Governor adapter jam nut elbow

GilbertRV · #1 11-08-2014, 06:38 PM

I can't seem to get the threads on the jam nut elbow (lycoming 74070) started in the governor adapter oil hole. It sounds crazy but he hole seems to be smaller than the fitting. Is it okay to use a standard MS20823-6 fitting on the governor adapter instead of the jam nut fitting?

Thanks,
Greg

GilbertRV · #2 11-09-2014, 09:11 AM

I felt really dumb posting this on here but I just can't get the elbow started. I think I've figured out why with this note in the conversion guide

I think my adapter oil port is 5/16 instead of 3/8. What does this mean? It sounds like I'm supposed to change to 3/8's but then it says to use -5 fittings?

The MS20823-6 seems to fit okay, since it is a tapered fitting it snugs up pretty good. Can I go this route or do I need to find another adapter?

rzbill · #3 11-09-2014, 12:11 PM

Need to use the fittings specified by Lycoming.

The "jam nut" fittings are "O"-ring boss fittings that have a straight thread. Do not screw a tapered NPT thread into a hole designed for an O ring boss fitting, it will damage the threads in the female hole.

az_gila · #4 11-09-2014, 01:10 PM

The Parts Manual for your engine says that the front crankcase fitting is a 1/4 tapered pipe thread to 3/8 flare 90 degree elbow, and the rear governor fitting is a 9/16-18 straight thread to 3/8 flare 45 degree angle elbow fitting.

Front one should be MS20822-6 and the rear fitting a 74070 Lycoming part.

I believe the 5/16 reference in your picture clip is for older engines that used a 5/16 steel tube instead of the later 3/8 tubing. The SI simply says to upgrade the older tubing to the new larger size. It does not reference the threads.

If you want to check the governor thread, go to ACE hardware and buy a 9/16 fine thread bolt (9/16-18) and use it as a test. As always, don't force anything...

GilbertRV · #5 11-09-2014, 01:19 PM

Thanks for the reply. I purchased the governor adapter second hand, my engine came with the cover plate. I believe it's off a 320, which I thought was ok since everything I could find showed the same adapter part number, 75153. I believe it must be off a really old 320 since the oil port is definitely 5/16 and not 3/8. I contacting the seller to see if I can change it out for a different one.

az_gila · #6 11-09-2014, 01:40 PM

Quote:

Originally Posted by GilbertRV

Thanks for the reply. I purchased the governor adapter second hand, my engine came with the cover plate. I believe it's off a 320, which I thought was ok since everything I could find showed the same adapter part number, 75153. I believe it must be off a really old 320 since the oil port is definitely 5/16 and not 3/8. I contacting the seller to see if I can change it out for a different one.

We are talking about the threads into the governor, not the tube end.

The choices should be 9/16-18 straight or 1/4 tapered pipe thread.

The 5/16 is the tube size of the oil line.

GilbertRV · #7 11-09-2014, 02:34 PM

I've probably got my terms messed up. I am referring to the governor end. The 74070 will not fit. I believe the female thread ID for a 9/16-18 should be .51, my female thread ID is .447 which would relate to a 1/2-20 with a 5/16 tube. I've checked it every way I can think of. I feel like I'm nuts because I can't find any reference to anything other than the 74070 elbow. I don't know how to make this work so I really appreciate the input.

,

I tried to get a picture of the caliper in the hole but the caliper has to be upside down and I'm not coordinated enough to get a good picture.

rzbill · #8 11-09-2014, 04:42 PM

The hole for an O-ring boss (ORB) fitting will have a slight counterbore on the face. That is for the O-ring to seat against. Look at the holes on your mechanical fuel pump as an example. Should be a perfectly flat surface for the O-ring.

I do not see that counterbore in the image you posted.

I am no historical expert on these. Maybe its time to do a couple of things. One, prove that it is an NPT thread and two, determine if these have ever been made with an NPT thread.

az_gila · #9 11-09-2014, 05:02 PM

These are the approx. pipe thread dimensions and the choice would only be 1/8 or 1/4. Your dimension does not sound like a pipe thread.

If you want to allow for the NPT taper -

The taper rate is 1 in 16 inch

GilbertRV · #10 11-09-2014, 06:16 PM

Thanks for all the feedback. I ran to Ace and picked up 1/2-20 and a 9/16-18 bolts. The 1/2-20 fits like a glove and the 9/16 is too big, like the elbow. I don't know what this governor adapter is but it's either really old or not standard or both.

The surface does have a slight counterbore for the o-ring to seat. I think I'd be fine if I could find a 1/2-20 ORB to a -6 tubing but that seems like a non-standard size and google hasn't turned up one yet.

Edit: I did order "45° Elbow, 3/8" JIC x 1/2" ORB Male" from Hydraulics Direct, although not for aviation it seems to have the correct specs.