'80 rear brake piston 41mm long

[bdento59]

Hi all, I'm rebuilding the rear brake on my '80. The old piston is 38mm dia x 41mm tall, but the replacement piston from Tims is 38x35mm? Any knowledge out there as to the reason why the original piston is 6mm longer than the replacement? I'm pretty sure that the caliper is the stock one. The bore measures 37.5mm deep from the lip to the bottom. Any insight appreciated.

[Rick Pope]

Bill,

Measure the thickness of new pads. I seem to remember that the front pads are a little thicker than the rears, so a little shorter piuston might make up the difference. Could you be so lucky?

Or, I might just be having a senior moment, and be totally wrong....

[daves79x]

Honda put two different rear calipers on the '80s. Bill used to note that difference. I actually thought it was the diameter that was different.

Dave

[EMS]

I'm not sure whether Honda put two different calipers in the 1980 bike, but they definitely put two different caliper pistons in the 79 and 80.
The original calipers are all carryover from the 1977/78 CB750F (SOHC!) model, while the psitons in the '79 are from the same bike, but in the 80, they were the same as the 79 CB750F DOHC
So, while the calipers are the same, the pistons differ between 79 and 80

[bdento59]

Honda put two different rear calipers on the '80s. Bill used to note that difference. I actually thought it was the diameter that was different.

Dave

Dave, you are correct. Bill (@ TIMS's CBX) does sell two different pistons for the '80 CB-X rear brake. One is a 38.1mm (1.50") dia x 35mm long, which equates to (current) OEM 45107-410-006. I've been told that this part number used to point to a piston with a 41mm length, but was phased out in deference to the 35mm variety. However, Bill also supplies a 42.81mm (1.685") dia x 41mm long piston which equates to OEM 45107-415-006.

[bdento59]

After spending some more time researching this topic today, I found some interesting factoids, some of which I'm sure you already know, but here goes anyway:

1) A 38.1mm (1.50") x 41.3mm (1 5/8") brake piston was a fairly common item in the early '80's, used on the 1980 CBX-A but also on 1979-80 CB750F and also 1975-77 GL1000 (front). There are probably others as well.

2) An identical piston was also apparently used quite extensively on early '80's Yamaha XS500, XS650, XS750, XS850, XS1100, and even XV920R (US market version of the TR-1).

The GL1000 part is still available at parts-n-more under OEM #45107-371-006 for $30 (chrome over steel). I went with a stainless steel pattern version from HVC Cycle out of Lincoln, NE for $27.95 + S/H of $10.95.

When I receive the product I will examine it's dimensions carefully and report back to the list. Thought you'd like to know.

[tevan]

Very good info. I like the stainless piston idea and the price is right. Please let us know if it works out.

[steve murdoch icoa #5322]

Bill, thanks for the follow-up.
$25.00 for the stainless sounds good.

[EMS]

The confusion originates from the fact that Honda issued two different rear calipers for the 1979 and 1980 CBX. Both calipers are interchangeable but they are by different manufacturers. 1979 caliper was 45201-410-006 with the corresponding -410- part-no for the piston. The 1980 CBX inherited the rear caliper from the '79 DOHC 750, which came with a different piston and the parts carry a -445- product code. The -415- parts are 1978 CX500 and, although Bill may sell them, were never used on the CBX, according to my records
Over the years, people have changed rear calipers believing 1979 and 1980 calipers were identical and thus needing different parts for spares.


Oh, and one more thing: If you guys would finally all use DOT5, all this would be a moot point, because you would NEVER have to rebuild your brakes again!

[Larry Zimmer]

At least not if you do a fluid flush every year!!