Vintage Dirt Bike Q & A

Got a problem, possibly major one.

Got a flywheel puller to try getting to the stator and points and the flywheel will not budge. Plus it stripped 1/8" of the threads off the bottom of the puller. Tried heating up the flywheel and it didn't seem to help any. I used an impact and bumped it a few times and it just started spinning.

Any way to salvage this? I tried to carefully use a 2 jaw puller after I got the other puller out of the way and it wasn't doing anything either. Is it possible the flywheel is just seized onto the crank?

It's likely either rusted on, wedged on by Woodruff-key deformation - or, like my MR175, both. I didn't destroy my flywheel getting it off, but it wasn't until I accepted the possibility of destruction that I applied sufficient heat & force to remove it.

If I recall correctly, after removing the nut I laid the bike over and kept filling up the crank/flywheel gap with PB Blaster (I don't think it did anything, but it made me feel better). Then I put on the puller and cranked it up tight (it's important to grease the bolt point - I always forget to), heated the flywheel center real hot at first (and nearly cherry red later) and started 'rocking' the flywheel with alternating left/right blows inward, along with direct sideways blows now and then. Occasionally I would apply more heat, and try and crank the puller a bit tighter. One thing I wish I had tried was using a drift punch in the keyway to shift the key slightly, which might have saved me a lot of time.

Normally stuck flywheels pop off with a cracking noise, but I didn't even know mine was loose until I started to see movement from the hammer blows. It turned out that the Woodruff key had "rolled' slightly in the groove and wedged it real good, also allowing water into the taper for some serious rust action. Every MR woodruff key I've seen (5 so far) has been toast, with some of them nearly sheared in half.

If enough heat is applied to the center it should break the rust bond, but only with the puller also yanking it off the taper (I'd use your 2-jaw). You might also have some luck heat-cycling it, using PB Blaster - or, if you're feeling brave, freeze spray - as coolant.

It might also be possible to use a chain vise on the outside of the flywheel or large pin spanner in the slots against the nut to try and spin the flywheel a tiny bit to break its grip, but it would really suck to snap off the flywheel threads.

Ray

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'74 CR125M (175cc), '75 MR175, '82 RM250Z, '08 YZ250F, '14 Zero FX electric, '14 Zero MX electric, '18 Alta MXR electric

Finally got time to try all this out yesterday at my buddy's shop. He's got a garage full of old Yamaha 2 strokes that run really well so I was hoping he had some insight to getting this thing off. He said he's never seen one that stuck ever.

We got the torch and heated it up almost to red, it was really cold yesterday and I think the heat was dissipating too fast maybe. Banged the hell out of the flywheel from all around, used a 2 jaw puller cranked as tight as I could get it with a big wrench, and still nothing. Burned up the stator, which I was going to replace anyway. Pretty sure the seal on that side is toast as well, but I am replacing it also. The only thing I couldn't try because we couldn't find a punch small enough was trying to get at that key way.

I looked down in there and it does look like maybe it's rolled and has it wedged on. I spent about 2 weeks filling the hole with PB Blaster, didn't seem to help either. Still looks rusty down in there. The only thing we could come up was trying to cut the flywheel off enough to get a better grip with a 3 jaw puller and have a better, more direct access to heat up the taper. The flywheel is pretty banged up and I have kind of come to the solution of just putting a PVL on there(if I can get this bloody flywheel off!). I think getting a new flywheel and stator, plus coil since mine is kind of knackered, will probably be about the same cost. I talked to a guy on here that put one on his MR and said he didn't notice a difference in torque getting rid of the big flywheel so why not.

Any other suggestions before I start cutting? I think I have some smaller punches that may get down to the key way, so I'm going to try that first. But if that doesn't work, I breaking out the cutter.

Well, I'm running out of suggestions.

Here's something I found online:

http://www.rustremoval.biz/rust-remover.htm

The 1 qt. 'Shooter' is only $17.50, and might work well. They recommend 10% or 20% solutions, but you might consider using it full strength.

I think if it were my bike, I would treat it full strength with this stuff, lie the bike down on its right side to let it soak in, put on the 2-jaw puller and crank it up tight, and then just leave it for a while, returning now and then to tighten the puller, whack on the key with a punch, or heat it up. You might come back one morning and find the flywheel popped loose. The good thing about a rust-frozen taper is that there's no sliding friction to deal with; it's only the direct bond strength of the rust (there's sliding friction on the key, though, which is why you want to try and move it slightly to break it free if you can).

If you need to get the flywheel off right now, I don't know what to tell you.

Ray

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'74 CR125M (175cc), '75 MR175, '82 RM250Z, '08 YZ250F, '14 Zero FX electric, '14 Zero MX electric, '18 Alta MXR electric

I don't need it off right away. I'll try that. The engine is out of the frame so it's not a big deal to sit in the corner and keep that going with the puller and see what happens. Thanks Ray.

I looked around and couldn't find a punch small enough to fit down in there. Hopefully the rust remover will help some. I may have a little screwdriver that'll fit down there, I haven't checked that yet. May be able to get it in there enough to do something.

