96 CR250 Crankshaft threads

Joined
Nov 22, 2006
Messages
26
Likes
0
#1
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:
Joined
Oct 19, 2006
Messages
2,142
Likes
1
#2
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.
 
Joined
Nov 22, 2006
Messages
26
Likes
0
#3
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
 
Joined
Nov 22, 2006
Messages
26
Likes
0
#5
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
 
Joined
Sep 11, 2006
Messages
156
Likes
0
#8
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.
 
Joined
Apr 26, 2007
Messages
147
Likes
0
#9
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 -
 
Joined
Feb 21, 2003
Messages
48
Likes
0
#10
100% agree... return or sell that one and order the correct part. Save yourself the headache and extra labor for a few bucks.
 
Joined
Nov 22, 2006
Messages
26
Likes
0
#11
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
 
Joined
Apr 26, 2007
Messages
147
Likes
0
#12
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 -
 
Joined
Nov 22, 2006
Messages
26
Likes
0
#13
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
 
Joined
Apr 26, 2007
Messages
147
Likes
0
#14
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 -