Hisso Engine update
As I mentioned in a previous post, I didn't want to post pictures every time we flew the airplane.
And I didn't.
But recently we had an issue with our Hispano Suiza engine. I wasn't going to post anything. The Jenny had flown and the website had a beginning and an end. But a good friend of mine suggested that I should post again because the issues we were having with our engine are Jenny related. Most of all, a future Jenny builder may benefit from our findings. He was right. Here goes.
This is how a small leak can turn into a major overhaul.
After several hours in the air, we noticed that oil was leaking out of the two camshaft towers in the back of the engine. Now I know having oil on your shirt after flying a Jenny all day was cool, but it should not have been leaking.
It was leaking so much, we were carrying a rag in the cockpit to wipe the windshield every hour. The tower has a clip (you can see it here) and a gland nut. We removed the clip, added some liquid gasket maker and screwed the gland nut back in place. The problem was that the clip had a 90 degree turn on the end, the point which dug into a hole. This kept the nut from spinning. When we tightened the nut, the old hole was nowhere to be seen. If we loosened the nut and found the hole, the nut was too loose. Hmmm....
After pouring through the Hisso manual, I found that the nut should indeed be tightened and a new hole should be drilled. I didn't like hearing that. I had visions of the drill shavings falling into the tower and clogging up some important oil passage. Another manual gave a better solution. You only drill half way into the tower. That's exactly what we did. The clip was re installed and the leak was fixed.
Lets fly some more.
We noticed another leak developing. This one was located at the crankshaft seal right behind the propeller. It started out as a slow leak, but the more we flew, the more the oil leaked.
That will be an easy fix, right? Just a prop seal change.
Off came the propeller. We removed the gland nut and the clip. Much to my surprise, there was no prop seal !! Did the guy who overhauled the engine forget it? No use crying about it now. Let's install a new seal. There was a race inside the crankcase. We figured a seal should go there. We purchased some graphite water pump packing material from MSC and pushed it in the race.
It was rather tricky because the packing had to fill the race but could not interfere with the gland nut. Next question : what is the proper torque on the gland nut?
The nut pushes against a large ball bearing on the crankshaft. The Hisso manual said you should tighten the nut "until the bearing will spin two revolutions on it's own after spinning by hand". Oh boy. There is no way we could test that. So we tightened the nut until it seemed right.
It was time to run the engine again. The prop was re installed. The primer cups filled with gas. Ignition on. Spin the propeller. Engine fires on the first blade......and oil pours out of the new front seal !!!
Now I'm getting worried. Ron Alexander always said sick engines give you signs. What was going on here?
There were many sleepless nights. More time was spent reading the manual. I learned that a crankshaft seal wasn't necessary. The gland nut was designed with grooves which forced oil back into the crankcase.
I talked to every Hisso expert I knew - Frank Shelling, John Saunders, Paul Daugherty, Mike Damiani and Jack Kearbey. Each one told me something to try. But we could not get the leak to stop.
"Check the breather. It may be blocked," everyone agreed. I pulled the breather. It was free and clear.
We made the decision not to fly the airplane until we figured it out.
Finally, John Saunders gave me the name of the man responsible for maintaining the Hisso engines for The Vintage Aviator Co. in New Zealand. Dave Cretchley offered the following suggestion.
"Do a compression check."
The next day we hooked up the differential compression tester and pumped 80 pounds of pressure into each cylinder. The results were abysmal. Not one cylinder held more than 50 pounds. Three had 30 pounds and one had 10 pounds ! Now we know the problem. The piston rings were bad allowing air to pressurize the case.
Air was rushing out the breather like a hair dryer. We had serious issues.
A decision was made to pull the engine apart. All we had to do was pull the cylinder banks, quickly hone the cylinder walls and put it back together, right?
We took the engine back to the shop and started removing the accessories - mags, water pump, intake tubes etc.
The entire cylinder bank comes off in one piece. We found out a normal wrench would not remove the nuts between the cylinders. They were too close together. The Hisso manual showed a special tool needed for the job. I borrowed one from Kevin Connor. Before long, the cylinder banks were removed.