location:
    Michigan
    i have had my 850 since 1990 and has been kept under cover for most of that time, before that it sat in a garage in Burbank California so it has had a relatively easy life.

    For the last year and a bit the front brakes have been a bit hinky when I have driven the car. The front brakes have never been touched beyond a new set of pads. A few years ago I bought new pistons and a rebuild kit from a vendor in Germany anticipating I would need to redo the calipers.

    I pulled the parts out today as the right front caliper was effectively locked. Pulled the caliper and used the brake master to push the piston out. Spent a long time cleaning the caliper up particularly the groove the seal lives in. The calipers are aluminum and have stood the test of time fairly well, the pistons were somewhat pitted so it was time to use those nice new pistons.

    The old pistons were chromed steel and thus could and did rust in spots. The new pistons are stainless steel and appear effectively the same as the OE ones. Until you go to install them. One of them I was able to install but as I rotated the part in the bore you could feel the steel galling on the bore of the aluminum caliper. The second one I couldn’t even get halfway down the bore.

    The old pistons when I measured them were 44.87mm but the new ones were 45.1mm. Seemingly a minor difference but one that apparently is keeping me from being able to use them. The parts are very nice replicating all the features of the existing parts aside from the diameter issue.

    I ended up cleaning up the existing pistons with a brass brush. The calipers are now working properly, the new seals in combination with the less than ideal pistons remains to be seen.

    As so often happens when dealing with old cars a one hour job ended up taking most of the day.

    As an aside I did have a brake hose that was not allowing easy movement of fluid with in it. I replaced it with a new one I had on hand.

    Tempting to stick the parts onto a lathe to reduce the diameter of the pistons.
     
    khnitz likes this.
  2. NM850

    NM850 True Classic

    Location:
    Albuquerque NM
    How about increasing the bore instead to clean up the Aluminum?
     
    khnitz likes this.
  3. Dr.Jeff

    Dr.Jeff True Classic

    Location:
    Sin City
    Agree, a better approach would be to hone the caliper bores. The seals ride on the pistons so they are critical. But the caliper bores do nothing functionally so not critical to get perfect bores. And being aluminum they will hone easily with a basic brake hone...no fancy lathe needed. I can see why you might not want to alter the calipers, but frankly if they can't be made to work with the new pistons (assuming the current old pistons do not hold up), then you will likely be buying new complete calipers anyway. I imagine the new SS pistons are a worthy upgrade and worth the effort to make them fit.

    By the way, if I understood correctly the bores in the two calipers are a slightly different diameter. That tells me they might have some built up corrosion or are warped or otherwise no longer perfectly round. So honing them may be a good idea anyway.
     
    khnitz likes this.
  4. kmead

    kmead Old enough to know better

    Location:
    Michigan
    Likely true.

    I didn’t measure both pistons so they might have some variance of their own.

    One issue with changing the diameter of the caliper interior bore is the seal seat diameter isn’t changing, the recess gets relatively shallower putting more stress on that part. One of the issues that often occurs with seals is a failure to clean the recess which makes the seal roll or have a raised area and wear that area first.

    If the old pistons end up having seal issues I will hone the existing calipers and fit the new pistons. I have an extra set of seals waiting.

    Thank you for the feedback, this helps.
     
  5. Dr.Jeff

    Dr.Jeff True Classic

    Location:
    Sin City
    Sounds good Karl. I doubt honing the bores will affect the seal grooves much. But I suppose that will depend on how much need to be removed from the bores. From the numbers I think you are looking at roughly .15mm? That would be .075 off the seal groove.
    I need to rebuild 8 calipers, wish me luck.
     
    khnitz and kmead like this.
  6. dnudelman

    dnudelman Low Mileage

    Location:
    Glendale AZ
    I cancelled my planned rebuild. For what AutoRicambi is selling new calipers for, why bother.
     
    khnitz and BrettM like this.
  7. kmead

    kmead Old enough to know better

    Location:
    Michigan
    AR has calipers for the 850? I will have to look.

    MWB sells them at 360 for the pair which I thought a bit dear.
     
  8. dnudelman

    dnudelman Low Mileage

    Location:
    Glendale AZ
    Not 850, 124.
     
    Last edited: Aug 25, 2019
  9. kmead

    kmead Old enough to know better

    Location:
    Michigan
    Did you buy that 71 850 Sport Coupe in Colorado at the recent sale?
     
  10. dnudelman

    dnudelman Low Mileage

    Location:
    Glendale AZ
    Both of them
     
    NM850 likes this.
  11. kmead

    kmead Old enough to know better

    Location:
    Michigan
    Nice! Congrats! It will be nice to see another Sport Coupe resurrected.
     
    NM850 likes this.
  12. dnudelman

    dnudelman Low Mileage

    Location:
    Glendale AZ
    it will be a long road
     
    NM850 likes this.
  13. kmead

    kmead Old enough to know better

    Location:
    Michigan
    It will be but it is a great little car. I just drove mine yesterday. Started right up after not being run for most of a year.

    Mine will be pulled off the road next year to get a full refurbishment, it really needs it as the paint is getting thin enough that rust is bleeding through in a few spots.

    Good luck
     

    Attached Files:

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  14. dnudelman

    dnudelman Low Mileage

    Location:
    Glendale AZ
    Beautiful car.
     
    khnitz likes this.
  15. Dr.Jeff

    Dr.Jeff True Classic

    Location:
    Sin City
    Gotta admit I like them also. A few years ago there was one for sale locally. Amazing condition body; no rust, no damage, very complete. Interior was rough and no engine/trans. At the time I was in a transition to quickly relocate unexpectedly (family reasons), so I passed it up at the asking price of $500. Very soon after that there was a 100% original 25,000 mile Yugo. Also no rust, no damage. Had been sitting for many years but everything was there. For the same reason I also passed it up at $500. I guess timing can be everything.
     
    khnitz and kmead like this.

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