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obsolete
obsolete GRM+ Memberand Reader
3/22/21 3:14 p.m.

So, the piston I just took out of a rear caliper from my 1988 Chrysler Conquest looks like this:

It's chrome-plated steel, and the plating has started to fail, so now I have pitting and rust on the sealing surface. The caliper wasn't leaking; I'm rebuilding it because the parking brake mechanism was FUBAR, but I know if I push this piston all the way in and install new pads, it will start leaking, because the pitting will be in the seal area.

Reman calipers are NLA. Replacement pistons are NLA. It's still possible to get lucky and find a used caliper with a good piston, but most of them out there are in similar condition.

As far as I can tell, my options are:

  • Reinstall the damaged piston into the rebuilt caliper, being careful not to push it in too far, and reuse the old pads, which only have a few mm left on them (least effort, least benefit).
  • Retrofit a caliper from another car with a custom bracket (most effort, most benefit).

In my build thread, TurnerX19 offered me a potential third option, somewhere between the other two in terms of effort and benefit:

TurnerX19 said:

In reply to obsolete :

Take accurate dimensions off that piston before you install it. You can make a sleeve of the correct outside diameter with a step on one end, and turn down the old piston to match the I.D. of the sleeve along with a relief for the step on the end that faces the fluid pressure. Interfearance fit and red Loctite during the install. Accurate lathe work is required, but it will absolutely work.

I really like this idea and want to try it. I know there are shops around that are capable of this kind of work, but I can't resist the temptation to go the grassroots way and try it myself first. The piston is just slightly smaller than 1.625", and I can buy 304 stainless tubing with that OD and .120" wall thickness, which should give me the correct depth lip at the end. So, all that's left is turning down the piston OD, turning down the pipe ID, pressing (possibly shrink fitting) them together with Loctite 620, and dressing/polishing the OD of the sleeve.

I'm not 100% clear on where the lip should go, though (please excuse the drawing quality, it's the work of a keyboard-jockey with the coffee shakes):

 

The bottom orientation would be preferable, right? I think I would want the piston pushing on the lip, rather than the reverse. Exposure of the joint between the piston and sleeve to brake fluid and pressure would be the same between the two, just 90 degrees off, since the pressure will exert force in all directions. Maybe it doesn't matter, since the surface area of the joint will be vanishingly small anyway.

Actually, why not just leave a lip on the caliper side of the piston itself (turn the piston down to a smaller OD until the point where the factory chrome plating ends), and make the sleeve a simple straight piece of stainless tube? I could just use a short piece of 16ga 1.625" OD then; I can even buy that pre-polished. Turn down the piston to match the sleeve ID, no lathe work on the sleeve at all except to square off the ends.

What does the GRM hive mind think? Any and all feedback and criticism would be appreciated.

stuart in mn
stuart in mn MegaDork
3/22/21 3:21 p.m.

Seems doable if  you have the equipment and skills, but personally I'd search one of the places that specializes in sleeving them.  White Post is one, but there are others.  https://whitepost.com/brake-sleeving-rebuilding-services/

 

stafford1500
stafford1500 GRM+ Memberand Dork
3/22/21 3:34 p.m.

There are companies that make sleeves for brake and clutch master cylinders in this same method. The direction you are working is absolutely doable. The tricky part would be having enough original piston wall thickness after turning the chrome plating off and still being sturdy enough to press a sleeve on. The lip on the caliper side of the piston gives you the easiest target measurements to hit and does not interfere with the pad side shape of the piston.

My approach would be to get a piece of stock that is a little bigger (more standard size like 1.75"OD). Cut the piston outer surface down. Use that to determine the bore depth and diameter for the new stock, minus whatever press/interference fit tolerance you are trying to hit. Press them together. Finally, turn the new material down to the finish size and polish. Install and never think about it again.

Alternatively, there is flame spray build-up, which still requires some lathe work and specialized equipment, but negates the cost of new stainless stock material. It also requires you find a shop that can do the flame spraying. It basically melts metal dust onto the surface which is then machined back to spec, but at temps low enough not to damage the base material.

dean1484
dean1484 GRM+ Memberand MegaDork
3/22/21 3:41 p.m.

Rock auto part # DPS85077 

 

obsolete
obsolete GRM+ Memberand Reader
3/22/21 3:52 p.m.
dean1484 said:

Rock auto part # DPS85077

Man, I wish. Try adding it to your cart and see what happens.

rslifkin
rslifkin UberDork
3/22/21 3:54 p.m.
obsolete said:
dean1484 said:

Rock auto part # DPS85077

Man, I wish. Try adding it to your cart and see what happens.

