MTFCA Forum

I ran across something I have never encountered. A standard 1/2" course thread bolt is 13 TPI, however today there are special 1/2" taps and dies that are 12 and 14 TPI. I have an old tap and die set that have the special threads 12 and 14 TPI. The set in the day appears to may have been a standard thread they are also 1/32" over size. When chasing threads it would be very easy to damage a 13 TPI bolt with an old 12 TPI die, they go on fairly easily for a short distance. The mismatched threads could be very easily mistaken for just damaged threads that need chasing.

While I am not sure about after 1928, Chevrolet used the 13-1/2 bolts for the head from 1928 back. Yes that is something that can trip one up not knowing the difference.

1/2-12 is British Standard Whitworth thread, the first standarization of threads in 1841. "Until then, the only standardization was what little had been done by individual people and companies". There is also a 12 tpi standard (12-UN and 12-UNR Series constant pitch threads) in several sizes starting with 9/16.
White motor company and others used a lot of odd threads throughout their vehicles. Whether it was for some special reason like strength or maximum thread area or just so they would be selling replacement bolts I do not know. Many of the odd size ones thread into aluminum alloy.

Just for fun, here is a portion of the special taps available from one company. (2009 prices)

Rich Eagle wrote: ↑

Mon Aug 31, 2020 11:37 am

1/2-12 is British Standard Whitworth thread, the first standarization of threads in 1841.

The 1/2-12 is both BSW and UNC. The BIG difference is the degree/angle of the thread. The British 1/2-12 is a 55 degree/angle thread and the U.S. 1/2-12 is a 60 degree/angle thread (mostly pre-WWI, but seen into the 1920's). The 1/2-12 is seen on a lot of antique machinery, my old Lodge and Shipley lathe is full of them.

Joseph, in Australia 1/2" UNC is 13 TPI, while Whitworth is 12 TPI. On sizes other than 1/2" the TPI is the same for each. I note that your 1/2" UNC 12 TPI may be confined to older machinery.
The angle of the threads is noted. However, in practice, it makes little difference, as nuts and bolts are easily mixed in general use. Henry's body hardware such as running board bolts and firewall bolts was 5/16" coarse thread coach bolts. Hardware shop bolts in Australia are 5/16 Whitworth and their nuts require a 1/4" Whitworth wrench, which is larger than the 1/2" wrench used on T bolts of the same size. To be the correct T size, I have to buy separate nuts of 5/16" UNC. Other than the size of the bolt heads and nuts, they are interchangeable, except when you get to the 1/2" ones.
Allan from down under.

From memory the cylinder head bolts on the Curved Dash Oldsmobile was 1/2”-12. These were designed in 1900-01 period and I had assumed (Ass U Me) that the now standard 1/2”-13 had not yet be established.
Correct me if I’m wrong but it does seems odd that Ransom Olds would choose an English thread when building a car in Michigan USA.

I have a set of early dies made by the A.B. Jardine Co. in Hespeler Ontario now Cambridge. The Company was founded in 1870. There is a story in the VINTAGE MACHINERY site about the company and the family Jardine was from Perthshire Scotland. Perhaps he was influenced by Whitworth and therefore designed the 1/2" 12 TPI die. A secondary note all the Jardine Dies are 1/32 over size ie 1/2 +1/32, 9/16 + 1/32 etc. The company failed just after WW2.

Hi All

I believe if you do some checking, what we now considered UNF and UNC standard US threads were the result of the Society of Automotive Engineers development of standardized thread sizes, focusing on one coarse and one fine for bolt diameter. If you find one of the bolt thread charts from before then (I have one buried somewhere) it shows several "standard" thread per inch for each of the more common bolt sizes.

1/2 -12 60 deg thread was common around Detroit in the early auto years. Leyland Falconer used this all over the single cylinder Cadillac engines that they built. And when the same car has 1/2 -13 on the Weston Mott axle, you have to watch what nuts and bolts go where to avoid an unpleasant experience.

Leyland Falconer also built single cylinder Oldsmobile engines along with the Dodge Brothers.

I am quite happy that the taps and dies are still readily available when needed.

Drive Safe
Jeff
Nova Scotia

My 1912 KisselKar manifold studs are 1/2-12 thread (both ends).

Broke one earlier this summer and a good friend carved me two new ones and a couple nuts from 4140 steel.