2009 Buell 1125R Stator Rewind and Rotor Modification

11 Jan 2015 | Author: | Comments Off on 2009 Buell 1125R Stator Rewind and Rotor Modification
Buell 1125R

2009 Buell 1125R Stator Rewind and Rotor Modification


The 2009 Buell 1125R has problems with overheating stators right out of the factory. The high electrical load from lights and radiator fans coupled with inadequate stator cooling tends to melt the insulation and short the stator windings out over time. The stator is definitely not bathed or sprayed with oil; it gets a tiny dribble from nearby bearings at best.

This means it is cooled mostly by conduction to the case and by air circulation inside the engine. This means that battery charging capability slowly decreases over time until the bike fails to start or run completely. I upgraded the voltage regulator to the FH012AA, but my stator failed anyway because this was still a maximum-current shunt-type regulator. It failed to keep the battery charged while running and charging voltages dropped into the mid-11V range.

With the stator disconnected and the bike running on the battery, I got 18V AC, 18V AC, and 1.5V AC from the three output phases, which confirmed the fried stator. That’s pretty poor performance from a 2-year-old bike with only 4000 miles on the odometer. My first order of business was to upgrade the voltage regulator again, but to the CE-605 SB, which is a minimum-current series-type regulator.

After reading about rewound stator failure and seeing the high price of 2008 or EBR replacement parts, I decided to rewind it myself to have complete control over quality and materials.


Winding Pattern

The three phases are connected together in a delta configuration. When viewed from the engine side, the poles are wound in a clockwise direction and each pole starts and finishes on the engine side. I sanded the enamel off of the wire and measured it with a micrometer, which showed that it was AWG 16 wire.

Each pole had 44 turns of wire, which was about 11 feet of wire when straightened out. So, 150 feet of wire should be plenty to rewind it. The following photo describes my rewind plan.


I have been unable to determine the insulation class or varnish/epoxy type used on the factory stator. My plan is to use 240C polyimide (ML) coated magnet wire in AWG 16 size with heavy build. Thankfully, MWS Wire provided me with a decent quote for more wire than I needed in 16 HML. You cannot get much higher temperature wire without moving to ceramic coated wire, but that requires much larger bend radius.

If the original stator was wound with polyimide-insulated wire. parts of the windings must have spent significant time at or above 270C/518F for it to fail at 4000 miles/hours. It is also possible that the original stator was wound with different insulation or a bad batch of polyimide that would fail at lower temperatures.


Special thermally conductive epoxy, like Duralco 132. has viscosity comparable to or greater than J-B Weld and it is expensive at $86/pint. Electrically resistant epoxy, like Duralco 4461. is good at penetrating windings, but expensive at $90/pint and not thermally conductive. I plan to use J-B Weld epoxy, which can withstand 500F/260C continuous, to hold the windings in place and protect against vibration, but I plan to use as little as possible.

The thermal conductivity of J-B Weld is 0.59 W/(m*C) or 2.36 BTU-in/(hr*ft^2*F), which is similar to water and a bit less than Cotronics Duralco 4460/4461 at 4 BTU-in/(hr*ft^2*f). I bought the J-B INDUSTRO WELD package that works out to $15/10oz or $24/pint. I tested some J-B Weld thinned with acetone, which would penetrate windings better, heated to 500F for 4 hours and it held up really well. It was still strong and hard as a rock afterward, but I did not like the surface microcracks.

The internal color was a lot more uniform than the photo, which over-emphasizes the fracture surface shadows, but I should have probably done a more thorough mixing job.

I tested some J-B Weld heated to 500F for 4 hours and it held up great. The embedded popsicle stick turned to charcoal. The very dark gray surface is due to either surface overheating from the radiant heating elements, surface oxidation at elevated temperature, or bread crumb smoke absorption.

Next I tested how J-B Weld flows when heated instead of thinned with a solvent. After I smeared the room-temperature J-B Weld on the 200F 4-layer 16 AWG test coil, it thinned, flowed over the coil, and dripped off the bottom. It flowed better with heat than with the recommended amount of acetone.

