ORIGINAL: Ken6PPC
ORIGINAL: Campgems
ORIGINAL: Ken6PPC
ORIGINAL: lnewqban
It could be that the thread on the shaft is slightly damaged, so the 1/4'' nuts don't fit all the way.
Wish you luck!
That would be my guess too! If so, before you buy a new crankshaft, try running a 1/4''-28 die over the threads. That might be enough to clean it up so that you can still use it.
NO No NO. Trying to chase the thread with a die will only result in screwing up the remaining threads. If it isn't 1/4-28 as suspected running a die along it will absolutleyruin the crank.Even if it is 1/4-28, and the treads on the end are damaged, trying to chase thethreads will most likely result in thedie establishing a different thread pattern. If the nut cant screw on, the die won't eaither an will makeits own path.
A close inspection will tell if the thread is damaged. If they are, a simple file is the best tool to use, Hold it at 30 degrees to the shaft, the same angle as the threads and file pushing the file to the center line of the shaft. Don't even do this though unless you can see that the threads are damaged. Idoubt that they are as
rsieminski sates that a nutwill screw on three turns prior to binding. That indicates thestart of the threads are good and what he is seeing is an example of thread pitch being different. As pointedout eariller, the 1mm pitch and the 28tpi are very close and it would take just about three turns to take up the normalthread clearance.
Don't fix something if it isn't broke.
Don
Don, he said the nut will thread on the crank, and then get tight. That means the first threads are fine, and the threads further up the crankshaft are the ones that are buggered up. A die will clean this up better than ANY file!
Unless of course, you think one can cut threads with a file better than machine tools can! [sm=75_75.gif]
Edited to add:
Don, I agree that a triangular file COULD be used to clean up the crankshaft threads, if it was used by someone who understood how threads are cut, and how they are supposed to look. Since the OP obviously doesn't posess those skills or knowledge (he doesn't have a thread pitch gauge, and didn't even know what thread pitch really was), do you REALLY think he will instinctively know how a thread is supposed to be cut (or this case, FILED)?
I think he is much better served to use a proper thread die to clean up his crankshaft. Actually, he would be BEST served by taking it to someone who understands threads and how they are measured and cut.
(No offense intended to the OP. He is trying to learn, and there is NOTHING wrong with that!)
Anyway, that is my opinion, and you are entitled to disagree!
Ken, if you read my post closely youi would see that Ididn't recomend filing as I'm sure damaged threads are not his problem. I just gave him directions on how to clean up the end of the screw as that is the most likely place for damage to happen.
To prove my point, I just went down to the Ace hardware and bought a 6-1 x 25mm metric hex head cap screw. Ipicked a 1/4-28 nut out of the bin and screwed it on to the screw It was a very loose fit at first, and the nut screwed on about10 MM before binding on the thread. At first, I though that may shoot down my theory on what was going on.Then I realized theat "hardaware" nuts and bolts are veryloose fitting. So,Ibought the screw and brought it home to see how the engine nuts fit. The first was of questionable origan and it fit about like the hardware store nut. I then pulled a fresh O.S nut and it screws on the hardware store screw almost exactly three turns before it jams up. 21 flats to be exact. My theory llives. Then I got out my best 1/4-28 die and had a go at the pristine screw. The first 2 1/2 turns went about like the prop nut, no interference and very loose. Then things tightened up and Iwent ahead, with cutting fluid for another 8 turns. The result was what Iexpected.Just before the point where the nut stoped, the tap started over cutting the scrw threads. At about the 12 mm mark, the die was cutting off the top of the threads. Itried the O.S. nut on again and again, it would go the three turns and lock up. At this point, the screw is destroyed, neither a 6mm-1 or a 1/4-28 nut will fit. You could bully the 1/4-28 onto the damaged threads, but as soon as you tried to tighten the nut against a prop, the threads on the nut would roll over and you would most likely have to cut the nut off the shaft.
The Pitch of the two threads are very close. 0.03937" per thread on the 1mm thread pitch, vs 0.03571 thread pitch for a 28tpi thread The pitch diameters are close also, but futher apart, 0.236" for 6mm and 0.025" for 1/4". The loose OD fit is what lets the nut go on for three turns prior to jaming up.
The first photo is the new screw with the O.S. 1/4-28 prop nut. Second photo shows how far that nut will screw onto the 6mm screw. The last is the results of "Chasing" the threads with the wrong size Die.
Added,
It is most likely that the engine in question has a 6-1 metric screw. Also, it is unlikely that the thread is damaged unless he followed some of the advise on this thread.
Don