Exhaust port oil leaks in the Thor engines

by Had Robinson

In general, it is a good idea to carefully identify the source and type of leak.  Do not proceed unless you know EXACTLY where the leak is coming from.  Generally, the area of the suspected leak must be scrubbed 100% clean, removing every trace of oil.  The oil can be unburned (light color) or burned (black color).

The Top 80 redrive leak page has helpful info on identifying the source of oil leaks.  If you have a Top 80, go to the Top 80 exhaust port page for instructions on how to correctly install the relevant parts.

For other engines, the following may help.

Exhaust port leaks on most engines are commonly a result of a defective gasket, improper torque of the flange nuts, and/or a lack of sealant.  Perform these steps and you will greatly reduce the leaks.

Supplies needed:

•  new exhaust gasket if the old one is not in perfect condition
•  tube of gray RTV

You cannot stop leaks unless you replace a defective gasket and/or repair the surface of a damaged cylinder port.  Badly damaged cylinders must be replaced.  Glopping on more RTV will not repair a defective gasket e.g. cracked or a deformed cylinder exhaust port.

1. Remove the muffler.

2. Remove the exhaust flange springs, the flange nuts, washers, and the flange.  Clean all parts with brake cleaner or equivalent and blow dry with air.

The studs and the inside surfaces of the nuts must be absolutely free of all traces of oil. Check to see that the studs are FIRMLY installed into the cylinder.  Use the two flange nuts (locked together on the stud) to ensure the studs are tight in the cylinder.  If there is any question, remove and thoroughly clean them and the threaded holes with brake cleaner.  Use air to remove the brake cleaner (and the dissolved oils) from the studs and holes.  Use a full drop of red threadlock on the studs and reinstall them to the proper torque.  2-3 Nm should be plenty for 5mm studs because it is the threadlock that will keep them from loosening, not the torque.  For larger studs e.g. 8mm, the torque value can be doubled.

3. Carefully clean the cylinder port.  Stuff tissue in the port so that cleaner and junk does not enter the cylinder.  If the mating surface of the port is not perfectly flush, it will leak.  With extreme caution, use a single edged razor blade to remove any deposits.  It is very easy to gouge the aluminum port if care is not used.  Once all material is removed, use brake cleaner and air to ensure the surface is clean.  The tissue will prevent the cleaner and debris from entering the engine.  Once all the parts and the port are clean, proceed.

4. Smear a light coating of gray RTV on both sides of the exhaust flange gasket with your fingers.

5. Immediately place the gasket on the studs, the flange, and the washers.  Finger tighten the nuts until the RTV oozes out between the pieces.  Wait an hour.  Remove the nuts and place a drop of blue threadlock on each of the studs at the washers.  Install the nuts and tighten them equally in stages to the correct torque (10 Nm or what is given in your engine's specifications).

Wait 5 minutes and RETIGHTEN the nuts to the correct torque.  Wait another 5 minutes and retighten again.

6. Reinstall the muffler system and the springs.  a. Install the springs first.  Use paraglider line to stretch the springs for replacement/removal.  Do not use Vise-Grips or pliers to stretch the springs because this will nick the tempered surface of the spring and weaken it.  Here is a video on how to properly do it.  Use PG line (best) or aircraft wire to secure the springs if they break.  b. Install the button-head screw underneath the carburetor FIRST and then Install the button-head screw underneath the springs.  Use blue threadlock on screws and tighten 3-4 Nm (just snug, let the threadlock do its job).

7. Wait at least 6 hours (overnight is best) before running the engine.  You must give the RTV time to thoroughly cure.

8. Start the engine and run it for 10 or 15 minutes.

If you did these steps correctly and waited for things to cure, you should be good to go.  Most exhaust systems have a flexible joint at the cylinder which will ALWAYS leak a little.  DO NOT CONFUSE LEAKS OF THE FLEXIBLE JOINT WITH LEAKS AT THE FLANGE/CYLINDER JOINT.