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M caliper question

Will the 1975 M calipers found '75 M caliper fit a 1973 911 t? I believe the offset to be 3 1/2. But I am not sure.
thanks in advance.
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'73 911T MFI - in process of being restored
'73 911T MFI - bare bones
'87 924S - Keep's the Porsche DNA in my system while the 911 is down.
aka "Wolf boy"
Old 01-10-2006, 12:55 PM
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Yes, both front and rear have the 3" bolt spacing on the mounts that you will need.

Don't mix the front and rear pairs up. They are the same externally but not internally.
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Old 01-10-2006, 01:07 PM
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You may want to check the mounting tabs on your front struts to be sure that the center-to-center mounting distance is 3 inches for 'M' calipers ... 3.5" sounds like 'A' or 'S' calipers.

You may want to ask for closeup pics of the pistons for the rear calipers in that set ... pitting on the outer one-third 'working' portion of the piston could mean a rebuild with those pistons will be a waste of time and money!
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Old 01-10-2006, 01:10 PM
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Man do I love pelican! Within twenty minutes two of best (IMH) pelicanites relpied.
Early S I know that I don't have A or S calipers. I probably just measured them the wrong way. Also what is this about "pitting" I have heard of the term before, but never quite understood.
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'73 911T MFI - in process of being restored
'73 911T MFI - bare bones
'87 924S - Keep's the Porsche DNA in my system while the 911 is down.
aka "Wolf boy"
Old 01-10-2006, 01:14 PM
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Either the bores or pistons can corrode leaving pits in their surfaces.

The dual causes are moisture the accumulates in brake fluid and damaged external seals.

the former is controlled by the additive packages in fresh brake fluid but eventually is used up. The lesson here is to change fluid often.

The external seals can become damaged allowing moisture and grit to accumulate in the working bore. Again damage from pitting to scoring will ensue. Regular inspection of the seals and and exercise of pistons is important.
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Old 01-10-2006, 02:24 PM
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Tom,

The caliper pistons need to be smooth and undamaged in the 'working' area in order for a rebuild to be practical or safe over the long term. Discolored old pistons should clean up with no roughness or surface pitting that can be felt by dragging a fingernail across any discoloration. Pitting that can be felt with a fingernail will chew up new caliper seals and cause leaks.

I use isopropyl alcohol and ultrafine Scotchbrite [maroon variety] to clean old used pistons, and then evaluate for rebuild suitability. I recommend the use of Dow Corning #4, #111, or #112 silicone grease on the cylinder walls and pistons when rebuilding calipers. It is fully compatible with all glycol-based brake fluids.

The rear set of pistons in the for sale posting have not been cleaned or are in questionable shape, and it was so mentioned with the ad.

The following pics are from a good 912 site caliper rebuild article http://www.hillmanimages.com/912/brake_pistons.html and I have added annotation to identify the critical area of the piston:

The piston in the bottom pic has cleaned up to a very satisfactory condition ...



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Last edited by Early_S_Man; 01-10-2006 at 03:14 PM..
Old 01-10-2006, 03:11 PM
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Well I think I now have all the parts I need to complete the Targa's brakes. I've got a new MC, new rotors, caliper rebuild kits, pads, Dow Corning grease, etc. The re-assembly starts tomorrow.

One question I still have is whether there is any advantage to using M-Calipers like the kind shown in Wayne's Projects book (no pin in center of piston bore, piston cup faces outward toward pad) and the M calipers with the pin in the piston bore, like the one shown below (picture from Dave's 912 site)?



The pistons for these calipers have an internal guide in the cup, which faces toward the caliper (away from the pad)

I have a complete set (two fronts and two rears) of both kinds of caliper--but only four rebuild kits, so I don't want to leave this to chance. The calipers with the pin in the bore came with the car, but I'm not sure whether they are original.

Are these different varieties of M calipers? They have the same bolt spacing and they are the same size.

Any thoughts?
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'73 911T MFI - in process of being restored
'73 911T MFI - bare bones
'87 924S - Keep's the Porsche DNA in my system while the 911 is down.
aka "Wolf boy"
Old 06-03-2006, 07:20 PM
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Tom,

The pins are part of the 'anti-knockback' mechanism that was introduced with the '64 356C and 356SC models in the fall of 1963, and carried over into 911 production in 1964. Yes, they were standard on your 1973 911.

The 'M' calipers without pins and knockback mechanism were probably 1976 models or later.

The 'anti-knockback' mechanism was designed in the days of standard 82% profile 165HR-15 tires ... and became irrelevant when 205 mm tire widths and 50-series profiles were standard.
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1973 911S Targa ... 'Annie'
1968 340S Barracuda ... 'Rolling Thunder'

Last edited by Early_S_Man; 06-04-2006 at 10:55 AM..
Old 06-04-2006, 08:49 AM
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Quote:
Originally posted by Early_S_Man


The 'anti-knockback' mechanism was designed in the days of standard 82% profile 165HR-15 tires ... and became irrelevant when 205 mm tire widths and 50-series profiles were standard.
I'm sorry to have to ask, but what in a tire makes a difference?
Old 06-04-2006, 02:01 PM
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Milt,

Cornering forces were lower with 165 mm section width tires, and the brakes were unboosted, meaning that more pedal travel than with power-boosted brakes was utilized taking up the slack between pad and rotor. A modified, higher-resistance anti-knockback mechanism would have been required with higher cornering force tires, but the power brakes basically eliminated the need.
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1973 911S Targa ... 'Annie'
1968 340S Barracuda ... 'Rolling Thunder'
Old 06-04-2006, 05:45 PM
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