Rear vacuum port size for Qjet
 

I am putting a '77 Pontiac Qjet on a '68 GP. The casting number is 17057262. I need to find a vacuum fitting that screws into the baseplate (for the power brake booster, hideaway headlights, etc). The fitting I ordered is too small. Does anyone know what size fitting works? I see some on ebay but they are expensive. I'm hoping I can find something else that will work.

Thanks in advance!

I am fairly certain the base plate hole you are referring to is 1/8"-27 NPT. Ames sells that fitting with several configurations containing branches for other vacuum accessories that may need manifold vacuum from that location. HVAC feed to the dash control and vacuum advance come to mind.

Confirm, yes it’s 1/8” pipe.

Some of the baseplate vacuum fittings are 1/4 NPT as well as 1/8 NPT:

https://forums.maxperformanceinc.com...d.php?t=866614

This particular carb, and all 1975-79 Pontiac M4M Quadrajets, has 1/4" NPS threads for a 1/4" NPT fitting.

1974 and earlier Pontiac 4M Quadrajets has 1/8" NPS threads for 1/8" NPT fittings.

FWIW

You guys are all awesome! Thanks for the replies. I am looking forward to getting a QJet back on this car. The PO had a Holley on there. The next step is to address the ignition "upgrades".