PAIR Chrome Pipes and Blocking Plate modifications ( ST1100 ) *

[KoTAOW]

Original article written by Tom Melnik, STOC 346:

PAIR Chrome Pipes and Blocking Plate modifications

While repairing a water leak under the carbs, I also decided to remove the PAIR system.  There are several articles in the AOW section for carb removal, coolant leak repair, hose replacement and PAIR removal.

After removing the PAIR system, I wanted to keep the look of the chrome pipes, but I needed to block off the exhaust ports.  There is no performance increase after removing the PAIR system.  It just reduces any potential for a vacuum leak under the carbs.  In doing this removal, I also found several screws and clips that I had lost over the years that had fallen under the rubber mat into the "black hole".

I'm too cheap to buy the blocking plates at $27.95 for a set of four at Intuitive Race Products.  I decided to use 1/8 inch aluminum for the plates because it's easy to work with and I already had some on hand in the garage.  Aluminum melts at 1200 degrees F.  Let's hope my exhaust temps don't get that high.

Here are some pictures I took during the manufacturing process.  I had already repaired the water leak(s) and re-installed the carbs and fuel tank.  I removed the lower plastic so that I could easily reach in and install the blocking plates and re-tighten the 10 mm bolts.

Right side chrome pipes cut and vacuum plugs installed.  
The pipes were cut about an inch from the existing bracket between the pipes.


Left side chrome pipes cut and vacuum plugs installed.
Pipes also cut about an inch from the existing bracket.


Close up view of left side chrome pipes showing vacuum plugs.


3/8 inch vacuum plugs. $1.85 for three at Advance Auto.


I'm hoping the rubber vacuum plugs are far enough away from the exhaust that they don't melt.  I used the rubber plugs just to keep the inside of the pipes free of road grime and fried bug guts. ( yum )

1/8 inch by 3/4 inch by 4 foot aluminum stock for blocking plates.
Only need about 8 inches to make 4 plates. About $5.00 at Lowe's


3/4 inch just wide enough for blocking plate. Use old gasket or make a template and trace onto aluminum.


Use hacksaw to cut off blocking plate(s).


I rounded off the corners with a grinder and hand file.

Use gasket or template and mark holes. Use uni-bit and drill to 5/16 inches for 10 mm bolt.
This leaves plenty of slop for easy bolt alignment.


I finally deburred the edges and holes and polished the flat surface with Scotchbrite.©

Permatex, Ultra Copper gasket maker. Good up to 700 degrees F. $6.50 a tube at Advance Auto.
If you can salvage the old gaskets, it's cheaper to just use them.


Left side PAIR tubes with blocking plates installed.  One can barely see the plates.  
Should be able to pass any "show n shine" inspection.


HTH.

~~~

Additional comments by Norm:

My story is similar in the wish to keep the stock look by retaining the pair pipes. I used a slightly different approach which may be easier for someone.

Rather than cutting aluminium plates to place behind the pair manifolds, some heavy exhaust gasket material was used. This is easily available from automotive parts houses although the easiest source usually an exhaust and muffler shop which will likely give one enough small pieces to make the gaskets. This material can be cut by heavy scissors and hole made by a gasket punch, step drill, or reamer.

To close the ends of the pipes, I simply cut the pipes where they would terminate out of sight and folded the ends over. IMO there is little advantage to closing the ends because the gaskets are unlikely to ever fail but wished to ensure that a stream of exhaust gases could not be directed through the pipes if the gasket failed. Using the vacuum caps would likely serve the same purpose because the dead space would prevent exhaust gas heat from contacting the caps.

One concern regarding the use of that type of vacuum cap is that they are very prone to becoming brittle and cracking which is a common failure for carbureted vehicles. If it is desired to use a rubber closure, I recommend use of a short piece of hose with a solid metal plug in the end of the hose.

One symptom which was reduced after removing the PAIR system was a reduction in exhaust popping on deceleration