1981 CB750C Front brake pistons.

[SmashEmUp]

Can anyone point me in the right direction to find some front brake pistons for a 1981 CB750C? I've looked and looked, even checked several of the links in the reference list here on this site. I can know that the support thins out when you go DOHC, and apparently even more so when you get to the C. Can anyone help me out here?

Thanks guys

[shinyribs]

I'm looking for the same thing. No luck so far,but if I do I'll let you know. I can get one from Honda,but it's $80 for just one piston.

[SmashEmUp]

Been browsing around when I've had time.... So far this is the best I could find. About $143 USD for a pair. Shipped.

http://www.cmsnl.com/cb750c-750-custom-1981-usa_model7238/piston_45107413003/

[shinyribs]

Same place I found. Their currency is listed in something I'm not familiar with ,either. Crowns,perhaps? I'm not even sure what the real price would be in US dollars.

I checked that link and they are now listed at € 33.50 a piece. The other day they were € 70.40 per piston. Strange. If that would be the price in US dollars then I'm cool with that. They are out of the Netherlands.Not sure what shipping costs would be to the US.

[shinyribs]

This is what I have found out so far.

 My forks are 1981 CB750C's.Apparently my calipers are the same as a 1978 F. I dont know if mine are mismatched,or if the 81 C's just use the same calipers as the 78 F's.

IF your calipers look like these and IF your pistons measure 38mm,or right at 1 1/2",then you should be able to use the 78 F parts. VintageCB750.com has the best deal on the caliper rebuild kits I have been able to find. $48 for the kit to do both calipers. They are out of stock on the pistons. Still looking for a good deal on the pistons.Cheapest I have found so far is about $45-50 shipped,each. Hope this helps.

Sorry about the crappy pic.It's the only one I had.

Disclaimer:The measurements work out,but I am waiting on the kits to show up to know 100% if they will work or not. I'll let you know what I find. Man,I hope they fit and I didn't just throw away $50.

SmashEmUp

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Jul 11, 2012; 4:21pm

Re: 1981 CB750C Front brake pistons.

69 posts

That's awesome man! Good to know! Never thought about cross referencing to check the other bikes. And yes, at least from the top view, my calipers look the same. Since you already spent the money, keep us posted! Lol

[shinyribs]

I made a discovery in the shop tonight I thought might help. I was disassembling a front caliper off of a '78 K. The pistons are the exact same diameter (38mm),but the K front piston is a tad shorter.The piston to caliper seal is also the same size. I tried the seal out of the K caliper and it fit perfectly in the '81 C caliper. So there is an option for a piston and a piston to caliper seal.

If you type "38mm caliper piston" into the search bar on eBay there is a company selling 38mm pistons with a seal for about $40 each.

Even though the 78K piston is shorter it does not seem like it would tend to cock in the piston bore and get stuck. I tried it at several different spots and it seemed to work OK.

DISCLAIMER: Since brake systems are the single most important item on any vehicle,I do not want to advise you to go this route,I'm just giving you options

To verify we are on the same page here are a few pics of my '81 C calipers. The piston with the groove around the edge is the original 81 C piston,the smooth piston is the one out of the 78K.

I'm still waiting on the rubber bits I ordered to show,so no word on that,but I hope this helps
My caliper I'm calling an 81 C

81C caliper with original piston inserted

81C caliper with 78K piston inserted

Length comparison of the two pistons.Again,81C piston is the one with the groove

[Hellbilly429]

Part#: 292893
APPLICATION: Honda
POSITION: Front or Rear
DIMENSIONS: 38 x 35 mm
NOTES: BCP-102
DESCRIPTION: Caliper Pistons
               
Price: $30.99

http://www.bentleysmotorcycles.com/productdetail.htm?productId=10808913

[Hellbilly429]

And an 81K has pretty much all the exact same parts with a few differences in appearances, but all the same guts. My caliper piston is the exact same as that 81C.

[shinyribs]

Now you tell me!  Thanks bud

[SmashEmUp]

I'm no engineer, and I wouldn't bet either of my motorcycles on it.... But, the important part is the diameter, right? Large enough to provide the proper deal, but not so large it gets stuck? And I'm guessing the shorter length would just require more fluid in the system? Such a small amount that it wouldn't be noticeable? Maybe? Lol

[shinyribs]

My thoughts exactly. Just a little more fluid.

[Hellbilly429]

I would agree, as long as the piston can move back and forth and seal properly the length shouldn't make too much of a difference, unless it's way to short and comes all the way out before it can push the pad into the rotor.

Sorry Shiny, didn't realise you needed that info before, my bad! I try to read as many posts as I have time to and answer ones I might have an answer too.... I just figure you are all knowing anyways bud

[shinyribs]

I know bud. Just missing with ya. Its all good now.:)

[shinyribs]

I got the brake rebuild kit today. It is for a 78F ( and whatever it lists as fitting that model). The piston to caliper seal,piston dust seal and the circlip for the piston seal fit fine. The rubber dust boots for the slider boots are not even close to whats needed. There only 4 in that kit,and these claipers require 8. Yet another stab in the dark with no results. Just letting you know.

[TOOLS1]

shinyribs wrote

I got the brake rebuild kit today. It is for a 78F ( and whatever it lists as fitting that model). The piston to caliper seal,piston dust seal and the circlip for the piston seal fit fine. The rubber dust boots for the slider boots are not even close to whats needed. There only 4 in that kit,and these claipers require 8. Yet another stab in the dark with no results. Just letting you know.

Huh?
TOOLS

[shinyribs]

I bought a caliper rebuild kit for a 78F (cuz the pistons match) and tried to see if it would work on the 81C calipers,since nobody sells a kit for them. Pricing the parts indiviually from Honda was gonna be over $140. Pistons are another $72 each. I know. You dont have to say it. But you can if you wanna

[TOOLS1]

Again Huh? 
TOOLS

[shinyribs]

Now I'm confused. Whats up bud?

[SmashEmUp]

The rubber dust boots... When you says they're not even close are you talking about just the quantity? Or the the size and fitment? Or the whole gambit? Lol