Clutch bolt thread and pitch

[Redbeard]

I pulled the clutch from my 2008 Vector and discovered you can thread a US 1/2" x 20 tpi bolt into the crankshaft. Is this true for all Yamaha clutch bolts?

 

[cannondale27]

M12.5 X 1.25

 

[Redbeard]

M12.5 X 1.25

The 973cc crank threads must have been different, as my stock clutch bolt mic's at 1/2" (12.7mm) diameter.

 

[cannondale27]

measure the thread itself. Is a oddball for sure. If it is US isn’t it 24tpi also?
If you did bung up crank thread be sure before running a tap in. Never saw us thread on a Yamaha before.

 

[Redbeard]

Fine thread for a US, 1/2" bolt is 20 tpi.

I agree, it's weird, but a US, 1/2" fine thread bolt threads all the way into the crank until it bottoms.

Thanks for the input, but my crank is fine. I only removed the stock clutch for replacement, as it's worn out. I just found my observation interesting.

 

[Winderallday!]

A 1/2-20 fine series thread may well thread just fine into a M12.5 x 1.25 threaded hole, they are very close in size, and with manufacturing tolerances this could be the reason it will thread into your clutch mount without an issue.

*M12.5 diameter equals 0.492"
*Metric 1.25 pitch equals 0.049" thread to thread

*1/2" nominal diameter equals 0.500"
*20 threads per inch equals 0.050" thread to thread

So the difference between the two is minimal.
Also, measuring the body of a bolt may be different than the threads, depending on how they are manufactured, they could be rolled threads instead of machine cut threads, this could account for a slightly different body diameter vs diameter measured over the threads.

If I were a betting man, the threads are metric on a Yamaha sled.

Cheers.

 

[darv]

all clutch Bolts are metric on yamaha. they have been using the same bolt for a number of years like a lot.

 

[Redbeard]

Call me stubborn if you will, but curiosity got the best of me. I stopped and asked my local, well respected machinist to mic and gauge the Yamaha clutch bolt, as my measuring equipment is generic.

What he found, under magnifying glass, is a metric 1.25 thread gauge is close, but not quite. He also tried other, more obscure metric gauges, which I didn't even know existed: 1.3 & 1.4. Again, close, but no cigar. The only thread gauge that matched perfectly was the 20 tpi, standard gauge. The bolt also mic'd .003" less than 1/2" and different maufactures bolts, of the same diameter, can vary as much as .006" in diameter. He also confirmed the correct way to determine bolt diameter is by measuring the unthreaded portion of the shank, not the threads themselves, as threads will always measure slightly smaller in diameter from the thread rolling or cutting process. Stamped threads are an exception.

So, at this point, I respectfully ask you guys, as a fellow sledder and friend, to get out your mic's, magnifying glasses and some 1/2-20 hardware to make a comparison and form your own analysis.

 

[cannondale27]

Call me stubborn if you will, but curiosity got the best of me. I stopped and asked my local, well respected machinist to mic and gauge the Yamaha clutch bolt, as my measuring equipment is generic.

What he found, under magnifying glass, is a metric 1.25 thread gauge is close, but not quite. He also tried other, more obscure metric gauges, which I didn't even know existed: 1.3 & 1.4. Again, close, but no cigar. The only thread gauge that matched perfectly was the 20 tpi, standard gauge. The bolt also mic'd .003" less than 1/2" and different maufactures bolts, of the same diameter, can vary as much as .006" in diameter. He also confirmed the correct way to determine bolt diameter is by measuring the unthreaded portion of the shank, not the threads themselves, as threads will always measure slightly smaller in diameter from the thread rolling or cutting process. Stamped threads are an exception.

So, at this point, I respectfully ask you guys, as a fellow sledder and friend, to get out your mic's, magnifying glasses and some 1/2-20 hardware to make a comparison and form your own analysis.

Then explain how I can put a 1/2in thread on a 3/4in square stock. Just saying.

 

[Redbeard]

That's why I started this thread, then ultimately went to a professional, Steve.

 

[cannondale27]

It is definitely weird. Clutch really doesn’t rely on the thread much except for initial torque. If guy had to looks like could use 1/2-20 in pinch. Good to know. Would bother me though!

 

[cannondale27]

Also I have always measured threads on thread when turning threads. I will check the bolt with a go no go guage.

 

[Redbeard]

The one thing I did not do, was mesh the threads on the Yamaha bolt and US 1/2-20 bolt together under magnification.

I think an accurate comparison would be using brand new bolts that have never been torqued or stretched.

It is time, however, for me to stop beating the horse.

 

[cannondale27]

Well to beat a dead horse...
I just checked clutch bolt out of 09 Nytro. Calibrated set of Go/No Go gauges. 1/2-20 is about perfect and available. So Northern Sledder even though us Yamaha purist cringe at thought of USA thread in our motors there in fact is. Or Metric version is just that close. There is a couple other thread sizes that are so close to Metric both meet all quality standards for a bolt. But you are right and I enjoy learning something new everyday even if its painful sometimes! Thanks Northernsledder.

 

[cannondale27]

We should get NosPro involved in this. That's his job. I bet he could find something.