Heat riser and exhaust pipe extension interference

[Michael B.]

I recently discovered that the heat riser on my '69 427 was not opening. Apparently its been like that for years. Upon removing it I discovered that heat riser assembly works fine. The issue is that the plate contacts the exhaust extension and doesn't allow it to open. Looking at the extension, it wasn't perfectly round so I ordered a new one. I heated the exhaust head pipe and dropped the new extension in place, making sure it was fully seated in the pipe. Checking with the new doughnut gasket installed, the extension height was level with the extension. I reassembled everything and guess what? There is still contacts and the valve doesn't open. Referencing the Assembly Manual, the right side exhaust components are not shown as clearly as the left side. I assume that an exhaust extension is to be installed on both sides.

I have two choices. One, grind the plate back 3/16" to resolve the interference issue. Two, leave the extension out, which will provide more room. Along with that thought, I'm wondering if the extension is supposed to even be installed on the right side. I assume it provides some heat protection to the doughnut. Any thoughts? And NO, I am not going to gut the valve or replace it with a spacer. I just need to finesse what the General provided.

[Joe L.]

Re: Heat riser and exhaust pipe extension interference

I recently discovered that the heat riser on my '69 427 was not opening. Apparently its been like that for years. Upon removing it I discovered that heat riser assembly works fine. The issue is that the plate contacts the exhaust extension and doesn't allow it to open. Looking at the extension, it wasn't perfectly round so I ordered a new one. I heated the exhaust head pipe and dropped the new extension in place, making sure it was fully seated in the pipe. Checking with the new doughnut gasket installed, the extension height was level with the extension. I reassembled everything and guess what? There is still contacts and the valve doesn't open. Referencing the Assembly Manual, the right side exhaust components are not shown as clearly as the left side. I assume that an exhaust extension is to be installed on both sides.

I have two choices. One, grind the plate back 3/16" to resolve the interference issue. Two, leave the extension out, which will provide more room. Along with that thought, I'm wondering if the extension is supposed to even be installed on the right side. I assume it provides some heat protection to the doughnut. Any thoughts? And NO, I am not going to gut the valve or replace it with a spacer. I just need to finesse what the General provided.

Michael------


Are your exhaust system forward pipes original GM or otherwise?
Are your extensions GM or otherwise?
Is your heat riser valve original GM and installed in the correct orientation?

[Leif A.]

Re: Heat riser and exhaust pipe extension interference

If your referring to the "donut sleeve" (that you're calling an extension), there isn't supposed to be one on the passenger side...only the heat riser between the exhaust header and the exhaust down pipe. Pictured is the donut sleeve that only goes on the driver's side.

[Michael B.]

Re: Heat riser and exhaust pipe extension interference

Replacement 2 1/2" Aluminized Pipes. Had the OEM extension but just replaced it with one from Corvette Central. OEM Heat Riser 3746844 stamped of the counterweight. Orientation is counterweight forward, only way it will fit without hitting the starter or the frame.

[Michael B.]

[Joe L.]

Re: Heat riser and exhaust pipe extension interference

That is exactly my question. Leaving it out would solve my problem. Like I said the Assembly Manual is unclear. The aftermarket head pipe has a matching diameter to take the extension but if installed interferes with the heat riser operation. If there is a consensus that it the extension sleeve should not be used on the heat riser side, I’ll leave it out. I thought it might be there to provide some temperature protection to the doughnut.

Michael-----

The extension is there to protect the donut and help insure against a leakage of exhaust gas at this point. On a small block, the extensions are used on both sides. However, I believe it is not used on the right side of big blocks. I used it on mine because I used a spacer to replace the heat riser valve.

[Mark E.]

Re: Heat riser and exhaust pipe extension interference

If your referring to the "donut sleeve" (that you're calling an extension), there isn't supposed to be one on the passenger side...only the heat riser between the exhaust header and the exhaust down pipe. Pictured is the donut sleeve that only goes on the driver's side.

Are you sure Leif? I've slept since I replaced the exhaust in my 454 but I think I installed a sleeve on both sides mostly because two sleeves came with the Corvette Central exhaust system. But I also used a spacer instead of a heat riser.

Michael- The good news is your 427 will perform MUCH better when the heat riser opens up.

[Patrick H.]

Re: Heat riser and exhaust pipe extension interference

Michael-----

The extension is there to protect the donut and help insure against a leakage of exhaust gas at this point. On a small block, the extensions are used on both sides. However, I believe it is not used on the right side of big blocks. I used it on mine because I used a spacer to replace the heat riser valve.

I've used them on both sides of small blocks. However, on the 63 it kept the exhaust valve from opening, so I had to grind the extension so the heat riser would work.

I have tried without, and the exhaust donuts degrade quickly.

[Leif A.]

Re: Heat riser and exhaust pipe extension interference

Are you sure Leif? I've slept since I replaced the exhaust in my 454 but I think I installed a sleeve on both sides mostly because two sleeves came with the Corvette Central exhaust system. But I also used a spacer instead of a heat riser.

Michael- The good news is your 427 will perform MUCH better when the heat riser opens up.

Michael,
Only as sure as my memory and it, sometimes, fails me. But, if I'm remembering correctly, my '69 427 (back in 1978) the passenger side did not have the sleeve.

[Joe L.]

Re: Heat riser and exhaust pipe extension interference

I've used them on both sides of small blocks. However, on the 63 it kept the exhaust valve from opening, so I had to grind the extension so the heat riser would work.

I have tried without, and the exhaust donuts degrade quickly.

Patrick------


I use only the sintered iron donut gaskets. This solves all problems associated with these gaskets.

[Patrick H.]

Re: Heat riser and exhaust pipe extension interference

Patrick------


I use only the sintered iron donut gaskets. This solves all problems associated with these gaskets.

I do too, but they can still have issues without the insert.

[Michael B.]

Re: Heat riser and exhaust pipe extension interference

Final results. I reduced the height of the installed extension by about 1/8" on the side near the block. I also removed about 3/16" from the side of the plate that extends down into the extension. During reassembly I realized how critical it is to get that head pipe aligned correctly. With the first pipe joint loose, it is possible to rotate the manifold to head pipe joint inward towards the block and still get a good seal at the doughnut. But doing that will cause the extension to protrude into the path of the heat riser plate. Careful alignment helps with the clearance. By the way, what a difference in acceleration in the upper RPM range.