This is just a quick tip for those enthusiasts who drive their classic hi-perf muscle cars on the street, taching on a considerable amount of street mileage on them, and who wish to use a PCV valve with a Holley carb which did NOT come equipped with a pressed-in vacuum port for the PCV hose. This is not to imply that the use of a PCV valve is mandatory on street cars, nor that this addition of a PCV vacuum port on the base of your carb is a "Must have" item. It's simply a matter of preference, and this quick tip is simply for those who choose to use a PCV valve. This isn't something that would usually apply to trailered drag cars which mostly get driven only 1320 feet at a time.
Some of the Holley 4150 carburetors, and just about all of the Holley 4500 (Dominator) carbs do not come equipped with the typical pressed-in vacuum port for hooking a PCV valve hose to, as some Holley and other brands of carbs do. This is no great surprise since these carbs are mostly intended for race applications. But with more and more guys driving big horsepower cars on the street, and using big displacement engines for the same, we see and hear of more and more street driven cars with big double pumper and even Dominator Holley carbs under the hoods, than we ever saw in the past.
For those Holleys that do not come equipped with an existing vacuum port, here's a fairly easy and simple modification and installation of a threaded vacuum port that can be used for a PCV hose. The following modification is easiest on Holley 4150 and 4500 carbs which have a boss with a round recess located on the base of the carb (usually on the primary side). Look at the first two pictures below....
what's more IMPORTANT
is what's on the other side of that boss. This is a Holley carb which has one wall of the base, which includes this raised boss, along with a double wall. There's an inner wall located in this area, and the other side of the base, has a straight wall, without a boss, and without that second inner wall inside.
IMPORTANT NOTE: This modification is for the Holley 4150, & 4500 carbs which have this double wall on atleast one of the sides of the base (either the front side, or the rear side). I cannot garantee nor vouch for your results if you perform this modification on Holley carbs which do not have this baflled double wall like the two pictures below clearly show....(it's the side that's in the top of both the pictures here).
In the picture above^ you'll notice the double wall on one side of the carburetor base (in the top of the picture). That is what you need to look for on your Holley carb, and it's at that location that you want to drill the hole needed for the vacuum port for your PCV hose. This recessed hole actc like a pilot hole for your drill bit while drilling the hole.
Below is another view of this (shown with the threaded vacuum port already installed)...
I performed this minor and simple modification on two Holley carbs, and this is one of them. Both had the double wall baffled area on the primary side.
You'll need to use an 11/32" drill bit to drill the hole, and a 1/8"-27 PIPE THREAD
tap. DO NOT use a standard thread tap that isn't for pipe threads.
The threaded port used for this modification has pipe threads which are tapered, and require a tapered thread. If your Holley carburetor has a raised boss on the base like seen in these pictures, it will measure close to a 1/2" in wall thickness at that point. If it doesn't have a raised boss, it still might have the double wall with the baffle inside, so you'll need to remove the carburetor from the intake manifold and take a look uderneath. ( On the base wall without a raised boss, the wall thickness will be about 3/8") (.375").
When drilling the hole for your threaded vacuum port, you need to be careful not to hit that back wall. So I measured out .500" on the end of the drill bit (not including the point) and made a mark with a marker on the bit, and then wrapped some duct tape around the drill bit right at that mark. That serves as a depth indicator, so you know where to stop. You'll obviously feel the drill bit bust through the first wall, but you want to clearly see when you're getting close, so that you're not leaning too hard on the drill motor when the bit is ready to break through the first wall. The base of the carburetor is only aluminum, so drilling is a breeze and it won't take long for your drill bit to bust through the outter wall. I suggest using oil on the drill bit so that it won't bind up in the hole when you break through the outter wall.
Next, you'll want to vacuum out all of the metal chips, and start threading the hole with your tap. I used a Tap handle since it makes it a whole lot easier to start the tap straight in the hole.
IMPORTANT: since this is a PIPE thread tap you're using, you cannot just run the tap through the hole as far as it will go. Doing so will open up the hole too much, and you will NOT be able to tighten up your threaded port. So only go as far as you need to go with the tap. No more. The deeper that you go into the hole with a pipe thread tap, the more it opens up the threads, and the looser the threaded port fitting will be. So measure a half inch (no more) on the end of the tap and mark it with a line and do not go past this mark with the tap into the hole. While tapping the hole, stop BEFORE the line you've made on the tap gets buried in the hole.
Here below is a picture of the finished result.The fitting was nice and snug. I used ARP thread sealant to make sure there won't be any air leaks although it might not even be needed... And you don't need to go cranking down real hard on the threaded port. Since the carburetor base is aluminum, you'll strip the hole out if you put to much muscle into tightening the fitting...(be sure to vacuum out the carb!)....
The threaded vacuum port is a 3/8" size, and is made by Performance Stainless Steel. It's part#1044 and you can purchase it directly from their website www.performancesst.com
or do what I did, and go to