brake master cilinder

[budgerian]

Is there anyone on the forum that has experience with both the 22.2 mm diameter brake master cilinder and the 23.8 mm version?

Depending on the type of brake servo attached to it, both versions were used on all E3's up to and including the 3.0 si (not sure about E9's, as I have a 3.0 Si). According to the diagrams on RealOEM the 23mm is used in conjunction with the booster that is shaped like, shall we say "a cookie box" (the front and rear of the booster make a 90 degree angle to the side) whereas the 22.2 is used with a slimmer but higher booster that has a somwhat tapered side and is also flanged on the side.

Reason for asking is that I can get a new 22.2mm version for very little money (contrary to prices for the 23.8 mm one, if you find one that is), and do have the correct booster, but my current master cilinder has a 23 mark cast in it..

Who is able to clear up if the 23 casting mark means I actually have the 23.8 mm bore size (looking at the picture the seller showed the bodies of both cilinders are identical, but i couldn't see the casting mark), and more importantly, who can tell me if there is a major difference in feel and braking effort between the two.

Now I am aware of the academic differences between larger and smaller bores, i would however like to know how it translates in everyday driving with the BMW e3/e9 setup.

Thanks for all your replies!

Roger

 

[MMercury]

Is there anyone on the forum that has experience with both the 22.2 mm diameter brake master cilinder and the 23.8 mm version?

I have no specific recollection about dealing with the 22.2 mm master cylinder to which you refer. The most general rule of thumb is that all things being equal, the smaller the bore = greater line pressure = less pedal effort = lower fluid volume. This can be good or bad to the extent bore size affects pedal "feel." This can be very subjective. The larger bore moves a greater volume of fluid = more pedal effort. This can be significant if you are using larger calipers.

The 1.6mm size difference you mention is, in the larger scheme of things, somewhat negligible. The smaller bore unit would probably be suitable for the subject vehicle to the extent its intended use is for ordinary street driving. Driving the car flat out on the Mulsanne straight, might lead to a different opinion.

http://www.airheart-brakes.com/pdfs/MasterCylinder_Selection.pdf
http://www.hotrodheaven.com/tech/brakes/

hth

 

[61porsche]

The bores equates to a volume of fluid necessary to react or cause a reaction for the wheel cylinders or calipers AND a given pedal reaction ( distance).

Changing to the smaller would be negligible. Adjusting the rod by an 1/8" longer or so would make up for any additional pedal travel. It's just a ratio that changes so the stroke equals the effort required.

If you changed brakes to brembos.. all bets are off.

Don't know about the boosters... maybe someone else can address. But as long as everything bolts up.. should be fine. E9s used two boosters as well, slim and large.