Pancar Differential maintenance

One of the most important parts on any Pancar is the rear axle with its Differential. A bad running Differential may get your car to a point where it feels undriveable, even without changing anything on its setup. In my opinion the most common errors are scratchy Diffs, Diffs with two tight and two loose spots or Diffs that were runned too tight.
Those who have driven 1/12 scale, World GT or Pro10 for some time will have there own receipt of maintaining their Diff. Maybe this is nothing new for you. I just wanted to give a review on how I maintain my Differential. Therefore I hope it’s a helping guide for those who are new to pancars or it provides a little hint here and there even for professionals.

Identifying Diff errors:

When is a service needed?

too much dirt inside the unit
crashing a bearing
if you ran to tight
if you ran to loose
replacing the spur gear
Saturday night

Scratchy Diff

This can be caused by dirt. Sometimes the dirt get spun out be itself, otherwise a service is needed.
Most common are worn out bearings. You should check all of them and replace some if necessary. In most cars the outer bearing of the Diff-Wheelhub is abused as a thrust bearing. Check this first, as it easily gets damaged by hitting a wall or tighten the Diff to much.
In case your Diff still feels scratchy after a well done maintenance try replacing the Diffballs or the thrust bearing, if you have a separate one.

Two tight spots and to lose spots

Especially new Diff-plates can be bend which is caused by a stamping out process during the production. I always sand my Diff-plates. If new plates are used start with a rough sandpaper like 320 and take your time. Carry on with 400, 600, 800 and finally 1000. It’s important to sand on a plane surface like on a massive windowsill in order to get the discs plane. By holding the discs up to a light you can see the dull, sanded areas as well as the shining, not sanded areas. Sand until the discs are evenly dull and do both sides. During the building of my new Asso Rc12R5.2 this took me 45 minutes for both plates.
A bended Diff-hub, a bended flange on your rear axle or incorrectly fitting D-Rings can also cause the plates not to run exactly parallel to the balls in the spur gear. After building my new Asso I had the feeling that the Diff-hub was a “Monday Production”. Not only I had to run my Diff slightly tighter than my Friend furthermore the left rear wheel seemed to vibrate. I bought a new hub.
An other issue might be a bended aluminum thread at the end of the axle. This causes the nut and cone to apply unevenly pressure against the thrust bearing.

Diff was driven too tight

All of the above mentioned failures will cause the Differential not run smooth and gently while being locked at the same time. First ensure that every single piece works properly and do a soft run-in.
Don’t lock a fresh maintained pancar diff (without thrust bearing) to much, as it will tighten itself during some rounds on the track. Otherwise don’t run it too loose or you will damage the Diff-balls. During the run-in I apply little throttle and just slow down one of the wheels a little. There is no need to keep hold of one of the wheels. This would mean a bigger burden than the car has to endure on the track.

How I maintain my Diff, Step by Step

After disassembling everything I check all the bearings. I rinse them with brake cleaner. While pushing them down a cone (tweezers or brush) I examine if they are scratchy while rotating under some pressure. The axle bearings in the ride high adjusters can get damaged by hitting something on the track. A damaged bearing on one side can cause the car not to go straight down the line. It will feel like it’s tweaked. Furthermore a worn out thrust bearing will cause a scratchy and purely locking Diff. To the undamaged bearings I apply some drops of lightweight bearing oil. Worn off ones get replaced.

Next step is to clean the balls and the spur gear. You want to ensure that there is no piece of dirt anywhere once you apply new lube and reassemble all parts.

With time the balls will work a groove into the plates which is important and needed. This groove enlarges the contact area of the balls on the plates increasing the stiction. This is the reason why a Ball differential can be locked while running smooth at the same time. Due to some crashes during a race the balls can get smashed into the plates, hammering little dulls into the grooves on the plates. Consider that tungsten or silicon nitride balls are even harder than the steel plates. This will cause the diff to snap in like a combination look of a safe. The only way to get rid of it is to sand away the groove and do a new run-in. Furthermore you have to sand the plates if you’re switching to a new spur gear. It can not be assured that the balls will run in the same groove than with the old spur gear.

Normally I use 600, 800 and 1000 sandpaper for this job. As a last step a apply some silicone oil to the 1000 sandpaper to get a dull finish on the plates. There is no need to polish the plates. Some roughness will be ok, because the ceramic balls will work an even groove into the plates during the run-in.

From time to time you should measure the thickness of the plates. My new ones had a thickness of 0,8mm. After sanding them to be plane they were 0,75mm. I think on the Asso 5.2 or 5.1 you can use them down to 0,65mm. If they become too thin the flange of the bearing in the spur gear can collide with the carrier on the axle. Also the flange of the inner bearing of the hub can collide with the bearing in the spur gear. This causes the diff to block while not being locked. I use the same plates for as long as it‘s possible and always sand them to have planar surfaces.

I apply a thin coat of Tamiya Antiwear Grease to the hub and the axle flange in order to make the plates stick to it. A light coat of silicone grease gets applied on the plates. I take some silicone grease and the diff balls in my palm to lube them. After placing them into the spur gear I put a little amount of silicone grease in every ball seat from both sides.

Some AW Grease goes on the other and the inner ring of the bearings. Be sure not to use too much an don’t let it get into the bearing. Because of the AW Grease the bearings will stick onto the axle and the hub. It can be assured that the bearings only run the way they should.

Once the rear axle is assembled it will be enough to tighten it with your fingers, put the rear wheels on and do the run-in. If the left rear wheel is disassembled there is no need to tighten the diff with a nut driver to get it locked.