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!

A trinagular file is good for dinged threads mid length. A very fine knife edgefile isbetter as it allows a finer cut to the coect angle.

A flat flile held near the 30 degree angele and cutting from the OD to the center will give you the best cleanup of the threads on the end of a screw. If I cut off a screw, I grind the end to the 30 degree angle, grinding so the flashing pulls to the center. Touch up with the triangle or knife edge file and you have a professional finished end on the screw. It gives you the same end profile that hitting the end of the shaft before threading it with the thread cutting bit on the lathe gives you. The file does a rougher, but acceptable version of this chamfer on the end of the screw.

Don

Quote:

ORIGINAL: Broken Wings

Allow me to correct you. The American Unified Standard Thread Series classifies UNC Unified National Course as the 1/4-20 size NOT UNEC
UNF is Unified National Fine 1/4-28 and UNEF Unified National Extra fine as 1/4-32

There is NO UNEC Unified National Extra Course designation for American Threads...

BW,


You are right about the nomenclature.

I could not find the UNEC thread title...


But between the UNF of 28 tpi and the UNC of 20 tpi, there is another tpi of 24... 1/4"x24 bolts, nuts and appropriate taps and dies do exist.

Here are [link=http://www.google.com/products?q=thread+tap+1/4%22+24&sourceid=navclient-ff&rlz=1B3RNFA_enIL241IL243&um=1&ie=UTF-8&ei=mC9YSoCXFI6InQPdiOXdCQ&sa=X&oi=product_result _group&ct=title&resnum=4]some taps[/link], for example. This size is quite common in Harley-Davidson motorcycles.


Common it is, even in hardware stores, but there seems not to be a title for it in charts... It is commonly called NS (National Standard)

Because of the huge difference between UNC and UNF, I was under the impression that the 24 tpi is worthy of the title UNC and that the 20 tpi is the UNEC...

I must have been wrong...

You're mistaken. You need to look up the difference between NS (National Standard) and UNS (Unified National Standard)

Common it is NOT!

The ONLY use of the 1/4-24 in a Harley Davidson Motor Cycle is to hold the lifter/tappet blocks to the engine case and the last year Harley used the 1/4-24 was 1972 on the Shovel Head 74 cubic inch engine. Previously the Pan Head 61 and 74 cubic inch engines used the same screw and the last year Pan Head was made in 1965.

They used a 1/4-24 Twelve Point head by 1" long screw.

You will NEVER find a 1/4-24 stocked in a hardware store in the USA, PERIOD...

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

[sm=75_75.gif]

Dar, might I suggest you invest in a somewhat later engineering hand book? Even my 1960 Caxton Engineers' Handbook doesn't list the 1/4x24 anymore. It was used in the early 1900's along with some other weird thread pitches like 27tpi (!) but their use was discouraged as far back as 1949 in the USA. The only exception to the normal 1/4-28 crankshaft that I know of is the old Merco 29/35 which uses a 1/4-26 BSF (Whitworth Fine). I have two of those engines and only one prop nut .

Oops, I see there were a couple more posts while I was typing this....agreed, you'll never find that 1/4-24 in your local hardware shop or probably in any engineering shop either. Have a look [link=http://www.sizes.com/tools/thread_american.htm]here[/link] Dar.

Quote:

ORIGINAL: downunder

...agreed, you'll never find that 1/4-24 in your local hardware shop or probably in any engineering shop either. Have a look [link=http://www.sizes.com/tools/thread_american.htm]here[/link] Dar.

Maybe so, Brian.

However, in the UNS tpi column, this dimension still appears.
Also, I don't have an American hardware outlet where I live, but do you think [link=http://www.google.com/products?q=thread+tap+1/4%22+24&sourceid=navclient-ff&rlz=1B3RNFA_enIL241IL243&um=1&ie=UTF-8&ei=IABbStWvBZXAmgOb18ndCQ&sa=X&oi=product_result _group&ct=title&resnum=4]all these tap offers[/link] are intended only for owners of Harley Knucklehead motorcycles?

They (the taps) seem to still be in production...

G'day Brian

Ah BSF. I was a long time restorer and driver of old Land Rovers. All my Series I Land Rovers were just about pure BSF and BSC. There were hundreds of 1/4 BSF nuts and bolts in them mostly holding the body together. If you want some 1/4 BSF nuts, I might still have a few around. I was even able to buy some in Orange a few years back but they were hand made and not the original "Sheridised" (Sp?) ones.

Mike

Mike and Brian, I hadthought the Witworth was mostly associated with motorcycles, That is where I learned to hate them. I about went nuts with not only the TPI, but also with the head sizes on the bolts. The one that comes to mind is the head bolts on a 350 BSAsingle. Later on, I drove a MG-GTcoupe and Ithink that there were a not of 5/16 and 7/16 screws where previous owners had bumped the size up allow for UNC threads. It's funny, we were just talking about this at the field this morning. Some of the equpment I worked on when I was on my apprenticeship back in the early 60's was manufactured in Germany and was orginally all metric, It was common practice to "Americanize" any thing that gave us trouble bolting it back together. It was a huge machine, and it was at least 10 to 15 years old and it saw maintance on about a weekly basis. Working on it required a set of metric as well a SAE wrenches. That was the only metric machine in the whole shop.

Some of the other machines I repaired were so old that you would run across a non standard screw now and then, Some of the machines were bolted together wiht Square head cap screws. Those you had to watch out for.

Don