I have tried several steps to correct my Bernina 160's bottom thread tension; too loose. I did occur after I cleaned out bobbin case & hook race,

Can be several causes. In typical probable order for rotary hook machines;
1.Top thread tension is too high (lower tension)
2.Top thread snagging at spool or elsewhere (for hoizontal spool use larger spool cap and/or machine not threaded per specific instructions)
3.Bent needle or barbed needle point (replace needle)
4.Needle not properly inserted into holder (see specific machine instructions)
5.Debris in shuttle hook/race (clean bobbin, hook, race)
6.Wrong bobbin (replace with proper machine bobbin)
7.Bobbin thread tension too high (reduce bobbin spring pressure, likewise top thread tension)
8.Poor quality or extremely light thread (if using cotton only thread, test sew with a polyester blend and see if problem goes away)
9.Bobbin case (replace bobbin case)
10.Machine timing. (check/adjust timing)
11.Burr in hook race, this can happen if a broken needle point jams in the race (remove burr or replace race)
12.Machine timing. (adjust timing)
13.Hook is worn or damaged (typically not a worn hook unless a few hundred hours of use, look for a barbed or bent hook point) Watch following Singer video... see testing top thread tension.
https://www.youtube.com/watch?v=O7N4XaIV6ms
and following animation on generic rotary hook sewing https://www.youtube.com/watch?v=zqRvljnNLFk

Is it the bobbin tension that is very loose? Try tightening it. Also check that the bobbin has been installed in the bobbin case, turning the correct direction and that the bobbin tension is threaded properly.

Bobbin Tension How and Why Tension is Adjusted

Understanding Bobbin Tension

Understanding Thread Tension Threads

If it is the upper tension that is loose and creating "thread nests" under the fabric, the machine is either misthreaded or possibly the thread is not seated completely in the tension disk.

Remove the upper thread from the machine.
ALWAYS RAISE the presser foot and rethread the upper thread from the beginning--verify the thread path is correct.
Set the upper tension to the midway point between high & low number (the factory standard--although it may need tweaking later for your particular machine).

Retest your machine. Hopefully, the issue has been resolved. If not, it may need service.

Tutorials Urban Threads Unique and Awesome Embroidery Designs

What causes Bird Nests Superior Threads

Sewing Machine Thread Bunching Up Here Why

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Try these suggestions
1. Change the needle--use a needle that's for the fabric being sewed
2. Rethread the machine--top --make sure to use every thread guide
3. try a different bobbin--make sure bobbin is inserted correctly into the bobbin case
& make sure the bobbin is turned the correct way in the bobbin case
4. make sure the thread has not come off the thread take up lever
5. Clean under the needle plate & in the bobbin area for lint or threads
6. Check to make sure the bobbin is not warped or damaged
7. Make sure the bobbin case is correctly in the machine,, if it's a top loading bobbin you may need to have the bobbin case checked to see if it's damaged or see if the hook and hook race has a problem
8. Make sure the top thread tension is on the correct setting for the fabric being sewed.
9.Also could be a timing problem, and the machine would need to be checked.

Try these suggestions
1. Change the needle--use a needle that's for the fabric being sewed
2. Rethread the machine--top --make sure to use every thread guide
3. try a different bobbin--make sure bobbin is inserted correctly into the bobbin case
& make sure the bobbin is turned the correct way in the bobbin case
4. make sure the thread has not come off the thread take up lever
5. Clean under the needle plate & in the bobbin area for lint or threads
6. Check to make sure the bobbin is not warped or damaged
7. Make sure the bobbin case is correctly in the machine,, if it's a top loading bobbin you may need to have the bobbin case checked to see if it's damaged or see if the hook and hook race has a problem
8. Make sure the top thread tension is on the correct setting for the fabric being sewed.
9.Also could be a timing problem, and the machine would need to be checked.

"Nesting or Rat's Nesting" on the bottom side of Bernina sewing machines (CB hook type) is generally the result of very loose top tension. If the problem is found in newer sewing machines such as machines made after 1986 (serial numbers beginning with 26xx xxxx), may have an older Bernina bobbin case in the machine. The problem with the older bobbin cases is that the rounded projection of the latch on the left side of the casing will catch thread, preventing easy thread escapement during the take-up lever cycling resulting in loops on the bottom side and or thread breakage, etc..
Note:....Timings and modified latch geometry's were applied to later models to allow for good thread escapement. Older Bernina cases cannot be used on newer machines but newer cases can be used on all CB hook Bernina machines.
Additionally, Chinese copies of the Bernina/Cerliani OEM bobbin are rampant and can cause sewing issues. Always ask for OEM Bernina cases.

Note the following trouble shooting logic:
1. The center tension disk that is visible through the thread slot in the top cover of most machines must be positioned exactly in the middle of the top cover slot. By loosening the fixing screws on the top tension mechanism, one can adjust the disk position to center.
2. It would be a very rare situation for the top tension to change whatsoever from the (red center marking) factory setting. The most likely cause of loose top tension under the normal center mark position, would be accumulated lint, thread, or a sticky substance between the disks. This would prevent the tension compression disks from applying an even tension on the sewing yarn (thread). Often this can be blown out with compressed air when the presser foot lever is lifted in the up position. In some cases, soft drinks and other sticky or corrosive liquids can find their way between the disks. In this case, the tension assembly must be removed, disassembled, and the disks cleaned and polished.
3. Check the facial surfaces of the bobbin case for corrosion, and sticky substances that prevents the thread from easily sliding over the bobbin case.
4. Check that the spindle of the hook body is not bent or warped, causing the bobbin case to wobble, thus creating an intermittent pressure on the hook race cover at the moment that the thread is trying to escape over the bobbin case finger. (Replace the hook body)
5. Inspect the undercut on the hook body spindle for thread wrappings and knots. This condition can cause the bobbin case to press hard against the hook race cover, hence hindered thread escapement .
6. Exceedingly tight bottom tension can cause some problems as well. ( If you don't know how to adjust Bernina bobbin cases to factory settings, please ask).
7. Hook timing, (loop lift) can be out enough to affect thread escapement. This is typical of newer machines made after 1986 since Bernina no longer made fixed timed machines like the earlier Swiss made models. The later made models have timing components that can slip under stressed machine mechanics.
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Are you sure you are getting the top thread between the tension discs? You should be able to tell the difference when you lower the pressed foot.

Are you threading the bobbin case correctly - the bobbin should rotate clockwise when pulling the thread out under the tension spring. Are you ensuring that the bobbin case is firmly clicked back into position when replacing it. Check your needle is OK. Did you remove the hook from the hook race when cleaning out - have you oiled the hook race ?

I would take it to a Bernina repairman, nice machine and well worth getting it serviced by a qualified technician.

I presume that you've tried the obvious things like a new needle and checking the bobbin case is in correctly.

There should be a small screw on the side of your bobbin case adjust it little by little until you get the correct tension.

I'm assuming the thread is looping on the bottom. If this is not the case, ignore the rest of this.
Perform a tension test. Tension assembly test. To do this test. Pull the thread through the tension assembly and test it to make sure the tension is working correctly.
(To do this test, adjust your tension setting to normal or medium or 5 or something in the middle. Raise the presser foot and pull the thread through the tension. It should pull easily!! Lower the presser foot and pull the thread. It should pull noticeably harder.)
If it doesn’t work this way then you probably have a piece of thread or lint stuck inside the tension discs. To remove it, turn the tension to 0 zero and raise the presser foot.Using a small screwdriver, open the space between the discs and spray with canned or compressed air. The stuff should come out. Do the tension test again to make sure you got everything and then try sewing again.
sewman7