Got a Duralast M51837 brake master cylinder off a buddy of mine. It's supposedly a 15/16. When I measured the bore it is a little over 1".

Would I be able to use this paired with a 280zx brake booster or is 1" too big?

Edited September 26, 2024Sep 26 by Z2 D2

Autozone says it's right for some ZX's.

https://www.autozone.com/brakes-and-traction-control/master-cylinder-brake-system/p/remanufactured-brake-master-cylinder-m51837/719096_0_0

9 hours ago, Z2 D2 said:

Got a Duralast M51837 brake master cylinder off a buddy of mine. It's supposedly a 15/16. When I measured the bore it is a little over 1".

The master cylinders are usually bored out larger in the area where the retaining clip goes. So if you are measuring the bore with all the guts installed, It's likely that you aren't measuring the actual working diameter of the cylinder. You are probably just measuring the diameter where the lead-in and retainer are located. 

My guess (from my chair) is that the bore down deeper in the cylinder is smaller than what you are measuring out at the end where you have the caliper.

1 hour ago, Captain Obvious said:

The master cylinders are usually bored out larger in the area where the retaining clip goes. So if you are measuring the bore with all the guts installed, It's likely that you aren't measuring the actual working diameter of the cylinder. You are probably just measuring the diameter where the lead-in and retainer are located. 

My guess (from my chair) is that the bore down deeper in the cylinder is smaller than what you are measuring out at the end where you have the caliper.

It does taper down into a smaller hole. Would the measurement be from the top of the taper or the bottom?

The measurement should be at the narrowest part of the taper. In order to get a good reading, you might have to push the brake piston in a little bit to expose the surface on which to put the calipers.

Or you might be able to just catch a little bit of the bore at the bottom of the taper even without pushing the piston in. And if you are just "reality checking" to make sure you have the right master cylinder for your car, that might be accurate enough.

And another complication will be that the retaining ring clip will be in the way of taking a measurement.

I'm assuming that the master doesn't have the size cast into the outside of the body? Cause if it did, you wouldn't have to go through any of this?

The rod from the booster fits in to the spherical pocket.  The internal bore doesn't matter for connection to the booster.  I'd worry more about getting the booster rod depth correctly set.  That's where problems happen.  If the internal bore has been enlarged it would affect pedal stroke slightly (more volume would give a shorter stroke) but, as CO points out, it's probably correct down where the piston is.

14 hours ago, Z2 D2 said:

Would I be able to use this paired with a 280zx brake booster

 p.s. it should work.  Probably best to match the year of booster.  Not clear what changed for 82 and 83 280ZX's, which are left out of the fit list.

1979,80,and 81 have verticle mounting. 82,83 horizontal.

On 9/26/2024 at 8:03 AM, Captain Obvious said:

And another complication will be that the retaining ring clip will be in the way of taking a measurement.

I'm assuming that the master doesn't have the size cast into the outside of the body? Cause if it did, you wouldn't have to go through any of this?

Wish it was that easy. There is a casting it just says 01 though.

Yeah, I had little doubt it wasn't that easy.  But I have to ask.  Haha!!

If you have one of these (commonly known as a layout caliper), you could push the piston in and reach behind the retaining ring to set this thing for the inside diameter. Then take it out and use your caliper to measure the jaw width.

If you don't have a layout caliper and are really really desperate, you could use a piece of coat hanger bent into a "U" shape. If you do it right, you should be able to get a reality check quality measurement.