What's correct? 1" bore or 7/8th" bore Master Cylinder
 

As part of my brake rebuild on my '66 Grand Prix, I wanted a new master cylinder.

Actually I was going to get my original one stainless sleeved but the cost is prohibitive. Here it is anyway.

So I called my friendly American Car Parts supplier who has been around forever. He asked all the expected questions, make, model, year, booster or no booster etc and yes he has one on the shelf. But then he threw me by saying his book tells him it has a 7/8th inch bore.

I say no, mine is a 1 inch bore. He asks if I'm sure, I say yes it is and I think his book is wrong. So he is sending me one with a 1" bore.

But am I correct? Is my cylinder correct? I've had the car for 25 years since i imported it and I know I put new rubbers in all the cylinders when I bought it but never touched it since.

The workshop manual does not specify the size of the rubbers.

Just wondering?

Ian:ausflag::ausflag::ausflag:

from the '66 Shop Manuals, Power Brakes used 7/8" diameter.

For what it's worth to you I checked the service manuals from 1962-1967 and they all said 1 inch bore.

1" vs 7/8" on these cars has been confusing for years. Recently I replaced a 7/8" MC with a 1" unit on a 66 Bonneville with power brakes. I could get a new 1" unit off the shelf at a local NAPA and the 7/8" one was going to take a week or more to order. The 1" MC with power brakes gave me a slightly higher pedal with slightly less pedal travel travel. Both the customer and I were satisfied with the results. Which is actually factory correct? Honestly not sure. I think the factory chassis manual for 66 says 7/8" for power brakes. But I would have to verify. You have a 1" unit coming and that's what you are used to. Should feel fine to you.

Side note: The new NAPA MC had a machining error on the MC piston hole where the pushrod installs. I had to remove a small ridge in the piston with a cartridge roll to allow the pushrod to install completely and bottom out in the piston like it is supposed to. The brakes would "self apply", drag and get the drums hot once the fluid got hot. The compensating port was blocked by this condition. You have to check everything!

Couple of pics of the MC ridge in the piston I mentioned in side note above. Pic shows ridge about 1/4" from bottom. Second Pic shows the ridge removed and polished allowing booster pushrod to bottom properly.

Thanks for all the replies. Yes, I was wrong in that I did miss the page in the '66 shop manual which gto4ben correctly stated that it should have a 7/8th bore but I'm GLAD I missed it because I might have agreed with my parts supplier when he said his books say 7/8th".

My new (made in Taiwan!!!) Raybestos master cylinder with a 1" bore has arrived and will have to wait till my booster is rebuilt before I can fit it up.

I just received the latest AMES catalogue and it doesn't actually list a new master cylinder for a 1966 car with Power Brakes? Nor does it specify the bore diameter.
And also it does not specify the bore diameter for the 1966 rebuild kit (PR201DK).

I'll write to AMES and ask some questions in the hope it might get some clarity.

All this is indeed very confusing I agree.

My NEW 1" MC I installed from NAPA was made in Taiwan as well. Probably the same manufacturer. That's the one that had the machining issue in the piston. So pay particular attention to that when fitting it to the booster.

My new MC is in a Raybestos box and is numbered MC36006
I was under the impression that Raybestos is an American company and my MC would be American made but I guess these are the times we are living in.

I’ll take on board the advice to disassemble and check before it’s installed.

But, all this is seriously leading me to the conclusion I should have just spent the extra and had my original sleeved and rebuilt. I may well decide to go that route. The people here who do this work are “old school”, with a strong reputation for highest quality work.

Thanks all.
Ian