Manifold Leak-Testing

Post Reply
Posts: 6913
Joined: Mon Aug 26, 2002 9:09 am
Location: Central Illinois
Has thanked: 100 times
Been thanked: 276 times

Manifold Leak-Testing


Post by Cotten » Thu Oct 27, 2011 7:32 am

Manifold Leak-Testing
Intake manifold leakage is the number one cause of maladies ranging from hard starting to holes in pistons. Shadetree tests to find leaks, such as spraying combustibles around the nuts, are often inconclusive, as well as sometimes risky. Ether should never be used!
The following is a method that shows exactly where leaks exist, as well as an indication of their size.
Inexpensive testplate, available for various manifold configurations

With two Chiefs in for service, I set about testing the manifolds before disassembly. The apparatus I use is a plate cut to fit the manifold as the carburetor does, although a simple cork or stopper with a tube through it would do:

You will note that I use a low pressure regulator and gauge, as no more than 15 psi is normally equivalent to complete vacuum. Usually 10 psi will demonstrate any troublesome leaks.
Testplate and regulator apparatus

A simple squirt bottle filled with diluted dish detergent is all that is needed to douse the manifold nuts and quickly show foam where any leaks might be.

With the intake valves shut and pressure introduced, it soon shows where the leaks are on this '51 Chief: The volume of leak was so great that it blew away the water faster than I could photograph it.

The same procedure on a '46 Chief showed a more typical result from a minor leak: Here you can see how the leak foams continuously on the left. Note also a another small leak on the right. Even this degree of leakage can cause tuning problems.

Another source of leakage that I have found occasionally is a casting flaw below the top bolthole on aluminum manifolds. Here it is found on a manifold that had also been previously fitted with brass sleeves to repair ferrule wear:
Leaktesting of '51 Chief manifold with detergent and water while under 15 psi of air.

This becomes apparent when grinding away the usual distortion from bolt torque, as can be seen as the shadow between the bottom holes. This particular manifold was easily ground to clean up these imperfections, but the others had such a deep flaw at the top hole leading into the bore that I feared the carb lip would bottom out if I removed too much stock. I filled the void with a urethane compound, and hope that a modern thick gasket will do the rest. Ultimately weld repair would be in order. Or a new manifold.

Grinding can easily be accomplished on a common stationary disc grinder, the kind that uses adhesive paper abrasive discs.

The most feared source of leakage is when it occurs around the nipple in the cylinder itself, which is unfortunately common with Sport Scouts. Leakage can even occur around the rivet, although I have found this rare with Indians.
Foaming manifold during leaktest on '46 Chief

These circumstances ultimately require tedious removal of the nipple. It is best to cut and crush the nipple to remove it, as backing it out can destroy the threads in the cylinder. The rivet often has distorted the nipple's threads, and causes this damage. Re-installing a new nipple and rivet is equally demanding, and a high temperature sealant such as "Seal-Lock" is recommended. A cam-ground anvil is used for peening the rivet:

With the rivet inserted from the inside, the anvil is then placed so that rotating it with the wrench will jam the rivet tightly against the nipple, and allow it to be peened without forcing it back into the bore.

The face of intake nipples must be smooth. Any flaws at all will prevent the ferrules from sealing. Often they can be dressed with a flat stone.
Casting flaw at top manifold bolthole

And of course, the manifold and ferrules themselves must be in serviceable condition. Brass ferrules work-harden, and should be annealed if re-used. Gland nuts that have been over-torqued until they are oval, or distorted from chisels and punches often prevent an adequate seal.
Anvil for peening intake nipple rivets

Use no sealers when reassembling the manifold, only grease or anti-seize compound upon the ferrules and threads. Align it with the carburetor on it's support, and center it between cylinders before tightening.

Following these simple steps for an airtight intake assembly will make starting and tuning much easier.
Typically damaged manifold spigots
You do not have the required permissions to view the files attached to this post.

Posts: 2
Joined: Mon Feb 04, 2013 11:44 pm
Bikes: 1957 flh, 1980 fxr 1988 flsftc 1992 flh
Location: Necedah, WI.
Has thanked: 1 time

Re: Manifold Leak-Testing


Post by BPaul » Tue Jun 11, 2013 3:08 pm

Hello Cotten, My name is Brian and i am a member of this site since i bought a 1957motor/58+frame Panhead. First, Thank you for all your information posted here. Second, I am having the hard start issue. but also it seems that after it warms up it dies before i can get out of the shop. then it really doesn't want to start. I am not a good mechanic. My question to you today is, Would you know anybody in central wisconsin that does this particular test and other panhead skills? Thank you again for all your help. Brian.

Posts: 6913
Joined: Mon Aug 26, 2002 9:09 am
Location: Central Illinois
Has thanked: 100 times
Been thanked: 276 times

Re: Manifold Leak-Testing


Post by Cotten » Wed Jun 12, 2013 11:32 am


I am certain there are lots of competent servicepersons in your state, however manifold testing is something that you may wish to do for yourself, as common O-rings are not as long-lived as they used to be. It may only be a season or two before another test is in order.


Post Reply

Return to “Miscellaneous”