Rock auto doesn't seem to actually have it, but Summit claims they can get one shipped from the manufacturer in ~3 weeks. 

obsolete
obsolete GRM+ Memberand Reader
3/22/21 4:02 p.m.
rslifkin said:
obsolete said:
dean1484 said:

Rock auto part # DPS85077

Man, I wish. Try adding it to your cart and see what happens.

Rock auto doesn't seem to actually have it, but Summit claims they can get one shipped from the manufacturer in ~3 weeks. 

Yeah, sorry for the terse reply above, posting from my phone. There's a weird bug or something with the RockAuto catalog, it shows up as available until you try adding it to your cart, then it tells you it's out of stock.

I already ordered both the Raybestos and Centric versions of the piston from Summit. They took my money and tell me the parts will ship mid-April, but I expect I'll see refunds when they find that the parts no longer exist. It was worth putting on the credit card just in case they find some dusty boxes on the back of a shelf somewhere, though.

Keith Tanner
Keith Tanner GRM+ Memberand MegaDork
3/22/21 4:23 p.m.

Is the shape of the piston so exotic that simply recreating it out of stainless on a lathe is out of the question? 

dean1484
dean1484 GRM+ Memberand MegaDork
3/22/21 4:29 p.m.

Just went and looked again and it has updated.  

 

TurnerX19
TurnerX19 UltraDork
3/22/21 5:14 p.m.

In reply to Keith Tanner :

These pistons have a scroll on the inside to work the parking brake. Yes it could be made, but the sleeve technique will be 1/4 the cost of making a single piston. I spend some time in a serious machine shop, this is my experience and why I suggested the sleeve. 

Keith Tanner
Keith Tanner GRM+ Memberand MegaDork
3/22/21 5:41 p.m.

Fair enough, that's why I asked. In the picture it looks like a fairly simple cylinder.

Boost_Crazy
Boost_Crazy Dork
3/22/21 5:54 p.m.

I might have a spare from my Lemons Conquest. I know we had spare calipers, I think we had spare rears too. If I find one you are welcome too it. 

Streetwiseguy
Streetwiseguy MegaDork
3/22/21 8:08 p.m.

Clean it up and slide that biatch back in.  If it does leak, it won't leak very much at all.  I've put far worse than that back into service.

If your old pads are half worn, it won't leak at all.

Pete. (l33t FS)
Pete. (l33t FS) GRM+ Memberand MegaDork
3/22/21 9:43 p.m.

Are you SURE that the seal sits that far out?  I've seen that kind of pitting often and it's always been outside the seal track.  Only a dry assembly would make this certain.  (Apologies if you've already done this)

 

Different tactic:  Who manufactured the caliper?  I wonder if there's a more available piston that has the correct diameter and internal mechanism but is different enough that they don't consider it an interchange.

obsolete
obsolete GRM+ Memberand Reader
3/23/21 11:46 a.m.

Thanks for all the great answers, guys. Yeah, my first thought was to go with a specialist rebuilder, but I have that annoying "I bet I could figure out how to do this" voice in my head that just won't go away...

The flame spray welding idea is neat. I've seen Abom79 do that in some YouTube videos. There is a guy here in town who could probably do the welding for me. Might be cheaper and easier than the sleeve, not sure. I would be a little worried about porosity.

Yeah, the female threads for the parking brake mechanism are in the piston. They're actually a separate piece that's dropped into the piston, then an inner lip in the piston is deformed to retain it. I can't come up with the right word for what that process is called, I want to say crimping or swaging but I'm not sure either of those is quite right. Here's what I'm talking about:

 

Pete, I've done a bunch of research, and I'm pretty sure it's an Akebono caliper. It has a "three triangles" logo in the casting that's different from the one I'm familiar with, but the design is so similar to DSM and SW20 rear calipers that have the familiar Akebono logo that it has to be the same.

Starion/Conquest:

 

DSM:

MR2:

 

The DSM piston is too small, but the MR2 piston is really close. My piston measures 41mm long and 41.2mm in diameter. The specs I found for the MR2 piston are 46mm long and 43mm diameter. SW20 MR2s are getting to the age where parts availability isn't great for them either, but I see somebody selling stainless pistons for them on eBay, so buying one of those and turning it down to Starion/Conquest size is theoretically possible.

Interesting thing about the MR2 aftermarket pistons, though: they don't include the parking brake parts. You are required to gut the factory piston and install the parking brake parts in the aftermarket piston with a wire clip: http://www.mr2turbo.info/pics/rearcaliper.html

So, if somebody wanted to machine a "simple" stainless aftermarket replacement Starquest piston, that would be a way to make it work.

obsolete
obsolete GRM+ Memberand Reader
3/23/21 11:57 a.m.

