I'm in the process of boring out a 17mm master cylinder and rebuilding it with a 19mm rebuild kit. Has anyone here ever done that?

Nobody's going to tell me it can't be done? Trailblazing isn't any fun if nobody says you can't do it.

It can't be done.
You'll die in a mashed up ball of flames!
Pox be upon you.

ya gonna die! ya gonna die! ya gonna die!



Just make sure you add some ss brake lines to make absolutely sure.


Don't ya just love us locals?

Why?

Why not get a boneyard 19mm and rebuild that?

That's better.

Boneyard 19mm master cylinders aren't free... I had the 17mm laying around, and I was bored at work. I make your average CSOB look luxuriant.

How ya gonna do that mister smarty pants? Are ya some kinda wize machinist dude or sumtin? Why stop at 19mm why not go 19.5mm just to be better than some of us ey? If i were that smart I'd make like a 100mm out of a 17mm thingy you are drilling your tool thingy into it like that your doing that thingy there.

It can't be done, it just can't I know it can't cuz the dude at Advanced said so. There you!

Are 19 mm master cylinders bored to some strange measurement (18.7 or something)????

It sounds like your time is worth nothing.

In my opinion, not that is counts for much, but why would you take a chance with something like brakes? I know I wouldnt want to chance it, what if it really cant be done and you think it's ok and you are flying and absolutely need to stop fast and it gives out? Then what? I just wouldnt do it. Again, that is just my opinion.

if your a machinist minded kinda guy, and you have the tools, why not.
Somewhere in the world there is a machinist boring them out for the final sizing from the cast product...so just make sure you know the spec.

It would be nice to measure a spare one that might be junk somewhere to see the proper size and the proper clearance...

I bet you could check ATE and see if there is a diagram.

Rich

I am basing the final size on the size of the piston. I'm basically making it the same amount larger than the 19mm piston as the 17mm bore was larger than the 17mm piston. I don't remember the exact measurement, but it was less than 19mm. I'm drilling it undersize and honing it to the final size to make sure it has a good finish. If it doesn't come out perfect, I won't run it.

Well, I say go for it but test it out thoroughly after you've got it done and then try it out carefully, look for leaks, etc. The thing is that if its done incorrectly it should be obvious immediately.

Just be careful.

And yes you will die. eventually

The casting may be bigger on the 19mm. I bought a brand new 19mm for my 6 instead of rebuilding it. You need the size of the casting. If it is bigger on the 19mm you should not dick with this. Used 19mms can be very cheep. I have one

Bob

i'll stand in back of his work... way in back

You'll have to bore it, then hone it. If you have the correct mandrel for the hone along with the appropriate sizing sleeve, then you stand a chance at success (a hand held hone is not nearly good enough). Remember, you're not trying to clean up a little bit of rust or pitting, but you are removing the radial machining ridges so the work must be correctly done. Of course you still have to determine if the 17mm casting has enough material to safely remove 2mm on the diameter.

Good luck,

Andys

The 19mm and the 17mm are identical castings, except for the "17" and the "19" cast into the bottom. I have a used 19mm in my other 914 and have held them side by side. (which is how I got the idea)

Personally, I'd rather have a rebuilt 19 made from a 17 than a used or rebuilt 19. You can never get the 19 as tight as when it was new, unless you sleeve it.

TWSS

Zing!!


All kiding aside though, I bet he can get a better product if he did it himself.

That or a horrable death, test it some place safe. Do you have a e-brake?

Don't forget that the master cylinders are inch sizes.
C# Metric_Inch__x/16
17- 17.46 0.6875 11 914/4
19- 19.05 0.7500 12 914/6, 911
20- 20.64 0.8125 13 911S
So the 19 is actually a 3/4" bore.
However, that is nominal. One would need to carefully measure one to determine what the exact size if the bore is, it may be oversize to provide operating clearance, or the piston may be undersized. However it works out, I'll be very interested. I have two 20mm masters that I'd like to rebuild. I have only one kit though. If the bores on either are poor, then a 19 could be overbored and made to work. In essence, this is duplicating factory production.

go for it- if i had some machining tools i'd try these things myself! i cant see why it wouldnt work as long as the honing finish is good.

After seeing the pictures of your cable shifter, I'm not about to tell you that something can't be done!

Turns out it can't be done... not the way I was doing it anyways.

I attempted to take off 1/64 of an inch with a hone. While theoretically possible, I nearly "used up" my brake cylinder hone trying. Then I started looking around, and found a 3/4" ream. I carefully hand reamed it, honed it smooth, and now it's a "new" 19mm master cylinder!

BTW, it is 19.05mm, I don't think my calipers were properly zeroed when I initially measured it to be under 19mm. The large multi-language sheet of paper that came with the ATE rebuild kit also lists the maximum serviceable diameter as 19.16mm. So now you know, and knowing is half the battle!