After curing for 24 hours, I cut it open at an angle to see how well it penetrated. It penetrated the first and second layers on the top side with gravity’s help. It barely penetrated the first layer on the bottom side with gravity working against it.

I think the J-B Weld pulled a significant amount of heat out of my tiny test coil, which also limited the penetration. The stator has significantly more thermal mass. If I heat it to 200F and keep my heat gun on it, I should get much better penetration.

I also plan to wipe any excess off for a very thin layer on the outside.

Rotor Modification

Removing the 8-magnet rotor was easy with my impact wrench and no crankshaft locking tool was necessary. I put the bike on my rear stand and put it in sixth gear with the rear tire off the ground. I used my propane torch to heat the rotor nut along all sides for about 20 seconds total.

Then it turns out my DeWALT DW059 cordless 1/2 impact wrench was absolutely perfect for this. It has a maximum torque rating of 300 ft-lbs and it took the nut right off. I’m sure someone will say that this put way too much stress on the transmission, but they would be mistaken because I did not use a breaker bar or torque wrench.

The rear wheel was free-floating and it did not move at all. The impact wrench worked against the inertia of everything (rotor, crankshaft, rods, pistons, transmission, etc) to loosen the nut. I imagine that the inertia of the rotor and crankshaft alone are enough for the impact wrench to reach 300 ft-lbs.

So, I’ll be using the same technique to reinstall the rotor since my impact wrench has a maximum torque rating equal to the revised torque spec. I’ll use Permatex High Temperature Threadlocker Red, which is essentially the same as the EBR-recommended Loctite 272 Red High Temperature Threadlocker. Months later, I helped Jsg4dfan with his stator and rotor; he removed and reinstalled his rotor nut with same impact wrench and with the transmission in neutral.

Neither one of us used the crankshaft locking tool.

EBR’s original fix for the overheating stator problem was a kit that contains a rotor with a precision machined oil jet in addition to a lower power rotor and stator. This oil jet sprays oil on all of the stator poles as the rotor rotates. That’s pretty cool, but I’m not willing to try that with my hand tools or spend that much money right now. One thing I noticed is the total lack of forced air cooling for the stator.

The stator fits tightly in the rotor and there is nothing forcing air in or out of the rotor. My idea, which is sure to be controversial, is to drill six small air cooling holes in the rotor between the magnet insert and the rotor flange. I did not find any oil in this area of the rotor when I removed the case, so this modification will not affect oil distribution. These holes will be small and will not interfere with the magnets, so I feel risk is minimal.

Also, the holes are symmetrical, so balance should not be affected much. As the rotor spins, these six holes will function as a crude impeller / centrifugal air pump and force air from inside the rotor to outside the rotor on the engine side, which must be replaced by air flowing past the stator and into the rotor on the open side. I started by punching the hole location to accurately locate the drill, used an 1/8 drill, and then finished up with a 3/16 drill.

The steel rotor is about 1/4 thick where I drilled the holes.

Almost a year after doing this rotor modification and debating its effectiveness on BadWeB, an interested reader informed me of some overheating stator problems on the BMW F800GS, which has a parallel-twin Rotax motor. The official fix from BMW/Rotax is, get this, an air-cooling rotor design. Hopefully this lends a little credibility to my hair-brained hand-drilled idea.

Notice the same 6 air holes spaced exactly half way between the 6 bolts. The main difference is that I placed my air-cooling holes at the rotor perimeter and BMW/Rotax chose the rotor flange. These flange holes are bigger, but I obviously thought perimeter holes would make better use of centrifugal force. Anyway, the original rotor construction looks remarkably similar to the 1125, which is no surprise.

Buell 1125R

Both this air-cooling design and my hand-drilled one are easier to manufacture than the precision machined oil jet in EBR’s rotor. Very interesting.


I first tried to hold the laminations in my left hand and wind with my right hand, with no gloves. Big mistake. I could not get the wire tight, my hands hurt, and the ridges in the lamination plastic coating made it very difficult to adjust the first layer of windings. So, I sanded the plastic coating around each of the corners. Then I rigged up some clamps and 2x4s to hold the laminations, which allowed me to wind using two gloved hands to pull the wire tight.

