Trouble Opening GMAC and seized motor

[Sally Vacuum]

Hello All...

Facing two challenges with a GMAC 10T motor here, and one cannot be addressed without first addressing the other. The main issue is that the motor is seized. Been searching the forums for info on this, but ... The secondary issue is that the inner jamnut is not coming off. Details on both below. If anyone has any feedback on either issue, it would be much appreciated.

18 Apr '26 Edit: See my post #25 for the explanation as to the cause of the seize.

The secondary:
The inner jamnut seems to be, no pun intended, jammed up against the bearings. The freehub still spins freely. The only tool that we can fit into the recess of the freehub is a pair of needle nose pliers. Unfortunately, we cannot grip the jamnut strongly enough to loosen it. The pliers just slip away. Thought about a universal socket, but in the only one I've looked at so far (Red Dog), the steel pins don't retract enough--it won't fit over the axle to get down to the jamnut.

Grin said that they used to use a homemade tool they made from a steel pipe slightly flattened in a vice to an ID of 15mm to fit the jamnut, a hole drilled in the other end of the pipe and a rod inserted through the hole to use as a handle. Nowadays, they use a custom tool they machined (or had forged...they didn't specify).

We live nomadically on bikes, in a tent in the forest, so we can't exactly make the steel pipe tool. Maybe, but there are some hurdles, none of which we can't handle. Just wondering if anyone had any other ideas for getting the motor opened up.

Reason for needing to open the motor, long story short (kinda):
The motor is seized. It will turn unloaded with throttle power, but trying to spin the wheel by hand or walking the bike: the wheel turns with great difficulty or just skids, respectively. When throttled unloaded, it makes a clackety sound reminiscent of vacuuming up cat littler, minus the "vroom" sound of the vacuum.

18 Apr '26 Edit: Forgot to mention--just before the seize, there was a squealing noise as of the sound of rubber and also a loud snap.

The seize came about when regen braking was enabled from around 8mph, maybe less. Bike was being ridden acoustic with CA turned on--no motor power. The motor has a history of the brakes not working, ever since it was new. With the tripwire engaged, the animation plays on the CA, but the motor's braking would cut out for a few seconds, with the animation still playing. The issue was intermittent and became more frequent and worse over time until a couple weeks ago when...complete seizure, skid to a stop.

Have some ideas what's going on, but need to open the motor to address it. Would love nothing more than to fix it and be on the way, but....can't get that jamnut off.

Other Details of the Build and Usage, if needed:
- pretty standard/simple build (?), didn't change many factory settings on the CA (just power limits, speed limits, lowered the Up rate, made some (also low powered) presets, set up the MF switch)
- baserunner
-under 2,000 miles
- under 2 years old
- mostly ridden without motor power with CA on to track stats, and generally only use 100-200W, except when going up steep grades fully loaded (around 275lbs including rider and weight of bike)
- use proportional regen regularly when able so as to conserve wear & tear on solar panel (we're 100% solar powered)
- rode for a year without torque sensor, installed torque sensor about 6 months before the seize incident

 

[offGridDownUnder]

We live nomadically on bikes, in a tent in the forest, so we can't exactly make the steel pipe tool.

I have some needle-nose locking pliers. I do not know they can fit, but it might be worth trying if available.

I have seen other needle-nose locking pliers in tool shops.

Another trick I'd try - given the rounded body of the jam-nut, a socket that will fit over the nut will not contact the flats - this would be a 17mm or larger socket. But what if you put two flat shims into the socket with some short-term adhesive? You would be narrowing two opposing flats inside the socket to the ~15mm span.

Or, take two strips of steel cut to a bit less then the width of one of the inner hex faces in the socket, but long enough to cover the depth of the inner hex face and also then bend outwards over the socket and along the outside to provide a way to hold these shims as you slide it onto the jam-nut. You would be bending these shims into J-shapes, with the short length inside the socket against the two opposing faces and the long lengths flat along the outer length of the socket to provide handles to keep them in place until inserted onto the jam-nut.

This would require a deep socket - I don't know the depth required to fit over the spindle. You might also find a socket that is bored through to pass over the spindle and which you could then apply a wrench or locking pliers to the outside of. With strong locking pliers, I'd expect this to grip even on sockets without any external knurling or hex surface.

I can try to work up a drawing if these ideas aren't clear.

 

[offGridDownUnder]

We live nomadically on bikes, in a tent in the forest

After you've solved you problem, I'd like to hear how you charge. The general idea of living very low footprint interests me - it's applicable even just for bicycle camping.

 

[Sally Vacuum]

The inner jam nut should not be that tight once you remove the outer nut.

