Brake Caliper Piston's - Changing piston sizes...is this possible?

devinbholmes · 05-18-2015, 06:15 PM

I posed this question to our resident brake expert Bill V. directly and am putting it out to the larger community to see if anyone has ever done this:

I have a set of front & rear 993TT calipers/rotors but really want front 993TT's and rear 993RS's. I was curious if one could take the rear TT calipers, disassemble them and have them bored out to 30/36 from the 28/28 to create an RS caliper.

Here's Bill's response to the question: "There are a bunch of us that have toyed w/ the idea of boring out the tt rears, you could go to the 30/36 Rs setup or an intermediate 30/34 setup. But i know of no one that has done it. It is certainly a theoretical possibility."

So...Has anyone ever done this or something similar with other calipers?

douglas bray · 05-18-2015, 07:36 PM

I know there are a dozen pairs of rear 930 calipers at Einmalig that someone needs to do this to. I was even toying with the idea but was able to pull a front set from a donor pho-turbo they had.

Love them.

KTL · 05-19-2015, 02:15 PM

Well you need to do numerous things to accomplish this:

Disassemble calipers
Bore piston hole
Re-cut pressure seal groove
Re-cut dust boot seal groove.

What's the intended brake setup front and rear? Reason I ask is because if you're shooting for a target hyd. bias ratio like the 44/36 front caliper divided by 993 RS rear caliper 30/36 pistons (5076 sq mm divided by 3450 sq mm), which is 1.471, you could accomplish nearly the same thing by using 930 front calipers with 38/38 pistons (4536 sq mm) and 993/965 rear calipers with 30/34 pistons (3230 sq mm) which equates to 1.404 HBR

Do you really need that large 322mm 993TT front disc? Often on the older, lighter cars the 993TT front caliper and rotor is a LOT of front brake for such a light car.

devinbholmes · 05-19-2015, 06:27 PM

Kevin - Great points.

The short answer is that my question was more of a theoretical one rather than one specific to a future brake upgrade plan.

The rambling answer is that this all started because I have the TT/RS combo on my track car and I have a front/rear TT full set that I was thinking whether I should install them on my 86 Cab M491 or not. If I was going to install them, I would really want the TT/RS setup not the TT front/rear setup which got me thinking...rather than selling the TT rear calipers and buying a pair of RS rears, why not just re-machine the TT's. That turned into the basis of the question: Can it be done?

Your point about the 930Fronts & 993/965 rears also brings up the similar question of machining 930 rears to a 30/34 setup rather than buying new calipers?

Basically I posted this to the group because I was curious if anyone had ever done anything like this and what their experiences were. There isn't really any intended brake setup in the near future for my car and I haven't decided what to do with the TT set I have (I may sell it off). The 930 calipers I have on my Cab work just fine despite the extra "heft" of the widebody.

I'm curious...What are you running on your SC?

KTL · 05-20-2015, 09:37 AM

I think the Big Red setup that many have installed over the years is really front biased. The TT rear calipers with the 28/28 pistons has a big influence on that front bias. All it takes is some more rear caliper bias/piston size and the braking improves dramatically.

Well the 930 caliper machining is a different animal. To use 34 pistons you'd have one piston with scraper seals (unmolested 30mm piston) and the other with boot seals. The seal itself is not the issue because who cares how its sealed from dust. But I don't know if there's enough room in the caliper body to create the 34mm installation when you consider how the pistons are positioned in the caliper. Looks like a tight fit out there at the edge if we increase the piston size? Here's a 930 rear caliper

I used to run 44/36 calipers in front (a hybrid setup with 993 black calipers and 309x32 Wilwood rotors) and 930 calipers and rotors in the rear. The bias was off a bit and I had trouble modulating the brakes. I had to be really careful braking or else i'd lock up the fronts. I switched to 993/965 rear calipers and the difference was quite surprising. Just that slight amount of increased fluid volume demand by the change in rear calipers allowed me to be much more aggressive with the brakes and I could brake much harder with a lot more confidence.

Car has been down for the count for almost 3 yrs now (engine blow, other projects "while i'm in there", moved to a new house, all sorts of poor excuses.....) and in the meantime I sourced some 930 front calipers. The plan is to test my theory with 930 fronts + slightly smaller 299x32 Wilwood rotors and the same 965 rear calipers with standard issue 930 309x28 rotors. One of these days/months/years i'll get my shyt together and back on the track. Until then, it's all theoretical talk.