Heat riser hole in my intake manifold?

[The_Handier_Man1]

From: poppinjohnnies  (Original Message)
Sent: 5/17/2005 11:17 PM

What is this threaded hole for?  It looks like it could cause some flow disruption in the manifold.  I bought a 3/4" pipe plug to screw into it, but it doesn't seem to fit.  Does anyone know what size and thread pitch this is?



From: chip
Sent: 5/18/2005 7:57 AM

it appears to be a straight thread and is not spot-faced [at least on that side] so i doubt that it could be "water tight". could you describe where it is located and what the manifold is from? it also appears to be new cast iron [?] so could it be a part for multiple applications?



From: poppinjohnnies
Sent: 5/18/2005 8:44 AM

I put a photo link on the bottom of the original post.  I couldn't figure out how to make the picture pop up automatically.  The picture explains a lot.  The hole is on the plenum floor on the right side of the intake.  It's directly under the right front venturi of the carb.  If you look down the center of the hole, it too has a floor under it.  It appears to be a core plug hole of some kind for the heat riser passage.  I thought about putting the impact wrench on the plug that I bought and try forcing this plug to fit, but I changed my mind.  I'm afraid that something might go wrong and a tool will end up inside a cylinder or something.



From: poppinjohnnies
Sent: 5/18/2005 8:45 AM

OOps -- I forgot to mention -- This is a 440 in my '78 Chieftain.



From: chip
Sent: 5/18/2005 4:39 PM

ok, let's try this. tell me the largest size twist drill you can slip into the threaded hole.



From: poppinjohnnies
Sent: 5/18/2005 5:22 PM

I don't have a drill bit that comes closeto being big enough to gauge it.  My 3/4 pipe plug looked like it was the perfect size but the thread pitch was just slightly different.  A friend had a 440 eengine in his barn with the carb off.  He said it had a bolt or external hex headed plug installed in the hole.  He said that it might be a tube thread.  I'm hoping that he'll give me the plug out of that engine, but you never know.  If he does, I'll get the thread size and pitch measured at the machine shop and post it in case someone else runs into this problem.



From: SeaRaySRV16O
Sent: 5/19/2005 10:18 AM

Hi,

IMO, if the hole is sealed at the bottom, and there is no air leak, I don't think I would trouble myself with fixin somethin' that ain't broke. On an intake manifold a little disruption to the flow is not a bad thing since it will aid in mixing the air & atomized fuel a little better. In the old days we use to only polish the exhaust side when doin' a port & polish job.

Regards,

Robert Donley
Jenison, MI



From: poppinjohnnies
Sent: 5/19/2005 12:25 PM

The problem is that I don't know where the passage goes.  I have a sneakin' suspicion that I'm losing vacuum through this thing.  I decided to get a 3/4 pipe tap and chase the threads with it.  Then I'll be able to install my plug.  I'll post the results tonight.



From: poppinjohnnies
Sent: 5/20/2005 12:02 AM

Well, here's another reply from me.  The guy who owns the land next to mine said that I could have the plug that's in the manifold of an engine in his barn.  Yippppeee!!! I'll have this off my mind!  I'll check the thread pitch and post it.
My first wife wants me to take her camping with the kids on Memorial Day weekend.  I want to get this thing ship-shape before we go.  hopefully, I'll have an improvement in gas mileage from my exhaust system and other minor improvements.  I'll let y'all know.
-- banker bry parrothead "poppinjohnnies" in SE Kansas --



From: HeavyHaulTrucker
Sent: 5/20/2005 9:21 AM

This hole is for a carburetor heater on some variations of this engine.  It leads down into the bowels of the manifold, but does not open into the engine at all.  It is a cavity that collects heat -- the mechanical choke also operates off of the same heat.

I did manage to plug mine off with a brass plug that I cross threaded into it (cast iron being much harder than brass), then found out that sealing off the cavity was a waste of time.

John



From: poppinjohnnies
Sent: 5/20/2005 10:29 PM

It turns out that the plug is a coarse thread bolt size.  I installed a factory plug that a neighbor gave me.  I didn't notice a huge vacuum increase, but it seemed to make a small difference.  I took the beast out on her maiden voyage (with the new exhaust) tonight.  What a difference!  It's so quiet inside that thing!  It seems to have a little more pep, too.



From: denison
Sent: 5/21/2005 1:13 PM

   I concluded that was merely an access hole, for whatever purpose, and closed off for normal operations, with a flat plug threaded in to fit.  Being open it would just allow some air interchange (back and forth) between the two "plenums" in the dual plenum manifold.   I would not expect much if any difference in performance with it open - its just letting the intake strem from the left primay throat to mix with the intake stream from the right side primary throat.