crossflow crankshaft thread sizes.

[schematic]

Hi
Does anyone know if crossflow v-4 crankshafts all use the same thread size for the flywheel nut? I purchased a used unknown year crank and the threads seem .006 larger OD than the original and my nut won't fit.

Thanks

 

[Chinewalker]

Re: crossflow crankshaft thread sizes.

I've never come across any that were different... Is it possible the end of the one you purchased is a little mushroomed due to someone banging on it while removing the flywheel? Might just need the threads chased...

 

[schematic]

Re: crossflow crankshaft thread sizes.

Hi Chinewalker
The threads are uniform and undamaged. The end of the crank is perfect. The thread pitch is the same as the original, but the thread is .006 larger OD for the full length of the shaft. I've tried different nuts....no luck. Is it possible they went with a metric thread on some models?

Thanks
Randal

 

[daselbee]

Re: crossflow crankshaft thread sizes.

NO metrics on OMC of "our vintage".

 

[Chinewalker]

Re: crossflow crankshaft thread sizes.

I've had crossflows from the 1960s to the 1990s and never noticed any change in the thread size... Are you sure your crankshaft is out of an OMC? Could it be a Yamaha crank? They had some 90-degree V4s back in the day that were of similar size (albeit looper rather than CF).

 

[Tim Frank]

Re: crossflow crankshaft thread sizes.

What size is thread OD?
Any chance the crankshaft was repaired?
i.e. built up and`rethreaded?

 

[schematic]

Re: crossflow crankshaft thread sizes.

daselbee - Thanks, that takes care of the metric possibility.

Chinewalker - Boy I sure hope its not a Yamaha. The only differences I could see are the rod journals are hollow, and the bottom seal ridge is approx. 1/4" longer. I hope its just newer than my 1981 crank.

Tim Frank - after your comment, I took a close up picture and zoomed in. Although the crank threads look perfect to the naked eye, zooming in show slight imperfections. I think you are right! It probably was lathed close to size but never finished with a die. Off to borrow a die I go!

Thanks gentlemen.

 

[etracer68]

Re: crossflow crankshaft thread sizes.

If the nut was over tightened, the threads could be pulled. Did you check with a thread gage, and does it fit in, all the way to the bottom. Generally if someone single pointed the threads on a lathe, they, or I, would have finished them. A die most likely wont run to the bottom, with a full thread. Im a tool maker for 40+ years, my thought would be to get a thread file ( a single file with correct pitch), and touch them up a bit. What does the od measure?

 

[schematic]

Re: crossflow crankshaft thread sizes.

Hi etracer68
Thats the puzzling part. The thread gauge fits perfect. The OD is uniform at .879 The original crank thread is .873 with the same pitch (14) I have tried a thread file but it just glides over. Do you think a adjustable die would be a safe bet. I have a thread chaser but I'm a little shy as it only wants to go on 1/2 a thread.

Thanks

 

[Tim Frank]

Re: crossflow crankshaft thread sizes.

Hi etracer68
Thats the puzzling part. The thread gauge fits perfect. The OD is uniform at .879 The original crank thread is .873 with the same pitch (14) I have tried a thread file but it just glides over. Do you think a adjustable die would be a safe bet. I have a thread chaser but I'm a little shy as it only wants to go on 1/2 a thread.

Thanks

It is supposed to be a 7/8-14UNF thread, not sure the class of fit, whether it would be spec'd as a class 2A or 3A.
In any case, 0,879" is simply too big....absolute max is 0.875".
I'd guess it was lathe-cut and the operator stopped short of the mark.
I'd also be concerned that the minor diameter is over-sized as well.

Your gauge fits because the thread form is OK, just the wrong pitch diameter.

Maybe : http://www.amazon.com/14-Carbon-Steel-Hex-Rethreading/dp/B002NZMSBY

 

[Tim Frank]

Re: crossflow crankshaft thread sizes.

Maybe : 7/8" -14 Carbon Steel Hex Rethreading Die - Amazon.com

On reflection, that probably won't work.
There is not a lot of extra mat'l to be removed to get on-size, but probably more than a "rethreading die can do.

 

[schematic]

Re: crossflow crankshaft thread sizes.

Well I tried an adjustable die. Wouldn't bite the crank threads. I then took a different approach. Since the threads look good and are uniformed throughout, I machined the nut .006 larger. I believe my nightmare is over.
Thanks to all who replied!

 

[etracer68]

Re: crossflow crankshaft thread sizes.

I will sometimes use a single sharp V shaped file. You can get them in a small file pack, they are only like 6 inches long, maybe 3 inches of file. I have used them to touch up pulled threads, when a die wont start. The file packs, have a bunch of differant shapes, and sizes. Hope you get it, as long as the minor dia is correct.

 

[Tim Frank]

Re: crossflow crankshaft thread sizes.

Well I tried an adjustable die. Wouldn't bite the crank threads. I then took a different approach. Since the threads look good and are uniformed throughout, I machined the nut .006 larger. I believe my nightmare is over.
Thanks to all who replied!

Did you re-cut the entire thread on the nut, or just bore out the ID?

 

[etracer68]

Re: crossflow crankshaft thread sizes.

Tim F,
I'm wondering the same thing. Seems if he just opened up the ID of the nut, the minor dia. of the stud may not have been cut deep enough. I think I would have started with a NEW nut, and worked the stud threads, until they were correct fit. But then again setting up the crank in the lathe, if it was already installed, would turn out to be more work. A split ajustable die may do the trick if its just the first few threads. Depending of the die, and how far the nut needs to go, with the flywheel installed.

Also, if the nut fits loose now, Im not sure it will hold 75ft lbs (I think thats what it is).

 

[schematic]

Re: crossflow crankshaft thread sizes.

Good question Tim Frank.
I re-cut the threads in the nut. I cut it .006 oversize. I first tried a split die, but the crank threads are so hard, it wouldn't touch it.
Nut torque is 105 ft lbs

Thanks

 

[Tim Frank]

Re: crossflow crankshaft thread sizes.

Good question Tim Frank.
I re-cut the threads in the nut. I cut it .006 oversize. I first tried a split die, but the crank threads are so hard, it wouldn't touch it.
Nut torque is 105 ft lbs

Thanks

If you have the tools and skill to chuck up a 7/8" nut and re-cut/single-point an internal thread 6 thou O/S, you didn't need any help or advice from me...
I love happy endings; nice job!

 

[schematic]

Re: crossflow crankshaft thread sizes.

Tim Frank. I value your advice. Thanks for your input. You are probably one of very few that appreciates the difficulty of performing this task. The engine is running today....like a swiss watch.

 

[Tim Frank]

Re: crossflow crankshaft thread sizes.

Tim Frank. I value your advice. Thanks for your input. You are probably one of very few that appreciates the difficulty of performing this task. The engine is running today....like a swiss watch.

etracer68 gets it too!
I';d guess that they either hard chromed the shaft and that's why it was so tough....or they built it up with welding....and it was effectively case-hardened.

You an amateur or a pro like etracer ?
I'm just a home-shop metal cruncher, but a 54" x 10" Atlas lets me do some pretty cost-saving repairs at home....

 

[etracer68]

Re: crossflow crankshaft thread sizes.

Sounds like you both, Tim, and OP, have the understanding, and skills. I've made a few piles of chips in the past 40 years. Good to hear all is good to go.