Maybe a short finishing nail might work to hit the key with, if the end were ground flat? or maybe leaving it pointed might work better. Or a small flathead screw... Both will probably bend quickly, but could be easily replaced with new ones for a few more smacks.

It might be wishful thinking on my part, but maybe the only thing holding the flywheel on is that key.

One more thought; immobilize the crank (I'd use wood blocks wrapped with rags against the crank throws, inside the crankcase) and try rotating the flywheel a little bit back and forth hoping for a tiny bit of movement around the crank, using either a chain wrench or a turning rod welded onto the flywheel. Combined with heat, the puller, and hitting the key if possible... who knows, it might even work.

Ray

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'74 CR125M (175cc), '75 MR175, '82 RM250Z, '08 YZ250F, '14 Zero FX electric, '14 Zero MX electric, '18 Alta MXR electric

Yeah, that's one thing I was thinking of also. Taking the cylinder off and sticking some aluminum bars under the piston that I use when putting barrels on pistons on my Triumph. Stick those under there so the crank can't move and try to see if that flywheel will move one way or another.

It's not just wishful thinking on your part, but mine also.

Just hope that it's as easy as I hope it will be doing all this other stuff.

note, the crankpin is pressed in the crankhalf, if you turn the flywheel with too much power the pin will turn in the crankhalf.
the lower rod bearing will get damaged too.
you have heaten the flywheel and pulled at the outside of it, so it is damaged and ready for the garbagecan. cut it off before you will damage more of your engine.

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80 CR250Ra
80 CR250Ra
78 CR250R
80 CR80
82 Bultaco Sherpa 340
78 Bultaco Pursang 250 MK12
76 Bultaco Frontera 370 MK10
76 Bultaco Alpina 250
73 Bultaco Lobito 125 MK6
76 Suzuki RM125A
90 Kawasaki KX250

Still working on cutting the flywheel off the taper. Don't trust the crank isn't damaged so if anyone has a crank they'd like to sell me, let me know. After 4+ hours of cutting and cussing, still don't have the thing off completely. Gotta put down the Dremel and angle grinder for a while and go to work, but I guess I'll come back at it in a day or two.

don´t know what you are doing at this 15min cuttingjob?
sounds like you are not very experienced by working with an angle grinder.

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80 CR250Ra
80 CR250Ra
78 CR250R
80 CR80
82 Bultaco Sherpa 340
78 Bultaco Pursang 250 MK12
76 Bultaco Frontera 370 MK10
76 Bultaco Alpina 250
73 Bultaco Lobito 125 MK6
76 Suzuki RM125A
90 Kawasaki KX250

Wow, didn't expect the condescending attitude, but you're partially correct.

The outside part of the flywheel has long been off but after trying at it some more with a puller, penetrating oil, and a few taps with the hammer, it still wouldn't budge. So I started cutting down the flywheel center with my steel cut off wheels. Trying to make a channel where the key way is to try and get to it that way. No, I'm not that experienced with this motor or 2 strokes in general. Plus I've never had to cut a flywheel off. I used what I had. Cut off wheels where the only thing I have that would get into that area. What do you use to cut these off? Magic?

Well, I'm sure you could have rented a plasma cutter and had it off in a few minutes, along with the molten crankshaft end. +1 on taking your time with it, as any sensible person would.

On a more helpful note... I wonder if you might be able to buy or borrow a huge nut splitter from a truck repair place? Since you're cutting your flywheel hub exactly where I would have - along the keyway, the thinnest/weakest spot- the splitter might keep you from having to cut all the way through to the key, or at least allow you to easily wedge open the finished cut rather than having to drive a chisel into it, saving your crank and cases from more pounding. Maybe the case won't allow the splitter access to the hub, though (it might be usable at an angle?).

I looked around for an MR250 crank, but didn't find anything - and I really don't know if a CR250 crank is a drop-in substitute. Here's a link to a thread entitled 'MR250 Crank':

http://www.vintagedirtbikeforums.com/vi ... 895&hilit=

...and here's a post from it - I realize it's over 2 years old, but you never know... seems like he had more than one:

"I may have a MR250 crank if you need one. You may need to use your rod since a couple I have have wasted rods. Also I have gasket sets for the MR250. Jerry at mailto:elsinorestuff@qwest.net."

And just in case the worst happens, you might consider a CR250 motor transplant, if you can find one for a good price. I'm always seeing early CR250 stuff on eBay.

Ray

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'74 CR125M (175cc), '75 MR175, '82 RM250Z, '08 YZ250F, '14 Zero FX electric, '14 Zero MX electric, '18 Alta MXR electric

Thanks Ray. Think I've read that thread before. Was wondering the same thing about the CR crank. Think at the rate I'm going I should have it off in a few more minutes of cutting. I've pulled the stator coils out of the way for a little bit better access so we'll see. Working 14 hrs today so I'll work on it tomorrow and see what happens. Take breaks to check eBay for cranks. Saw one, but it didn't look much better than mine.

The 73-74 cr250m crank will not work nor will the mt250 crank they have smaller bearings, I dont know about the 75-76 cr250 tho.

i got mine off with a 2 prong puller

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Dylan