O-320 Engine Overhaul


Page 1 - The Cub Project
Page 2 - Fabric
Page 3 - Firewall Forward
Page 4 - Firewall Forward 2
Page 6 - Final Assembly (2010)
Page 7 - Final Assembly (2011 page 2)
Page 8 - Final Assembly (2011 page 3)
Page 9 - Completed Aircraft
Page 10 - Later Updates and Modifications

Thanksgiving  (November) 2010 -  I bought an O-320-E2G from a wrecked Grumman Cheatah.  The engine
has 1855 SNEW, but that was spread out over 35 years.  All of the cylinders have been replaced at various
times, which tells me this engine didn't get to fly a lot and likely suffered some corrosion issues.


1/17/2011 - The hangar is starting to look like an overhaul shop.  The near engine is the O-235-L2C that I
was going to use on the Cub.  The next engine is the O-200 that I just finished that is destined for a Pietenpol.  
The far engine in the middle of the hangar is the O-320 I'm about to tear down to overhaul for the Cub.


The O-320 as received from Fletch Air.


O-320 with accessories removed.


Oil Pan removed.  Hmm.  Isn't there supposed to be a screen over this inlet?


Oh, there's the suction tube screen laying on the bottom of the sump.


Accessory case removed.


Accessory case.


Disassembled cylinder parts.  The end of the connecting rod on the left front that is ash colored is from #3
cylinder, which has apparently been running extremely hot.


Cylinders.  You'll note the broken exhaust stud on the right front cylinder.  Also note the aluminum color of the
heads, all except for the back on on the left.  That's #3 cylinder which again appear to have been very hot.  The
cylinder bore was showing a blue tint to it, which also indicates exposure to extreme heat.


First look at the crank looks good.


Uh oh.  That is a corroded cam follower for the front pair of intakes.  Both cam followers for #1 and #2
cylinders were showing some corrosion and the shared cam lobe between them was spauled.  # 1 and #2
intakes were lacking about 3/16" of lift.  The cam is junk and at least two of the cam followers are beyond help.

My analysis following the engine tear down today.  #3 cylinder looks like it has been running extremely hot.  
The steel in the barrel is showing some heat discoloration, the wrist pin is showing some heat gauling, and even
the piston end of the connecting rod looks like it has been so hot that the oil on it was getting scorched.  I am
concerned about what kind of condition the head will be in as that whole cylinder assembly has clearly been
badly overheated.  Additionally, there is some cam spauling, so the cam and some of the followers are junk.  

However, what I have seen of the crank so far looks good.  This engine is very much exactly what I expected
to see, so I am satisfied that it's a good buy.  For now, the case, the crank, and the cylinders are in the hands
of my good friend and machinist Doug to be miked and closely examined so we can decide exactly what needs
to be done to put this engine back into new condition.

February 4, 2011 update - After measuring all the parts, the crank is on the cusp of meeting spec, so may or may
not end up being ground.  It has some service bulletins due, so will be sent out for work regardless.  There is also
some minor corrosion on the aft side of the prop flange that will be ground out and the flange cad plated.  The
cylinders are all well beyond spec for std, so will have to be bored.  The exhaust valves are worn out.  The cam is
junk.  Two cam followers are junk and 6 can be reground.  This is pretty much normal wear for a high time O-320
that spent a lot of time sitting.  The work up looks like it's going to run $7500 - $8000 to overhaul this engine.  

February 10, 2011 - Still no pictures to show, but progress is being made.  The Crankshaft is now in the hands of
Aircraft Specialties.  Bad news on the case as the bearing saddles didn't pass muster.  They were beat out .002 - .003"
beyond spec.  Divco has the case so they can mill the halves and line bore the crank bearing saddles and the cam
bearings.  Add another $800 to the estimate, so now it's looking like $8300 - $8800 for the engine.

February 23, 2011 - Aircraft Specialties will grind the crank .003 undersized.  Brown Cylinder has the cylinders
for boring, install new studs and replace the valve guides.  Divco has the case for machining.  The 6 repairable cam
followers have been reground and the rods magnafluxed and sized.  I expect to have all parts back in about 3 weeks,
so hope to be putting the engine back together in mid to late March and hang it on the plane in early April.

