OX-5 #3

We had a great time at EAA again this year.  Hope all others who attended did as well.  I've been a little bit busy doing annual inspections since getting back, but finally had some time to get more done on this project.  

The first item to accomplish was polish the crankshaft so it could be fitted in the bearings to check clearances.  I've been involving my friend Mark on the OX-5 - he's got lots of oddball engine experience plus, a big enough lathe to spin the long, spindly crankshaft.  I was a little apprehensive on how it would clean up.  It's been sitting since 1929!  Using typical polishing materials and techniques used today: polishing cloths, crocus paper, etc., we were pleasantly surprised how well it came out.  Actually, very surprised - it looks new!

Prior to polishing after sitting 85 years!

All polished up

You'll notice the crank sitting on a block of wood, laying evenly on all the main journal cheeks.  Early Curtiss manuals warn that supporting the crankshaft on the timing gear and thrust bearing will cause the crankshaft to take a set due to its length and the soft material it is machined from.  Prior to me getting it, the crank has been hanging vertically most of its life.

In the crankcase, ready to check fit with the main bearings

Next, it was time to check the main bearing clearances.  The main bearings actually control 3 things in this engine:  the thrust bearing clearance in the crankcase, the timing gear clearance between the crank and camshaft, and the clearances between the crank and main bearings.  This fit-up now, only checked the crank to bearing clearances and thrust bearing to crankcase clearance.  Manuals call for .0015" - .002" clearance between the bearings and shaft.  We didn't anticipate any problems here, as the engine supposedly has about 50 hours on it.  Our Plastigage confirmed this with all main bearing clearances between .0015" and .002" - right were it needs to be!  The outer race of the thrust bearing is also supposed to rotate in the crankcase and this was the case.  In an interesting article written about Parks College in St. Louis in 1928 (they operated a fleet of OX-5 powered aircraft and claimed 700-1000 hours between major overhauls) said that a properly fit crankshaft would spin 3 revolutions if hit with a fist.  Well, I didn't hit it with my fist, but did give it a good spin and easily got 3 revolutions!  Quite remarkable when you think that this was all machined with 100 year-old tools and practices on a very long crankcase!

Main bearing caps and shells

A little about the bearings...both the main and rod bearings are babbitted, insert-type shells. The cam bearings are split aluminum bushings that fit tight into the crankcase, but have the same .0015" - .002" clearance on the cam.  It is very important the cam to bearing clearance is not too loose.  The OX-5 has a rather unconventional oiling system in that the oil first goes thru the cam bearings and cam and then to the crankshaft and connecting rods.  Kind of backwards in todays thinking.  Large clearances in the cam bearings would directly lead to low oil pressure and improper oiling of the main bearings.

We next took a look at the rod bearings and were disappointed to find that most of the babbitt was cracked on the rod side of the bearing.  We suspected maybe excessive clearance was the culprit, but they all checked OK.  Not sure why this happened - possibly the accident, where maybe the broken propeller pounded against the ground for several revolutions - who knows...  The crankshaft, by the way, is straight and I will magneflux it to check for cracks.  So, I have no choice but to re-babbitt the rod bearings and machine them to the proper size.  Not the end of the world.

Also got a start on the cam.  It will be polished the same way as the crank, and looks like it will come out just as well.  We also checked a couple of cam bearings and they show no wear with nice, tight clearances.  So it looks like the rod bearings are all that will require attention on the lower end.  Not too bad for an engine of this age!

Camshaft mounted on lathe for polishing

Back to wing work for a little while.  The lower LH wing is over 50% done and going quite well.  I'm ready to glue on the leading edge and trailing edge ribs.