Help with changing bore size from 13/16 to3/4

[San segrest]

I want to change the wheel cylinder  bore size from 13/ 16 to 3/4 do i have to change to a different master cylinder. Can it be done. Help some body.

[datzenmike]

Not necessarily. A larger wheel cylinder will need more fluid moved to get the same pressure and stopping performance. Your master can easily enough do that, but it will have to travel farther towards the floor. The amount??? probably not much but it may seem sort of mushy after, just be aware of that. 

[mklotz70]

I agree with Mike.  Also.....along with more pedal travel, it should require less leg effort because it creates the effect of more leverage.....which is why it can feel "mushy" as Mike stated.  As Mike pointed out, the wheel cyls will require more fluid.....for just one wheel cyl, pretty insignificant, but if you're changing all 4, it could be a substantial change.  Basically, you'll have to change all 4, or at least both fronts or both backs.  Assuming you're still trying to use the stock m/c, you'll have to change all 4.  You'll also need to make sure that your drums and shoes are in great shape and adjusted perfectly. Any extra travel from mis-adjustment could require too much pedal travel and be unsafe.  

[Crashtd420]

Umm... 

Isnt 13/16 (.8125)

Bigger than 3/4 (.750)

So he reducing the diameter not increasing....

[mklotz70]

4 minutes ago, Crashtd420 said:

Umm... 

Isnt 13/16 (.8125)

Bigger than 3/4 (.750)

So he reducing the diameter not increasing....

DOH!!!  Okay....pretty much the opposite then.   Downsizing the wheel cyls shouldn't cause much of an issue.....just more leg effort.   Thanks Crash. :)

 

[banzai510(hainz)]

I have a wheel cylinder that different and its minute amount of difference if that and would worry about it as it needed it right now and installed it. Just adjust the shoes and youll be fine

 

13 /16

3/4  = 12/16  not much to worry about

 

just make sure its for a 521 as they are SAE threads I believe on a lot of stuff .. I don't mix up with 620 stuff as I assume they all metric

Edited March 11, 2020 by banzai510(hainz)

[Stoffregen Motorsports]

43 minutes ago, banzai510(hainz) said:

I have a wheel cylinder that different and its minute amount of difference if that and would worry about it as it needed it right now and installed it. Just adjust the shoes and youll be fine

 

13 /16

3/4  = 12/16  not much to worry about

 

just make sure its for a 521 as they are SAE threads I believe on a lot of stuff .. I don't mix up with 620 stuff as I assume they all metric

You can swap from SAE to metric. You just need metric banjo bolts and metric bolts for where it mounts to the backing plate.

[datzenmike]

Was only on my first coffee this morning. As soon as  I read the title just now I could see the glaring mistake. OK.... 

 

Going to a smaller wheel cylinders will require less fluid so the pedal will not need to travel as far. It may feel stiffer. 

[DanielC]

Going from 13/16 to 3/4 wheel cylinders in the rear will also slightly reduce the tendency of the rear wheels to lock up under hard braking.  If you rarely haul anything in your 521 this may be a good thing.

 

I have seen both 3/4 and 13/16  wheel cylinders, and I also have seen wheel cylinders marked wrong, I.E. a cylinder marked 13/16 that was actually 3/4 or the other way around.