I used an old wire brush handle to manipulate the windings and to press the first finished layer flat. The first three layers were all about 13 turns with the fourth layer 5 turns or less; 13*3+5=44. I was happy to complete the four poles of the first phase.

I did one phase per day for three days and finally twisted the proper wires together for the delta configuration. Check each phase for continuity and ground isolation as you go.

Next, I heated the stator in our toaster oven on convection bake, which uses an internal air circulation fan, at 200F for one hour to make sure it was heated completely through. I marked the inside of a bathroom cup with a pencil using tablespoons of water as a guideline. I mixed up two tablespoons (1 oz) of J-B Weld using one tablespoon of resin, one tablespoon of hardener, and a Popsicle stick.

As soon as I felt it was mixed thoroughly, I removed the stator from the oven and set it on a 2.25 hole saw on top of some newspaper. I applied J-B Weld to the engine side first. It had a thick and almost pasty consistency at room temperature (70F), but it thinned, ran, and dripped after a few seconds on the hot stator coils. The drips did not form J-B Weld stalactites on the bottom of the stator, so I know that was a result of small thermal mass in my penetration test earlier.

The larger thermal mass of the stator allowed its temperature to stay elevated for much longer, which allowed the J-B Weld to remain thinner for longer. I did not need my heat gun. I applied two coats on the engine side, but that was probably not necessary. A side effect of the heat was extremely fast cure time.

A few minutes after I finished coating the engine side, it was no longer tacky and I flipped the stator over to coat the cover side. After pulling the delta wires through to the cover side, I applied one coat on the cover side and you can see it is thinner. In most places the engine side and cover side flowed enough to run and meet in between poles, but some windings were exposed in several places.

I was not looking for perfect coverage, just enough to keep things from vibrating and to help conduct heat between layers. With my rotor modification, any exposed copper between poles will function as cooling fins. The side of a pole shown below with exposed windings was the one with the least side coverage; all the rest had better coverage.

Based on my lower-temperature penetration test and the fact that the fourth stator layer was only 5 turns with huge gaps, winding layers 2-4 should be well bonded together at the top and bottom of the poles. The elevated temperature and longer duration may have even penetrated into the first layer, but I’d rather not disassemble it just to find out. After doing both sides of the stator and waiting for it to set up (20 minutes), the J-B Weld in the mixing cup was still usable.

In hindsight, a combination of acetone and heat may have been the best approach for this application with this particular epoxy. That would have thinned the J-B Weld even more for better penetration and it would have lengthened the set-up time at this elevated temperature for better penetration. However, my stator is complete, I’m itching to ride, and I’m done testing for now.

The polyimide coating is incredibly tough. I tried hand sanding the ends of the wire, but that got me nowhere fast. I used my mini torch to heat the ends of the wire until they glowed orange, which charred and destroyed the polyimide. Then I removed the charred remains of the coating with a wire wheel and my drill.

I used self-fusing silicone tape, aka Rescue Tape. aka F4 Tape. for electrical tape. It is good to 500F, a great electrical insulator, chemical resistant, and very tough. In order to try to replicate the original attachment method, I used a strand of wire to wrap each lead to each pair of delta windings.

Then I soldered it, taped it all up, put down a bead of Permatex Ultra Black RTV Silicone, and mounted it back in the ignition cover. Again, check for continuity between phases and isolation from ground.

Hot Idle Testing

I installed the stator. At cold idle, the stator produced 21V AC on each phase. At cold 3000 rpm, the stator produced about 40V AC on each phase. Then I connected it to the new CE-605 SB series rectifier regulator and did some testing.

I idled the bike in various states on my rear stand for about an hour. The voltage never dipped below 12.1 with both cooling fans running and the high beams on; even a slight increase in rpm brought the voltage above 13. The voltage never dipped below 12.6 with both cooling fans running and the high beams off. Here are the results:

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