Osco manifold 1970 straight 6

[itstim]

I have an overheating problem and have narrowed it down to the manifold and I have a couple of questions: the riser has these 3 quarter inch allen head bolts (that snapped off!) that appear to hold the riser onto the manifold...is that correct? riser has these 4 1/2 inch allen head plugs on it...what are they for? Manifold # CY 1867. riser # 1816=7. 1970 gm 250 straight 6.

 

[yoced]

Re: Osco manifold 1970 straight 6

You are correct; those three set screws (1/4-20x5/8) hold the riser onto the manifold. The endplate for the manifold has a machined V notch that the set screws run into, holding the riser into place. <br /><br />

riser has these 4 1/2 inch allen head plugs on it

Other than the three 5/8" long bolts that hold the riser on to the manifold this riser should have no more bolts at all. You will have up to (4) 3/4" pipe plugs in the riser body itself, and an additional 3/4" hole where your water inlet is.<br /><br />

Manifold # CY 1867. riser # 1816=7

Manifold is number CY618. <br />Riser will be number 1816-7xB25 if you have a 3 inch exhaust, or 1816-7xB20 if you have a 2-1/2 inch exhaust.

 

[itstim]

Re: Osco manifold 1970 straight 6

What are the 4 3/4' plugs for? Basically, how does a riser work? Why a riser? What's inside a riser?<br /><br />Thanks!

 

[yoced]

Re: Osco manifold 1970 straight 6

The plugs are just to fill the holes left by the casting and machining processes. Some pieces have expansion plugs to fill those holes, some pieces have various sizes of pipe plugs.<br /><br />The riser is there to keep water from back-tracking into your engine through the exhaust outlet. The higher the rise the harder it is for water to backwash up through the exhaust into your engine. Typical design spec may call for 16" of rise above the waterline.<br /><br />Inside the riser is your exhaust from the manifold and a water jacket which keeps it cool. On your system water from the manifold is piped into the riser, where it mixes with the exhaust stream and goes overboard through your outdrive.

 

[itstim]

Re: Osco manifold 1970 straight 6

Oh...so the discharge water doesn't go through the manifold but goes from the hose on the fore end of the manifold straight to the riser? Also, when looking throught the hole of the riser that connects horizonally to the manifold there appears to be corrosion of what looks like a wall that used to be there. Was there a partition that originally rose up there? I can look through that hole and see the inside of the aft wall of the riser. hope that makes sense! Trying to call Osco but the line is constantly busy.<br /><br />Thanks!

 

[yoced]

Re: Osco manifold 1970 straight 6

No, your water makes two passes through the manifold: down one way and back the other, then out to the riser.

 

[itstim]

Re: Osco manifold 1970 straight 6

Thanks...What about the partition in the body? Is the body of the riser hollow with the exception of the double-wall where the discharge water enters the riser from the fore-end of the manifold or is there a partition wall that prevents water from entering the manifold at the aft end of the manifold?

 

[yoced]

Re: Osco manifold 1970 straight 6

Ok, I understand what you are looking for now. the interior of the unit looks like a large vertical hairpin turn (think turn 2 at Laguna Seca). A single wall of iron is what separates the two legs of the hairpin. It sounds like your exhaust has eaten through that wall. If that is the case then you must replace that riser, as its entire function has disappeared with that wall.

 

[itstim]

Re: Osco manifold 1970 straight 6

Got it! So when that wall goes water can come back into the manifold and into the cylinders...and then " The Dawn of Destruction" will take place in short order! Now I understand risers. Thank you so much for your help! Geez...I'm glad I found this out!