lorollinls

Member
Nov 22, 2006
26
0
what is the thread size on a 96 honda cr250 crankshaft..... i picked up a flywheel weight used and the threads are wrong :pissed: ....so im gonna see if i can tap out the flywheel weight to make it work i dont know if they are just boogered up on one end or what but gonna try it nevertheless. So if anyone knows that thread size please help me out! :bang:
 
Last edited:

76GMC1500

Uhhh...
Oct 19, 2006
2,142
1
What brand of weight is it? Are you absolutely sure it's the correct one for a 96 CR250? There should be no need to make any modifications for it to fit.
 

lorollinls

Member
Nov 22, 2006
26
0
steahly, and no im not absolutely sure its the correct one for my bike, but that was what he advertised it as in the **** auction.....i know it shouldnt require anything, but it looks like im gonna have to do somehtin
 

lorollinls

Member
Nov 22, 2006
26
0
that is def an option, but im tryin to figure out how to make it work first by maybe tapping out the threads first and see if that helps, i think the threads may be just damaged alittle, not sure, i was just wondering if anyone else had any ideas like this one...

thanks
 

oilspot

Member
Sep 11, 2006
156
0
Measure the outside diameter of the shaft on the threads with a dial caliper or micrometer. That will tell you the size. Then you have to figure out the thread pitch. Then use a pitch guage (looks like a really small comb) to figure out the pitch of the threads. If you don't have a pitch guage, just go to the hardware store and buy the different size nuts for that thread size. If you can't find a nut with the right thread pitch then you will not be able to find a tap.
If the existing threaded hole on the flywheel weight you have now is the correct size, but not the correct thread pitch you will not be able to retap it. The hole has to be too small so that you can drill it with the correct drill bit, and then tap it to size.
Theres nothing wrong with modifying the weight to work. But if you do it wrong you will end up with a destroyed crankshaft and possibly a lot of other damage.
 

Bunya

Member
Apr 26, 2007
147
0
Keep in mind we're dealing with metric threads. Metric threads are specified by it's major diameter x thread spacing. A typical spec would be 12x1.25 which means the OD is approx. 12mm and there is 1.25 mm from the top of one thread to the top of the next.

When you measure OD of the threads, they are going to come out an oddball number like 11.86 mm (you lose a little material when the threads are cut). For male threads, round your reading up to the next whole number, in my example 12 mm. For the pitch of the thread, a thread gage makes it quick and easy, but it's not absolutely necassary if you have a caliper or a good rule. Since metric thread pitches are specified by their spacing, measuring the distance from 1 crest to the next will tell you the pitch. It gets a little tricky on smaller sizes where the pitch may be .7, .75, or .8. But the crankshaft is probably going to be 12 mm so a pitch of 1.25 or 1.5 is typical. Measuring the distance between 10 crests then dividing by 10 will make things easier and less prone to error (there's little difference between 1.25 and 1.5 but a big difference between 12.5 and 15). If you don't have 10, use as many as you have then divide the result by how ever many you counted.

The bigger problem is determining what the female threads are. You can measure the ID to get the size, but you really have to use a thread gage to determine the pitch. An option would be pull to the flywheel off and see if you can put the weight on backwards. If the threads are a screwed up, it's likely only the first few threads are so the outside ones should be fine. If it fits correctly, then you'll know it's just buggered threads. If it's a common size, you might be able to find a tap locally. OTOH, if it's an oddball, try someone like http://www.jlindustrial.com/ that serves metalworking industries.

Finally, if it turns out to be the incorrect thread, don't attempt to alter it. It's obviously close in size which means you can't safely re-tap it a different thread. You'll be removing way too much material for it to be safe. When it fails, it could ruin the threads on the crank and take out the cover $$$$$. :eek: It's not worth the risk. If you got it on Fleabay, contact the seller and get a refund because it's not what they said it was. Otherwise, either try to resell it or chalk it up to experience and purchase the correct one. One of the important lessons in life is: Cheap is Expensive!

