[brian450]

I feel like the default answer is rightfully "no" when someone asks if they can use some old, rusted, pitted pistons...but I'm going to ask this anyway and hope that I'm the exception to the rule 😋.

This is on an '85 CB450SC with the dual piston front disc brake. This bike sat for about 6 years so I figured it was worth pulling it completely apart and assessing everything in the brake system. I was able to get the pistons out using a method similar to this video on YouTube since I didn't have an appropriate air compressor attachment to do it (basically using a master cylinder to push them out gradually; I used the one on my bike). One thing I did not realize for a bit was that I really needed to keep the bleeder valve on the calipers as high as possible (relative to the rest of the caliper, but below the bleeding kit/vacuum thing) to get all of the air out of the system, as well as tapping the brake hose for good measure. Eventually I got enough pressure in the system to push out the pistons.

Anyway, here are the pistons themselves. I cleaned them with brake cleaner and then some Barkeeper's Friend. (Side note: does anyone else use BKF to clean motorcycle parts? It's a powder with oxalic acid in it and is super gentle on pretty much any surface, but does an amazing job at getting rust and grime off with a little bit of water and elbow grease.)

The left piston seems fine, but the right one has some small pitting on it. I have some random aftermarket (I think) piston I bought years ago that's basically new, but the thickness is noticeably smaller than these (I think OEM) ones, so there's always the option of using that.

 

[Boomer343]

Pistons first .... I would replace both if I couldn't match the good one and yes you need to replace the pitted one at least.
I replace the CB350G piston with a much lighter one from a recent model of bike and I feel the brakes respond better.
As to BKF yes it is a very good cleaning product, I use the liquid cleaner as it is more convenient and rinses easily.

 

[brian450]

Pistons first .... I would replace both if I couldn't match the good one and yes you need to replace the pitted one at least.

Bummer. I'll try to find some. These are the only stainless aftermarket ones I can find. OEM ones are about the same price on eBay so I may just get a couple of the aftermarket ones.

As to BKF yes it is a very good cleaning product, I use the liquid cleaner as it is more convenient and rinses easily.

Glad to hear someone else uses it 🙂. I have some of the spray too and I feel like it doesn't do nearly as good a job as the powder.

 

[Alan F.]

Here you go: 1982 HONDA CB750K FOUR K - FRONT BRAKE CALIPER STAINLESS STEEL PISTON | eBay

According to Partzilla's related fitment section it's the same part

Judge for yourself:
Honda 45107-MA5-771 - PISTON | Partzilla.com

Cheers.

 

[brian450]

Here you go: 1982 HONDA CB750K FOUR K - FRONT BRAKE CALIPER STAINLESS STEEL PISTON | eBay

According to Partzilla's related fitment section it's the same part

Judge for yourself:
Honda 45107-MA5-771 - PISTON | Partzilla.com

Cheers.

Yep, grabbed those same ones direct from their website. Thanks.

I think I read somewhere that the OEM pistons are iron coated with something? That would explain why they're so heavy. Can't wait to get this front brake put together! I picked up a stainless steel red brake hose from Galfer for a nice subtle color highlight 😎.

Oh and I pulled out the rubber dust seals and stuff from the caliper last night too. They seem to be in good shape (no cracking and they're still flexible and stuff) so I'm planning on reusing them.

 

[Alan F.]

Since you've got them out I'd replace them now, that way you know they'll be good for a long time.

You can probably reuse the old ones, but they wont last forever. It's easy enough to get back in there next time I suppose.

Just make sure the seal grooves are very clean, you may have to scrape with a metal pick.

 

[longdistancerider]

Reusing old rubber parts is a waste of time/effort. They're 35 years old and have lost some elasticity not to mention that there are microscopic tears where the pitted piston has moved across the seal. Replace all the rubber bits and be done.

 

[brian450]

Since you've got them out I'd replace them now, that way you know they'll be good for a long time.

Reusing old rubber parts is a waste of time/effort. They're 35 years old and have lost some elasticity not to mention that there are microscopic tears where the pitted piston has moved across the seal. Replace all the rubber bits and be done.

Sigh, you all convinced me. I'm going to spend more money fixing this bike up than I paid for it 😋.

I took out the seals and cleaned out the grooves a bunch, so the caliper's nice and clean. Just grabbed a set of OEM rubber dust and piston seals on eBay. The seals that are in there now might be new as of ~8 years ago but I can't remember for sure if me and a friend replaced them at the time or left the originals in there. I guess new ones can't hurt anything other than my wallet (and might prevent a lot more than that from being hurt in the long run).

 

[Alan F.]

Well, like a lot of parts on a lot of bikes, if you don't know how old they are...you should replace them.

Essentially where will you be when these old seals fail? Probably riding or you'd never notice they failed until you spot a puddle of brake fluid under your front tire.

 

[ancientdad]

Sigh, you all convinced me. I'm going to spend more money fixing this bike up than I paid for it 😋.

You DID sign up for vintage Honda twin ownership... that comes with the territory

 

[Alan F.]

If $15 isn't in the budget for brakes, take it out of the beer budget.

 

[brian450]

If $15 isn't in the budget for brakes, take it out of the beer budget.

Ended up being closer to $30 but oh well. Peace of mind and safety are always good.

 

[longdistancerider]

Even if you get a free bike, by the time you have fixed it's faults and done everything to bring it up to being roadworthy, you exceed the value in most cases. There's a reason bikes are sold off cheap, owners don't want to spend the money to fix them right.

 

[brian450]

Even if you get a free bike, by the time you have fixed it's faults and done everything to bring it up to being roadworthy, you exceed the value in most cases. There's a reason bikes are sold off cheap, owners don't want to spend the money to fix them right.

Very true. I started adding up all the purchases I've made to fix up this bike in an Excel spreadsheet and I was kind of shocked haha. All those little replacement nuts and washers and gaskets and seals really do add up!

 

[longdistancerider]

Danger Will Robinson, danger.
That's always a scary thing to do. I have a stack of receipts for the CL, I just keep adding to it. Maybe one day I'll start adding. NOT.

 

[ancientdad]

I kept almost all my receipts for the 450 and stopped counting (closely) when it exceeded the $6000 I had planned to spend (including the used bike purchase of $1900)... I'm only guessing today when I say it's around $8500, but $2500 of that is in the engine

Still - (first the shock, then the pondering, then the aftershock, then the indifference...)