Early 63 rear wheel cylinders with metric threads on bleeder ?

[Tim T.]

I have rear wheel cylinders 5450649 that supposedly can off a very early 63. They were sold as a package with front wheel cylinders and master cylinder. Master cylinder dated H 1 62 (Aug 1 1962). These wheel cylinders appear to have never been rebuild as they have the original Delco Moraine dust boots and piston seals. In rebuilding them I found the bleeder screws to be metric 8 X 1.25 threads and this has been verified by a Corvette restoration expert. This is smaller than the typical 3/8 X 24 bleeder screws so they have not been retapped. Any ideas ?
Tim Tschanz

[Joe L.]

Re: Early 63 rear wheel cylinders with metric threads on bleeder ?

I have rear wheel cylinders 5450649 that supposedly can off a very early 63. They were sold as a package with front wheel cylinders and master cylinder. Master cylinder dated H 1 62 (Aug 1 1962). These wheel cylinders appear to have never been rebuild as they have the original Delco Moraine dust boots and piston seals. In rebuilding them I found the bleeder screws to be metric 8 X 1.25 threads and this has been verified by a Corvette restoration expert. This is smaller than the typical 3/8 X 24 bleeder screws so they have not been retapped. Any ideas ?
Tim Tschanz

Tim------


Are the mounting bolt tappings also metric?

[Tim T.]

Re: Early 63 rear wheel cylinders with metric threads on bleeder ?

Joe,
No they are SAE
Tim

[Edward J.]

Re: Early 63 rear wheel cylinders with metric threads on bleeder ?

Tim , metric was only a thought until about 1978 for Gm and then it was only partly metric not all bolts and nuts were metric. My thought is someone re-tapped the bleeder, or maybe there may have been something different with the cars that went Europe. just food for thought. I was a GM tech for many years back when the switch over came around and remember have to buy all new wrenches to conform to the metric system.

[Tim T.]

Re: Early 63 rear wheel cylinders with metric threads on bleeder ?

Edward,
Typical bleeder screw for this casting is 3/8 X 24 as I stated, which is larger than 8 X 1.25 metric which is about 5/16 SAE.
And no, they do not have a helicoil or other insert in them.
Tim

[Edward J.]

Re: Early 63 rear wheel cylinders with metric threads on bleeder ?

Tim I agree on the bleeder size that most will see. I have no way of knowing why it would be this size. could they just possibly be a service replacement that was still around in the 80's when metric started to come around?? Joe L. could enlighten us. Could GM ship parts overseas to countries that were using the metric system??? I am sure you'll agree its a rare part you have there.

[Joe L.]

Re: Early 63 rear wheel cylinders with metric threads on bleeder ?

Joe,
No they are SAE
Tim

Tim------

I see two possible explanations:

1) Beginning about 1978 some Delco brake parts, both PRODUCTION and SERVICE, began to use metric thread sizes for some threadings. It was also possible for there to be a MIX of SAE and metric on the same component (e.g. a master cylinder, wheel cylinder, caliper, etc.). So, it's possible that the wheel cylinders you have were manufactured in the 1978+ period. The unusual thing would be the casting number. By the 1978+ period I would have expected 8 digit numbers, most likely beginning with "18". But, it is possible that earlier casting numbers were still being used;

2) It's possible that, even back in the 60's, the same castings were used for non-USA destined components which were tapped for metric. However, that seems unlikely in this case if the mounting bolt holes are tapped for SAE. Are you sure the mounting bolts are SAE? As I recall, one of the 10mm thread pitches will accept 3/8-24. In other words, I think you can thread a 3/8-24 bolt into the metric thread and it will seem like it's right.

[Tim T.]

Re: Early 63 rear wheel cylinders with metric threads on bleeder ?

Joe,
The mounting bolt holes are 5/16 X 18 which is similar in diameter to metric 8 X 1.25. However, the metric thread 1.25 is about half way between 5/16 X 18 and 5/16 X 24 and the holes show no sign of cross threading, i.e. definitely 5/16 X 18.
A real mystery.
Tim

[Joe L.]

Re: Early 63 rear wheel cylinders with metric threads on bleeder ?

Joe,
The mounting bolt holes are 5/16 X 18 which is similar in diameter to metric 8 X 1.25. However, the metric thread 1.25 is about half way between 5/16 X 18 and 5/16 X 24 and the holes show no sign of cross threading, i.e. definitely 5/16 X 18.
A real mystery.
Tim

Tim------


Then I'd say that my possible explanation #1 is what the situation is here.

As a matter of fact, calipers for 1980-82 Corvettes used metric bleeder valves while the caliper half bolts and caliper retaining bolts were SAE.

[Bob J.]

Re: Early 63 rear wheel cylinders with metric threads on bleeder ?

I have rear wheel cylinders 5450649 that supposedly can off a very early 63. They were sold as a package with front wheel cylinders and master cylinder. Master cylinder dated H 1 62 (Aug 1 1962).
These wheel cylinders appear to have never been rebuild as they have the original Delco Moraine dust boots and piston seals.
In rebuilding them I found the bleeder screws to be metric 8 X 1.25 threads and this has been verified by a Corvette restoration expert. This is smaller than the typical 3/8 X 24 bleeder screws so they have not been retapped. Any ideas ?
Tim Tschanz

Tim,
please describe or post a pic of the wheel cylinder pistons.
Bob Jorjorian

[Tim T.]

Re: Early 63 rear wheel cylinders with metric threads on bleeder ?

Bob,
I have rebuilt them and they are installed on my 63, unfortunately no photos.
However, the pistons are no different than on an old set of Wagner made in the USA wheel cylinders.
To me there was nothing unusual about the wheel cylinders, I rebuilt them and installed them. But the bleeder screws were so short (7/8 " overall length) I was having difficulty bleeding the brakes. I went looking for longer bleeder screws, that is when I realized they were metric.
I did then remove the mounting screws and found them to be SAE, looked for any sign of insert and rethreaded bleeder also found every thing normal.
Tim

[Bob J.]

Re: Early 63 rear wheel cylinders with metric threads on bleeder ?

Tim,
if the pistons looked like Wagner pistons, they are either later service cylinders or have been rebuilt before.
Later GM service wheel cylinders (and aftermarket) use a plain piston that is die cast.
The later GM/Delco pistons usually have the Delco eye on the outboard side.
Original pistons were aluminum with radial fins.

My guess would be they are later service cylinders since you stated they had Delco boots.
Bob Jorjorian

[Tim T.]

Re: Early 63 rear wheel cylinders with metric threads on bleeder ?

Bob,
They are not aluminum, and do not remember any Delco Eye.
I am now thinking that Joe Luca is correct that they are late service replacements as GM went to metric threads .
Sounds most logical.
Thanks for the reply.
Tim