I agree...it shouldn't be, but indeed it is. It's kinda smashed up against the bearings. It was also installed incorrectly, with the beveled side facing the bearings. Thanks for the link to the video--have already watched it a few times prior to posting.

Grin has parts.

We purchased a spare planet when we initially bought this kit.

Thanks for the tips about building the tool and what specifically to look for at the hardware shop. Not sure about the WD-40 though. Long story short, I have never once had it work in a scenario like this. Not that I mind the suggestion--just sharing my experience.

 

[Sally Vacuum]

Thanks for the ideas there. I think I can visualize it just fine. We've got a long 17mm socket for the axle nuts, so this may work out! Might try zip ties for shims...hey, why not? (We solve plenty of problems with zip ties).

After you've solved you problem, I'd like to hear how you charge. The general idea of living very low footprint interests me - it's applicable even just for bicycle camping.

We've got two solar panels, a 36W and a 30W, each with a DC port. It's pretty simple: we bought st3 extension cables with the kits (we got matchy matchy GMACs) and chopped off the non st3 end then crimped on andersons. I believe they started selling st3 to Anderson cables right after we did this. (It was good experience though).

The simplest way is to then get a DC to Anderson cable (got ours from ebay). This goes from the panel to the MPPT step-up controller (we got the Elejoy from Grin). Then the st3 goes from the MPPT to the battery.

The 30W panel came with a DC to MC4 cable, so what we did was get Grin's MC4 to Anderson cable which then goes into the MPPT.

The nice thing about these latter two cables is that it enables us to use both panels on one battery which we do seldom. To do this we also got an MC4 to Anderson cable (at the same time from the same seller on ebay) and hook it up in series. It's a bit of a nest of cables to do this, but I'll try to make it make sense:

36W panel --> DC to Anderson --> Anderson to MC4
30W panel --> DC to MC4 --> conjoined MC4s to Anderson --> MPPT

We don't often use both panels on one battery at once, but we can if we need to.

We get anywhere from 1.5 to <3V from a full day out in the sun, depending on which panel is used. To preserve the longevity of the battery, we typically charge them only to 49.4V (give or take). We also rarely let the battery level drop past 46V from throttling or pedal assist.

 

[Sally Vacuum]

The WD is only used to flush debris - because it's safe on plastic & pressurized w/handy nozzle- no other reason. CRC works too, but avoid anything that attacks the plastic planets or bearing seals.

I understand, and it's certainly an option, but the outer jamnut nut came off easily, as did the washer (obviously), and the axle is clean as a whistle. We'll see about shimming up our socket and go from there. It's just extremely tightened down...and backward.

 

[offGridDownUnder]

We'll see about shimming up our socket and go from there

I don't have my GMAC here (stored in a friend's garage), but memory and your photo suggest a 17mm or 18mm spark plug socket might fit over the cylinder of the jam-nut, and then the metal from a food can, maybe doubled by folding, could shim the difference. You need shears to cut the can into strips.

If you can fit a spark plug socket over the cylinder and the axle some of them come with hex flats so you can turn the socket without a socket handle.

Don't forget the other tools while you're shopping.

 

[offGridDownUnder]

A 10mm axle is slightly larger than a 3/8", so a 3/8" drive socket might slide over the axle. A 1/2" drive socket will almost certainly clear the axle. Have this in mind when you are choosing sockets, particularly if you can't find a socket deep enough to fit over the protruding axle.

Can you take the GMAC with you when you are shopping?

The closer the fit over the cylindrical part of the jam-nut, the better (less wobble), but this is a one-off, so just folding the shims over several times to make up the difference is worth a try if you find only a larger one.

 

[offGridDownUnder]

do you have a hammer?

Yeah, that solves everything...

 

[offGridDownUnder]

The alumin(i)um drink can for shim is likely strong enough, and you can cut that with the scissors on a Swiss Army Knife.

 

[offGridDownUnder]

And your point is?

Just lightening the mood.

 

[offGridDownUnder]

we got the Elejoy from Grin

I bought one of these:
Sun200

It's not needed with your set up, in fact you could probably up-size your panels with your set up. It also uses the Elejoy, but starts with a 36V panel.

I set it to charge to 52.4V with my 48V battery, to reach ~85% charge.

A note: you may already know this, but many BMS systems will not balance the cells in a battery unless the charge goes to 100% - the small differences are only apparent in the final fractions of a volt. For this reason I do charge to 100% once or twice a month to maintain battery health.

 

[Sally Vacuum]

but memory and your photo suggest

To what photo are you referring? I didn't post a photo...

