If you are Lowes, grab a 1/2" 20 bolt and try it in the radiator....
The thread count per inch is listed by the manufacturer as 20...thats where the 1/2" 20 comes from in the listing
1/2" diameter
20 TPI
The standard hydraulic flare angle is 37 degrees...Parker fittings make some that are 30/37....mainly for aircraft use....
If you want to do some reading on AN vs standard flare...see....
http://blog.parker.com/an-37-flare-v...the-difference
I checked with a tech guy at Jegs that I know well...
The Earls fitting listed and what you purchased goes into the transmission itself....not the radiator....he told me they should update the text of the part description to reflect that...
He also said that most folk now dont use the radiator cooler at all...they close off the bungs with plugs and run external coolers.....
If the project I was working on was high horsepower, that's the way I would also go...
The current projects are both "stockers"...around 350 / 350...both early 350's...
