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Following on from my previous question as to what size adapter I need for my Holley carb, I thought it best to also check what size the original fitting was that is connected to the hard line going to the brakes booster that screws into the base plate at the rear of the Quadrajet.
I think the holley port may be a 3/8 NPT but the qjet is bigger. Still need to confirm the Holley size.
From: At my Bar drinking and wrenching in Lafayette Colorado
Originally Posted by '81 Vette
I thought it best to also check what size the original fitting was that is connected to the hard line going to the brakes booster that screws into the base plate at the rear of the Quadrajet. I think the holley port may be a 3/8 NPT but the qjet is bigger. Still need to confirm the Holley size. Thanks.
The Chevy Q-Jets are tapped for a 1/4" NPT fitting in the rear of the throttle plate. The female threads in the fitting are 5/8"-18 inverted flare for 3/8" tube. Most Holley carbs have a single 3/8" tube nipple coming out of the rear of the throttle plate, but the ones that have threads are 1/8" NPT. No carb ever used a 3/8 NPT port - that would be bigger than the thickness of the throttle plate - a 3/8" NPT thread measures 5/8" in diameter. Do you understand the dimensions of NPT threads..?
Original Q-Jet fitting. The fitting has 1/4" NPT male threads and 5/8-18 female threads:
Available from Earl's under part number 02074ERL
Indeed you did but I was unsure of the female thread size. I'm trying to source something here in the UK and needed that info as well. Thank you, I appreciate your help and input.
Originally Posted by 7T1vette
I think it is 1/4" NPT...but not positive.
Thank you.
Originally Posted by lars
The Chevy Q-Jets are tapped for a 1/4" NPT fitting in the rear of the throttle plate. The female threads in the fitting are 5/8"-18 inverted flare for 3/8" tube. Most Holley carbs have a single 3/8" tube nipple coming out of the rear of the throttle plate, but the ones that have threads are 1/8" NPT. No carb ever used a 3/8 NPT port - that would be bigger than the thickness of the throttle plate - a 3/8" NPT thread measures 5/8" in diameter. Do you understand the dimensions of NPT threads..?
Original Q-Jet fitting. The fitting has 1/4" NPT male threads and 5/8-18 female threads:
Available from Earl's under part number 02074ERL
Lars
Thanks Lars, really appreciate all the tech info. That's exactly what I needed so I can find something here in the UK. Sorry, not sure about the NPT thread sizes. I'm more used to classic British cars.
qjet power brake fitting