 

[Sally Vacuum]

Plan 'B'... (assuming cassette still installed)

Tools.. 3/16" punch - small hammer (Full disclosure: I have used a 'disposable' screwdriver, but strongly discourage such tactics

No, the cassette is not still on. We have several bike tools proper (edit: because we do all our own bike work).

In spite of your disclaimers on the hammer punching technique, this seems like a great way to damage threads (edit to add: and nut and bearings), and I would only resort to it as a very last option if the nut was seized, and I was desperate and had no other options, but maybe that's just me. To each their own!

In this case, however, the jamnut is not seized. Rather it is overtightened, per my next post.

edited to add link to next post

 

[Sally Vacuum]

Through careful observation and contemplation, the mystery of the seize has been solved.

I mentioned earlier that the inner jamnut had been installed backward--with the beveled side facing the bearings. I'm not sure whether that part of the motors is assembled in China by MAC or in Canada by Grin, but this is the first time this motor has been attempted to be opened by the end user.

Inspection of the inner jamnut and bearings shows clearly that the jamnut has rotationally cut into the rubber seal of the cartridge such that the jamnut is embedded into the seal by at least one millimeter, likely more. This has warped the seal and exposed the race and balls inside. One ball even appears to be missing from its home in the race.

I forgot to mention this in my original post, but just before the seize, there was a squealing noise as of the sound of rubber and also a loud snap.

"So this inner jamnut has a beveled cutout on the inside, and it's really important that...that bevel is not facing the ball bearing or it risks actually crushing the seal of the ball bearing..." --Justin Lemire-Elmore (GMAC Hub Motor Disassembly Tutorial)

The inner jamnut being installed backward allowed it to contact the seal and be dragged along rotationally as the bearings spun around the axle. Thus, the jamnut gradually tightened more and more until it was pulled far enough into the bearings so as to cause damage to the bearings and then motor failure. Had it been installed properly, with the flat side facing the bearings, the flat side would have rested on the metal lip at the inner-diameter of the bearings and not caused this damage.

Edit to add: Although this explains the squeal, the snap, and the seize, I'm not convinced this goes so far as to explain the regen braking malfunctions.


Second edit to add:
Furthermore, the axle nylock nuts were not overtightened so as to cause this. If this were the case, the jamnuts and washer between them, rather than being tightened rotationally, would have been forced laterally along the axle by the bike frame's dropout. There would be damage to the threads of axle, jamnuts inner and outer all. As it is, the axle threads are in excellent like-new condition, and the outer jamnut came off normally and by hand.

In the matter of the seize, Grin has been, from the start, blaming damaged wiring and possible shorts and continues to blame damaged wiring when it is clearly not the wiring, even having seen a photo and admitting that bearing damage would cause a motor seize, saying nothing of the damaged bearings in the emailed photo.

If both sides of the jamnut were flat on both sides, I could see how orientation is irrelevant, but this jamnut has been pulled rotationally into the bearing seal causing it to crush the bearings, exactly as the video describes.

 

[offGridDownUnder]

Have you been able to disassemble it now?
Do you have replacement bearings?

Given the contradicting assertions of what the issue is, please let us know what you find as you proceed.

 

[Sally Vacuum]

I respectfully disagree.

There is no unrestricted lateral path for the dropouts/nuts to successfully compress the jam nuts into the outer bearing (specifically, the bearing you claim is damaged). Why?

I'm saying that tightening the axle nuts would *not* have done this. So I think we agree on that point.

Have you been able to disassemble it now?
Do you have replacement bearings?

Given the contradicting assertions of what the issue is, please let us know what you find as you proceed.

The 17mm socket doesn't fit over the jamnut (just checked it against Papa's pictured 18mm measurement upthread to be sure before buying a new socket). The socket I got is not deep enough to fit over the axle with the ratchet still in it, and I'd rather have the longer one. Long story short, the half-inch drive 6-point sockets are two bays down at the big-box hardware store, and I forgot about that until I got home and realized the socket I got (a 3/8” drive + adapter) is too short, even though it is still a deep socket. I can go back and exchange it no problem.

We don't have replacement bearings, but they are likely obtained easily enough by matching the part number.

The motor of discussion is actually my boyfriend's motor.

I took it upon myself to register for the ES forums partly because I just felt like it. We are DIYers and skilled improvisers, but electric hubs are a brand new field for us. We'd love nothing more than to repair the motor and be both rolling again, in spite of numerous electrical issues (some of them serious safety concerns) that I haven't mentioned in this thread.

My boyfriend has been the one exchanging emails with Grin, so he is more intimately familiar with the details of the written exchanges. He decided to write up a summary of these, and so I am copy/pasting it here:

Your suggestions for makeshift tools have been great and I believe we could unscrew the jamnut and take apart the rest of the motor with one of these methods.