Streetwiseguy said:

Clean it up and slide that biatch back in.  If it does leak, it won't leak very much at all.  I've put far worse than that back into service.

If your old pads are half worn, it won't leak at all.

Pete. (l33t FS) said:

Are you SURE that the seal sits that far out?  I've seen that kind of pitting often and it's always been outside the seal track.  Only a dry assembly would make this certain.  (Apologies if you've already done this)

Valid points, guys. I'll pull the dust boot out of the caliper, grab some pads, and measure the rotor thickness so I can see exactly what I'm working with. Unfortunately the old pads are way past half worn, they're almost dead.

obsolete
obsolete GRM+ Memberand Reader
3/23/21 12:06 p.m.
Boost_Crazy said:

I might have a spare from my Lemons Conquest. I know we had spare calipers, I think we had spare rears too. If I find one you are welcome too it.

Cool, thanks! Send me an e-mail through the forum if you have any parts to unload. Which one was yours? I know there have been a couple over the years.

obsolete
obsolete GRM+ Memberand Reader
3/23/21 12:28 p.m.

I think I have a solution! That MR2 caliper rebuild article was the key. I remember seeing a post from a guy on one of the Starquest groups on Facebook who tried ordering replacement rear caliper pistons from Latvia a couple months ago. He was disappointed when they arrived because although the dimensions were correct, they were missing the internal parking brake parts. This is the part, FRENKIT P414101: https://www.frenkit.es/en/catalogo/catalogo-online/P414101. I see a bunch of listings for them on eBay, they all ship from Latvia, Lithuania, or Germany. His conclusion was that they were the wrong parts.

The cutaway diagram in the eBay listing shows a groove for a retaining clip. I bet these FRENKIT pistons will work just fine with the factory parking brake guts transplanted into them. I'm going to try to get in touch with the guy and see if he still has them.

wspohn
wspohn SuperDork
3/23/21 12:58 p.m.

On calipers that have the seal in a groove in the cylinder, you can just take the rusted piston and have it ground, hard chromed and reground to original spec, unless as Keith suggested just machining a new one up from scratch isn't cheaper.

The tougher ones are where the seal sits on the piston so the condition of the interior surface of the caliper is critical. I've had that issue on my 4 wheel disc brake old MGs and Jags, and you have to hone out the cylinder, hard chrome them and then grind to spec - more expensive than new pistons.

dean1484
dean1484 GRM+ Memberand MegaDork
3/23/21 2:11 p.m.

We use to just hone them out and then take emery cloth to the pistons and slap them back together.  I think all but one worked fine doing this and that was the rear cylinder on a Renault Alliance that my friend had.

TurnerX19
TurnerX19 UltraDork
3/23/21 6:31 p.m.

Yeah, the female threads for the parking brake mechanism are in the piston. They're actually a separate piece that's dropped into the piston, then an inner lip in the piston is deformed to retain it. I can't come up with the right word for what that process is called, I want to say crimping or swaging but I'm not sure either of those is quite right. Here's what I'm talking about:

the correct name for this process is staking.

obsolete
obsolete GRM+ Memberand Reader
3/24/21 7:15 a.m.
TurnerX19 said:

the correct name for this process is staking.

Yes, that's it! The parking brake parts are staked into the piston. Thanks.

dean1484
dean1484 GRM+ Memberand MegaDork
3/24/21 8:06 a.m.

I was looking on CL at some questionable choices and found this

https://providence.craigslist.org/cto/d/bristol-conquest-starion-collection/7284238019.html

may be worth a call to see if he has some he wants to part with. 

obsolete
obsolete GRM+ Memberand Reader
4/14/21 5:34 p.m.

obsolete said:

I bet these FRENKIT pistons will work just fine with the factory parking brake guts transplanted into them.

Well, I did it!

First, I took some measurements and did some math to confirm that the rust pitting on the old piston would actually be in the seal area with a new rotor and pads (47mm is the sum of an 18mm thick rotor and two 14.5mm thick pads). Yep, bad news:

 

So, time to operate. New piston on the left, old piston on the right:

 

Some angle grinding later...

 

Cleaned up, dropped into the new piston, couple snap rings, ready to go:

 

And a fully rebuilt caliper:

 

I wrote up a big long caliper rebuild thread at the Starquest Club forum with all the gritty details. Took the car for its first drive of the year today and the brakes work great. Should be done with maintenance on it through the end of the summer, I hope. With these cars, you never really know.

Toyman01 + Sized and
Toyman01 + Sized and MegaDork
4/14/21 5:42 p.m.

Nice!

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