Curtiss Jenny Restoration

Welcome! We hope you enjoy following the restoration process of a 1918 Curtiss JN4D Jenny. Once completed, the aeroplane will be flown and displayed at the Candler Field Museum in Williamson GA (30 miles south of Atlanta). You can contact me below by clicking on "VIEW MY PROFILE"

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Location: Peachtree City, Georgia, United States

Sunday, November 09, 2014

Hisso Engine update

You know, I kinda stopped posting on this website because the Jenny has flown and I figured it's purpose had been complete.  Remember, this was not a shrine to me, or Ron, or Brian Eberle, but rather a resource for other future Jenny builders.

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.

Luckily, we had purchased two spare cylinder banks (seen above on the left and right).  After taking off the banks, we started to wonder.  How was the rest of the engine?  We had our doubts.  The Jenny was a valuable aeroplane.  We had poured seven and a half years of love into the restoration.  Did we just want to throw this engine back together and go flying again?

I have a builders mentality.  My wife rolls her eyes all the time whenever I look at something and say "I can fix that."  I started off wanting to repair this engine on my own.  But the little guy who sits on your shoulder and whispers in your ear told me to swallow my pride and seek professional help. Do I have a valve seat cutter?  Nope. How about a 4.75 in hone?  I do not.  Can I make a new valve guide?   Maybe, but it would be my first.

I had been watching (with interest) John Gaertner at Blue Swallow Aircraft overhauling two tired OX-5 engines.  These boat anchors left his shop in better than new condition.  I wanted the same for our Hisso.

That's enough for one blog entry.  I'll post another one soon starting with the arrival of our engine at John's shop.  What he found was very interesting.

Till then....enjoy



Blogger Liberschnitzelgruber said...

I'm actually commenting on the description of the engine problem in your most recent post. I guess my shaky understanding of how engines work has been exposed; even with such a careful explanation I'm still not sure what the cause of the leaks was, or why the engine lost compression. Is there an explanation for why the rings failed? Will an engine with only 10 pounds of compression in one cylinder run smoothly?
It's interesting to me how much more difficult it must have been to keep planes in flying condition, 75 years ago, since the engines had none of the technology warning of problems; ignore small leaks at your peril.
Lastly, I'm pretty sure anyone who's happened on this website will agree; you are one incredibly talented and very smart guy. I have such respect for your accomplishment building this Jenny. Chris

2:22 PM  
Blogger Brian Karli said...

Hi Chris,

Nice to meet you. Thanks for the kind words about the project.

A set of good rings would hold pressure in the cylinders. See, when the piston moves down in the cylinder, it sucks a fuel/air mixture into the empty space. Then the cylinder comes back up and compresses everything, making it nice and dense for ignition.

However if the rings are worn out, as the piston comes up the pressure leaks past the sides of the piston and into the crankcase.

The crank case is not normally pressurized. Inside the case, oil is free to flow around the bearings and other movable parts. But when the case is pressurized, the air pushes the oil out through whatever openings it can find. In our case, it was the front nose seal.

Yes, the engine will run smoothly in this condition and would have probably continued to do so forever, but I didn't like the leaky oil seal. It wasn't supposed to leak there and that bothered me.

Thanks for asking and again thanks for the kind words. I'm nobody special - just someone who wanted to build a Jenny.



12:07 PM  

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