March 11, 2011 - Still nothing to show for an engine, but there has been progress.  Most of the bottom end parts
are sitting at Doug's shop in Santa Fe.  The case has been welded, milled and line boared by Divco, then finished
with an acid bath and alodyne treatment.  The crank was found to be cracked, so the new/used one is here.  Polished
at std, and epoxy coated in the inner bore per SB 505.  Brown cylinder is going to trade me a set of std nitrided
160 hp cylinders in exchange for the cost of reworking my old 150 hp cylinders.  For those interested in doing the
150 to 160 hp conversion,  the cylinders, pistons, wrist pins and rings are all different.  Make sure you get the right
parts.   I would get started assembling the engine this weekend, but am currently tied up with the real job that provides
the funding for this project.  In another week or so, you'll start seeing engine assembly photos on this page.


April 2, 2011 -  We finally have all the bottom end parts.  Doug came up and spent the day helping me get
the bottom end of the engine put back together.


Case halves.  I love that alodined finish.


Case halves mated together with the new crank and new cam, then set on the stand.


Accessory case about ready to install.  Sharp eyed readers will note that I have the fuel pump push
rod installed upside down.  Yes, I did make that mistake, and yes, it did get corrected.


Doug installing the through studs.


Rods are installed, and accessory case is installed.


I fabricated covers for the vacuum pump and fuel pump mounting pads since I won't need either.  The tach
transducer doesn't clear the side of the new Casper Labs angle oil filter adapter.  Doug will mill away some
of the oil filter adapter and we'll see if we can't get it all jammed in there.  

Note: I ended up buying a 1 1/2" oil filter adapter extension through Vans to get it all fitted correctly.  I could
have just as easily bought a 90 degree adapter for the transducer.  Either way, the cost was $70.  This problem
got a bit mroe complex when I went to mount the engine.  The extension moved the oil filter too far so it didn't
clear the firewall.  I removed the oil filter extension and ordered a 90 degree drive for the tach.  It was HUGE.
No way was it going to fit.  I ended up sending the tach back to UMA in exchange for an identical tach that
uses a different pulse count and got a transducer that screwed into the access hole on a magneto, so it counts
pulses as the magnet in the magneto passes by.  It works really well and is absolutely accurate.


Bottom view of the engine minus the oil pan.


April 9, 2011 - Oil pan installed.


Case masked off for painting.


Painted intake and valve cover drain tube.


Painted valve covers, dipstick, intake flanges, and mag clamps.


April 15, 2011 - Still no cylinders, but I was notified that they shipped today, so maybe Doug will have them
ready for me to paint by next weekend.  I shot the case black today.




April 17, 2011 - With no cylinders in hand, instead I installed the accessories.  Today it was magnetos, dip
stick, alternator, starter, carburetor, and air box.  That's all I can do until I get the cylinders, probably or
Friday or Saturday.

Update: There is a 1 1/2" extension installed in the oil filter adapter to make room for the tach drive transducer.  
It had to be removed as it put the oil temp tranducer into the firewall.  Now the tach transducer won't fit.  UMA
is sending me a transducer that screws into the Slick Mag vent hole, but will also require swapping the tach for
one that is programmed for a different pulse count.


April 22, 2011 - I've finally got the cylinders in hand. Doug did a nice job with the valves and seats, so everything
is ready to go once they are painted to match the engine.  I've painted the steel sleeves yellow to match the plane,
then have them masked off in preparation for shooting the black.


My paint drying set up.  Two heaters blowing across the cylinders.  It was enough heat for me to cure the
yellow cylinders, then shoot the heads black in one day.



April 23, 2011 - The paint is cured after sitting in front of the heaters overnight, so I'm moving them to the
other hanger for assembly. 



OK, I was a bit remiss about taking assembly photos.  It's hard to do when you hands are always covered
with either assembly lube or gasket sealer.  Here we are hanging the completed engine on the airplane.



The hoist crew.  They also helped me roll the hoist up from the other hangar with the engine hanging under it.


The engine overhaul is now completed, so this is the end of this web page.  Future updates will be on the
completed plane page.


Page 1 - The Cub Project
Page 2 - Fabric
Page 3 - Firewall Forward
Page 4 - Firewall Forward 2
Page 6 - Final Assembly (2010)
Page 7 - Final Assembly (2011 page 2)
Page 8 - Final Assembly (2011 page 3)
Page 9 - Completed Aircraft
Page 10 - Later Updates and Modifications