BearKiller

I am finding conflicting information as to the correct NPT size of the boost-tube fitting that is screwed into the intake manifold.

One source quotes it as being 1/4-NPT and the other source says 1/8-NPT.

Which is it ??


Plans are to replace the rigid tube with flexible air-line and Parker Prestolok fittings.

As best my memory serves me, the factory fitting in the intake is a 90* EL, correct ??

Can/should I likewise get a 90* air-line fitting for that particular spot.

I see most everyone else uses a straight fitting in that hole.

Thanks.

jimbo486

Yes, the fitting in the intake/head is a 90. You could replace it with whatever fitting you'd like.

As for the AFC fitting, I've never heard a definitive answer myself. I don't believe it's NPT because it uses copper washer as a seal. I believe it's a straight and not tapered thread. I've heard of some who simply drill and tap the top to use whatever sized fitting they choose.

I'm doing some work on my truck tomorrow. I'll pull the banjo bolt and see if I can't come up with the size and thread pitch for you.

BearKiller

The banjo-bolt is M8 x 1.0 and can be substituted with 1/16-NPT.

It is the fitting in the intake where I am getting the conflicting information.

Some say it is 1/8-NPT; others say it is 1/4-NPT.

Thanks.

Jim Lane

Would Cummins tap a NPT thread into their engine if they tapped every other fastener using a metric pitch?

I think it is metric.

The pipe tap into the intake manifold is closest to a 1/8 NPT but not quite.

Jim

Hey what are you doing awake?

dzl_damon

I've never heard of a 1/16" npt. I used the elbow coming off the intake plenum. I took the elbow to the parts store and found a compression fitting that fit it. On the other end, I used an adapter for the same compression fitting with a 1/8"NPT Going into the AFC housing. It went in relatively easy for about 2 turns or more. I used a small amount of teflon tape on the threads and gave it another turn with a wrench. The soft brass gave way. I've had no leaks or issues for 2 years and have taken it off many times. I used copper tubing since I had it on hand, nylon would be easier to work with.

JD730

I say its 1/8-NPT, that's what I used on mine when I plumbed my valet switch in.

CaptainChrysler

I cannot explain why, but these motors are metric threads and standard NPT ports. Until you get into the 98+ years, then they are metric oring fittings. (STORM block)

I will second that the ports in the side of the intake are 1/8" NPT. I drilled and tapped the AFC cover to 1/8" as well. I put a straight fitting coming out of the head. The reason being, with a 90* and bumped timing the fitting will not come out without moving or disassembling the pump. So I just stuck a straight one in there.

Tater93

The port in the head is definitely 1/8 npt and I drilled the afc housing out to 1/8 npt. Made it easier to find fittings and of coarse a bigger hole means better airflow which makes the afc more responsive. I also used push lock air line fittings and 1/4 nylon air tubing.

BearKiller

Thanks for the information, especially the bit about why not to use the 90 that I had in mind.

I do have several 1/8-NPT taps and may end up just drilling/tapping the banjo hole.

There is such an animal as 1/16-NPT, but nobody in this little one-light town carries it.


Thanks everyone.

BearKiller

Couldn't sleep; I was watching for that school-bus-sized asteroid gizmo that is headed toward earth.