Marc -
 

lorollinls

Member
Nov 22, 2006
26
0
cool thanx for the replies....the weight is evidently correct, thread size and pitch is correct, threads did look alittle boogered so i just ran the proper sized M12 X 1.25 tap through it to clean up the threads alittle bit and it still didnt help it thread on correctly. even tried putting it on backwards like Bunya said and it still threaded on crooked after the first full turn soooo....i think it may possibly be the threads on the crankshaft, but i dont know what ill do about this becuz i can not find a die in that size (M12 X 1.25). So im still back to the drawing board...gonna study it over some tonight to see what i can come up with.

thanks alot for your help guys
 

Bunya

Member
Apr 26, 2007
147
0
Does the flywheel nut spin on freely? It really sounds like the pitch is different on the crank. Both 1.5 and 1.25 are considered 'fine' for a 12 mm shaft so it's possible the crank might be 1.5. If the flywheel nut spins freely on the crank, it would indicate that the crank threads are in good condition. If that's the case then they're likely 2 different pitches. If you need it, the die is commonly available:
12 x 1.25 Die

Incidently, for chasing existing threads, the Carbon steel hex die is fine but not for cutting new threads. For new threads you have to use the HSS (high speed steel) die.

A word of caution! Before I'd go putting a die on the crank I'd triple check the thread size to be sure it's the same. If you run a die down the crank and it's different, the crank will be junk. You could buy quite a few used flywheel weights for the cost of a crank!

Marc -
 

lorollinls

Member
Nov 22, 2006
26
0
no doubt, thanx for the wise words bunya.....the dealership couldn't tell me the exact pitch of the threads on the crank....but going by the nut that was on there wont be accurate because i bought the bike used a few months ago and im rebuilding it....the previous owner obviously put the wrong nut back on the crank because the threads are messed up in the nut although it does spin on freely.....ive been thinking like you that the crankshaft pitch might actually be 1.5 or 1.75. But I did triple check with steahly off road that the cr250 flywheel weight was M12 X 1.25..... so i dunno

thanx again
 

Bunya

Member
Apr 26, 2007
147
0
Well you do have a bit of a nightmare on your hands. The previous owner should be kicked in the rear for doing that - I'll never understand the deeds of morons. It certainly would explain the problems you are having. If Steahly is telling you the crankshaft threads on your bike are 1.25, I'd go by that. After all, it is their business to know.

Just to be safe, the route I'd take would be to order the correct nut from Honda - you're going to need it anyways. Once I had the correct nut in my hand, I'd determine it's thread pitch and hopefully verify M12x1.25. I would then obtain the correct die from J&L to clean up the threads on the crank. You'll have to be extremely careful and be sure you're starting the die on the original thread. Proceed very slowly at first, start just a touch then back it off and check with a magnifiying glass to see where it's starting to cut. If it appears to be in the original thread then turn it little further. Then back off again and check. I would repeat that procedure until I was able verify it was correctly tracking the original thread. With some luck the incorrect nut was a lot softer than the crank and absorbed most of the deformation.

Marc -
 

2strokerfun

Member
May 19, 2006
1,500
1
Bunya said:
With some luck the incorrect nut was a lot softer than the crank and absorbed most of the deformation.

It should be as the crank threads will have a radius in between. But it darm sure didn't do the crank any good.
 

Bunya

Member
Apr 26, 2007
147
0
From my previous post:
Bunya said:
Incidently, for chasing existing threads, the Carbon steel hex die is fine but not for cutting new threads. For new threads you have to use the HSS (high speed steel) die.

The carbon will be fine and easier to use if you don't have the proper die stock.

Marc -
 

lorollinls

Member
Nov 22, 2006
26
0
sorry it took so long to follow up, well i figured out whats up, the threads were wrong, they were 1.75 on the crankshaft AND the end of the crankshaft JUST where the threads are is bent alittle, so i tapped the weight to 1.75 and threaded it on and it wont even mount up to the flywheel flush due to the crankshaft being bent.... how would this happen, anyone have an idea ? I just bought the bike back in november, i took the flywheel off correctly with a flywheel puller no problems at all but the threads were already boogered so ill bet it happened before i got it.......
 

