FMX box clunk
Stupid question time, my FMX box changes seamlessly and drives fine, but putting it into D or R causes it to jerk / clunk.
I changed the oil and filter last month ( it was very clean ad not burnt at all) but i didn't get the 5 litres out i was led to believe would come out, nearer 4 litres.
The question is "what does the oil look like on the dipstick when you check it, does it show a thick oil line because mine just shows a smear about 3/8" above full.
I think the oil is low causing the clunk and want to know if I will damage it by putting another litre in just to see if that cures it ( not driving it up the road, just trying it into and out of D and R)
When I had the sump off I put a drain plug in so it's easy to top up or drain it.
Is that smear of oil film the same on both sides of the dipstick?  If so, it is probably accurate.

The torque converter also has a drain plug.  But then you will need closer to 12 quarts of oil.

Did it clunk before the oil change?
Are the universal joints in good condition?
Is there a lot of wear in the rear axle because that will definitely cause clunking when going from drive to reverse or reverse to drive?

Sadly, the aftermarket never produced much in performance parts for the FMX.  So it was never known for being a performance transmission.  But, believe me, they are tough and can take a lot of abuse.

If the fluid is truly low, even just one quart, the trans will act "wonky" going into and out of gear, etc.  I don't think it will cause an internal clunking noise.  What type of trans fluid did you use?
I'd better explain the "clunking" is a proper jerking and gets worse the hotter the box gets.
I've put a litre of type F in and it seems better, i might drain a small amount out tomorrow and think that will give the correct level on the stick, as for it clunking when it was put into gear it was doing that before the fluid change 
The UJs feel ok but the diff could well be 130,000 mls, cant say that it whines though but I sucked out the old oil ( more of a sludge!) refilled it ran it for 200 miles then sucked it out again and refilled it so if the "sludge " was taking up any slack it's not anymore.
Cannot rule out wear in an older trans.  But, the rear axle is probably an open differential (non Traction Lock) or (non-Posi traction as some say in the Chevy world)  The back lash can add up significantly as they age, ring and pinion back lash, side gear to spider gear back lash, side gear to axle back lash and on and on.

The trans should be filled with Type F fluid.  Might be worth it to do a complete fluid change including draining the torque converter just so you know it is filled with the correct fluid.  Will that help, I don't know?  But for now it's a bit of a guess whether or not it has the correct fluid.  If you do drain the converter, do not restart the motor without first putting 6-7 qts. of fluid in the trans.  Then after starting the motor it's kind of a race against time to quickly fill the trans while running the shifter through the gears so nothing runs dry inside.  Hopefully JTS can chime in and confirm this.  He seems to be more experienced than I am.

For rear axle gear oil, I've had best success with Lucus heavy duty conventional gear oil.  It tends to cling to surfaces and operate quieter.  They also have a conventional oil stabilizer that works great for gear oils and motor oils.  That might quiet any backlash noise in the rear axle.  I use a little of it in the 351W of my daily driver F150 with 150K miles.  Lucus has some great oil products.
After thinking about this. I can't remember many issues, other then you must use Type F only, with the FMX, or Cruise O Matic as it was known, and the Ford O Matic before that. The trans has been in existence since the early 50's, with refinements in 58 to the Cruise O matic, till about 68 when it became the FMX. They are all similar in design, and a lot of parts will interchange. That being said it was a solid, and reliable trans. All transmissions are capable of failure. Severe use, neutral drops, backing up, and slamming it in gear to do a burn out, you name it. With a trans as old as yours you have no idea what someone may have done to it. I would first check the rear end for excessive slop. With the car raised up enough to slide under it, and the rear tires still on the ground. Block the wheels so it doesn't roll over you. Turn the driveshaft in the direction of normal travel. Now turn it backwards, it should go back no more then about an 1/8 of a turn, any more then that, and you have to much slop / wear in the rear end, gears, spider gears, bearings etc. A telltale sign of excessive wear can be a leaking pinion seal. If it drips on the floor then its probably due to excessive wear. That could be the clunk, check the U joints as they will also cause a clunk when going from forward to reverse, and vice versa. I would suspect the trans last. And as I said anything can fail. But it is the most unlikely, as a trans that clunks is broken somewhere inside, and usually won't shift, and drive properly. And I don't remember you saying anything about it acting up. Hope it helps, and you find the cause. Let us know 

 "But it is the most unlikely, as a trans that clunks is broken somewhere inside, and usually won't shift, and drive properly"

What a good point JT  Thumbup
Apart from shifting into gear it is a seamless drive, change, kickdown etc.

" The back lash can add up significantly as they age, ring and pinion back lash, side gear to spider gear back lash, side gear to axle back lash and on and on."

Another good point  Thumbup  always have to remember she is 51 years old and I remember how worn out I felt at 51 ( a long time ago now!)

Off topic - Do I need to refit the PCV pipe or can I just have a breather cap ( no rules for old cars over here, no yearly MOT test etc) as that would let me fit an 1/2" carb spacer ( as suggested and I've found one over here at a reasonable price) without having to tap into the air filter of manifold?
It's not required but it's best to use a PCV system.  It will help evacuate vapors in the crankcase caused by combustion gases seeping passed the piston rings.  It will also help remove moisture from the crankcase and keep the oil cleaner.

Some run two breathers and no PCV when the motor has a very aggressive cam and doesn't develop enough vacuum for a PCV valve to work correctly.

I use a PCV setup on my old cars.

I am guessing you have an OEM style Holley carb instead of an aftermarket universal Holley carb?  That would explain why it doesn't have a vacuum port for a PCV valve.

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