Initially Grin suggested that cable damage causing a phase short was at issue. I sent them a photo of the cable outside the motor showing that there was no exterior phase short. At this time I also sent them a photo of the jamnut and damaged bearing cartridge.

In their reply they made no mention of the damaged bearing cartridge and, instead, suggested that the problem was caused by a phase short in a part of the cable that can not be seen, even if the motor were disassembled.

I had to do my own research on this, but the internet suggested that the symptoms of the issue, motor seizure followed by significantly increased resistance when turning the motor accompanied with a ticking noise as the motor turns, are all telltale signs of bearing damage.

When I told Grin of these symptoms they made no mention of bearing damage and continued to suggest that there was an invisible phase short. When I informed them that the motor turns when the throttle is pressed and asked if that would be possible if there were a phase short I was told "no", a phase short would prevent the throttle from activating the motor.

They said that it must be that an invisible phase short caused the seizure and the fact that the throttle turns the motor now is likely that the phase short was alleviated when I fiddled with the cable when unplugging it. That is, the invisible part of the cable must have been jostled when I unplugged it and the phase short was massaged away.

Despite the fact that this does not explain why there is significant resistance and a ticking noise when turning the wheel, no mention was made that bearing damage could be the case.

Finally, I mentioned the bearing cartridge damage clearly visible in the photo I sent them and asked if this could have caused the issue. I was told yes, this is likely the cause. And they seemed to suggest that the jamnut was forced into the bearing cartridge by overtightening the axle locknuts. I believe this is unlikely because, as Papa stated, there is no unrestricted lateral path along the axle for the jamnut to simply have been forced into the cartridge by overtightening the locknuts.

Not without stripping the threads, which are in like new condition. Not to mention the outer jamnut was easily removed, unscrewing it by hand. Which I had already mentioned to them.

It appears as though they are trying hard to find any reason that this is not a warranty issue and I'm concerned if I remove the inner jamnut with these makeshift solutions, further damage could be caused to the bearing cartridge at which point Grin will say that I caused the damage to the bearings when removing the jamnut.

 

[offGridDownUnder]

It appears as though they are trying hard to find any reason that this is not a warranty issue

I am sad to hear this. I hope it works out.

I went back through some of my own correspondence with Grin, and I found this information for the GMAC:

Bearings:

Cassette side 69020 (inner), 6901D (outer)
Brake side 68040

Do correlate this with the bearings you have, perhaps something has changed.

 

[Papa]

So help me out here Sally... please.
I'm not interested in your communication(s) with Grin's CS, especially not warranties - that's between you and them. My ONLY interest is helping you and your significant other achieve roadability, that's it.

...the outer jamnut was easily removed, unscrewing it by hand.

Be specific... Was any tool(s) used prior to hand/fingers on the OUTER jamnut? In other words, I only want to know how tight, initially, the OUTER jam nut was prior to using your fingers? Again, please be specific.

I'm concerned if I remove the inner jamnut with these makeshift solutions...

Is the INNER jam nut still installed?... or did you successfully remove it? If the inner jam nut is still installed, lets try a quick tool-less experiment.

1.) Lay the wheel on the ground, table or chair or whatever is convenient... with the torque arm positioned down, and the freehub positioned skyward.

2.) You or your BF reach under the wheel and firmly grab the torque arm while the other slowly rotates the wheel COUNTER-CLOCKWISE (as in reverse), while observing the lower jam nut. Hopefully this will effectively 'unwind" the jam nut. If not, use your needle nose pliers to help coax the jam nut to rotate the same CC direction while rotating the wheel. Do not allow the torque arm to rotate.

 

[Jack Klompus]

So help me out here Sally... please.
I'm not interested in your communication(s) with Grin's CS, especially not warranties.

How dare you speak to myself or my girlfriend like that. I am asking you not to post in this thread again. Do not reply to me, do not reply to Sally Vacuum, do not reply to anyone. You are banned.

Now, forums do not generally allow thread posters to ban people from their thread. So, as a gesture of good will, I am asking you to honor this ban.

P.S. There is no need for me to open this motor. If I keep using it I will eventually need to buy replacement parts from Grin and they are not getting one more cent from me.

 

[OlderGiantFan]

I'll leave this one for the mods to sort out. Glad I'm not a mod

 

[Apartunis]

here is no need for me to open this motor. If I keep using it I will eventually need to buy replacement parts from Grin and they are not getting one more cent from me.

No need to solicit help here then. Enjoy the ride.

 

[OlderGiantFan]

Looks like Greta has left the chat?