Bunya

Member
Apr 26, 2007
147
0
It may not be that the crankshaft is bent, it might be the threads you tapped in the weight are not straight. We advised you not to attempt to retap the weight a different thread as it will turn it into junk. :coocoo: It's possible the tap either started crooked or became crooked as it interacted with the existing threads.

If it's definitely the crank that's bent, it was probably the result of someone hitting the end of the crank with a hammer to get the flywheel off. They likely had the nut out at the end of the shaft to protect the threads. They probably caught the edge of the nut rather than flat on the face. This would act like a small lever and could bend the end of the shaft. It would also account for the damage to the threads and why a different nut was used.

Either way, the weight is certainly ruined. It could be used on a different bike with a 14 mm or larger crank where the existing threads are drilled out and it's retapped to the larger thread. If the existing threads are crooked, it would have to be bored to correct the alignment because a drill will just follow the crooked threads. You really should have taken the advice to sell that one if the thread was incorrect. :bang:

Once again, Cheap is Expensive!

Marc -
 

lorollinls

Member
Nov 22, 2006
26
0
its ok, i didnt loose any money on the weight....... and also the crank IS bent where the threads are where it connects to the smooth part, you can turn the motor over slowly and watch it wobble........the nut threads on fine but it is just about 1mm crooked, if that, but tightens down fine, the nut is not heavy enuff to make a difference and make it feel weird but the weight will not mate up the flywheel flat and the flywheel weight was tapped STRAIGHT, you can thread a bolt into it and it goes in straight as pie.......see what im sayin?
 

Bunya

Member
Apr 26, 2007
147
0
You did lose money. The weight is junk. You can't safely use it. There isn't a whole lot of difference between 1.25 and 1.75. When you retapped it you removed a lot of material. The amount thats left is not strong enough to safely retain the weight. It's not going to be perfectly balanced, especially after retapping it. The dynamic forces that act upon it can be very large, especially at RPM's that match up to resonant frequency of the crank system as a whole. Have you ever seen the film of the Tacoma Narrows Bridge rip itself apart? That's a good example of what happens when something hits a resonant frequency. If you research flywheel weights, some people have had weights trash their engines. It's quite possible that they were a bit out of balance and hit a resonant point one too many times. Under normal circumstances, these forces are well within the limits of the thread, but not after retapping it like you did. Whatever you could have sold it for on Fleabay is what you're out. That would have offset or even paid for whatever the correct weight is going to cost you. See what I'm saying?

Anyway, to solve your problem. Since you tapped the weight I'm assuming you have a M12x1.75 tap. Find a piece of bar stock, probably at least 3/4" and a foot or so long. Drill a hole in the end & tap it to match the crank. If the bend isn't too severe, you can then thread it on the end of the shaft as far as you can and use it to carefully bend the shaft back. As it straigtens, you should be able to thread the bar on further. Always keep it as tight as possible. Go slow and keep checking with the flywheel weight to see how close you are. Don't use the old weight you retapped to check it with!!! Even though threading a bolt into it looks straight does'nt mean it is. If it is crooked, then you'll still have a problem when you put the new weight on. Wait until you have the new weight and use that for checking the bend on the shaft. Even if you don't put a weight on, the shaft should still be straightened for the nut to fit properly or there's a chance it will loosen up and ruin your day.
 

lorollinls

Member
Nov 22, 2006
26
0
no bro, i believe i would know whether i lost any money on it or not, i bought and SOLD the damn thing to a friend for use on something else actually for the same price....also i do understand a few things about resonant frequency seeing as how i am an acoustical engineer and electrical engineer. i design home and car audio components such as subwoofers that can destroy things at the right frequencies as well. but i do understand what you are saying about the exact rpm that will match up with the weight's resonant frequecy and cause it to fly off and destroy anything in its way inside there. i just decided to not run the thing now, i dont NEED it, but will get a new weight and crank when i rebuild the bottom end next winter or so. Thanks for the straightening tips and thanks for all of your help.
 
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