"Red" is "New Type" piston w/ notches for stamped SS pad clip for 84-87 Fiero and likely others.
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84-87 Fiero rear piston Is Not same total diam as 88. Only same in the guts for E-brake function.
Piston Size: 47.75mm (1.88") 88 Front and Rear
Piston Size: 47.57mm (1.87") 84-87 Rear
Piston Size: 49.05mm (1.93") 84-87 Front
From Cardone.com
88 MC size: 24mm (0.9448818898") per ACDelco
(Note that these numbers can change a bit in different "books" used. I have older rear listed as 45 then "updated" to 48mm on one page in my cave because of catalog data. Some cat's are just errors, others round up to whole numbers. If you follow rounding rules, both are 48mm but isn't for people designing or working on them.)
May seem to work and Yes, Seems very small dif but 88 shell w/ 84-87 piston can have seal leaks etc because of total size diff.
GM et al make the main piston seal to fit a specified bore
AND the piston in that bore. Tiny piston size change makes the seal compress more or less. Less here old piston w/ 88 seal. Tight/loose seal can effect piston self adjust etc.
Plus will make rear brakes weaker because of hydraulic rules but you might not tell for normal driving.
Lower Performance? How bad? See my Cave,
Brake Upgrade DL excel sheet and adjust to used 88 MC size and both pistons.
Unprotect the sheet then just enter diam and sheet should do the rest.
I get...
code:
Setup MC mm MC area Cal mm Cal area Ratio Change vs Stock
Rear
OE MC & caliper 24 452.389 47.75 1790.757 3.958 0.000
OE MC & Old piston 24 452.389 47.57 1777.281 3.929 -0.030
At the very minimum... If you run older piston you need to run them on both sides on rear axle or likely have brake steering problems, one wheel locking more/easier, etc.
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Dr. Ian Malcolm: Yeah, but your scientists were so preoccupied with whether or not they could, they didn't stop to think if they should.
(Jurassic Park)The Ogre's Fiero Cave[This message has been edited by theogre (edited 11-18-2019).]