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a near fit thread question: 27mm X 2mm in a 1.065 X 14 recut to 27mm X 2mm?

justashooter

Plastic
Joined
Mar 5, 2005
Location
pennsylvania
Frank Dehass says that Carcano rifles have a 1.065 X approximately 14 pitch barrel to receiver ring thread mate. I see a source for cheap barrels that are threaded 27mm x 2mm, and see a source for 27mm X 2mm taps. The threaded interface in a carcano receiver ring is about .650" shank length. The threaded shank on the potential barrel is about .500".

27mm is 1.06299.
2mm pitch is about 12.7 TPI.
14 pitch would be 1.814mm thread
1.75mm pitch would be 14.514" pitch

I have a spare carcano barrel at home, but can't get to it to determine with a caliper exactly what "approximately 14 pitch" means; whether it is less than or more than 14 pitch.


How does the forum feel about containing a 35 Remington chamber loaded to 40KSI standard factory ballistics with such a re-threaded receiver ring?
 
If your looking for advice I would recommend you forget the idea of bastardizing this project. If you can't see spending the money to rebarrel the correct way, then drop that idea.
 
It is much better to thread the barrel properly. The thread has to be lengthened anyway. If the barrel has enough meat to shorten and re-thread, use it If not, forget it. What is the source of barrels.
 
If your looking for advice I would recommend you forget the idea of bastardizing this project. If you can't see spending the money to rebarrel the correct way, then drop that idea.

I believe he is talking about chasing the receiver threads with a tap that matches the barrel threads. In that case, one could probably come up with some calculations to determine just how strong the resultant thread-form will be after it is partially removed. I wouldn't call that a bastardization as much as modifying things to suit his needs.

If the strength is there and it's a personal firearm that he knew/understood the limitations of there is no cause for concern.

That being said, my concern with the above mentioned chasing the threads process is the fullest form thread will be those that the tap is first started into and as the tap works back the pitch will be progressively further and further off, removing more and more of the thread-form. If he lengthens the shank without threading the lengthened part (just adding a relief) the only threaded part of the shank will not be interfacing with the best threads in the barrel; that would need to be accounted for.

IMHO, I would find out for sure what the pitch is. I don't know much about Carcano rifles, is it possible the Carcano is actually a metric thread and somebody has bastardized the thread callout into a standard thread? In that case "about 14 tpi" might, in fact, be 2mm pitch.
 
Usally you try to modify the cheeper part, or the non-serial numbered part first. In this case the barrel. It is easier to turn barrel threads to fit the receiver, than to try and retap a receiver with possible hard spots in it.
 
I tried to look up what an actual Carcano barrel thread is and all I can find is it is close to 14TPI. If it is metric, 1.75 is close to 14TPI (14.514 to be exact). 2mm pitch would be 12.7 TPI which is a lot further from 14 TPI. The receiver is variously described as strong and weak.
The 35 Remington cartridge has a good reputation, if you can adapt a way to feed it in the Carcano receiver you might have a good outcome.
I am with the others here, if I could not thread the barrel to whatever matched the receiver I would get some one else to do it for me or give up on the idea.
A former member here had some "mechanical drawings" that showed the barrel threads of various guns, his name is speerchucker30X3 perhaps you could find the post. Also another member posted some ones hand drawn specs on many different barrels. I could find neither but they are here somewhere.
 
Bolt Action Rifles by Dehaas.. Has the hand drawn thread specs, AND the approx 14 TPI for Carcano and the Italian training carbines.. And his articles In NRA Gunsmithing Guide show same dimensions.

Back to square 1..

It looks like he did not have a metric thread gage then. All are either inch or approx inch.

I would make a test plug at 1.75mm and try it...

Or simply see how well a 1.75 mm Thread gage fits thread.

Amazon.com: Metric Thread Gauge

Change the barrel threads or get a new barrel blank.

Consider costs of this. Barrel, reamer, headspace gages, stock, sights, funky scope mount. Odd cartridge clip..

Why spend twice as much as a good used rifle, to build one that will be worth half as much as a used one.?

Get a used Mauser 98 Large Ring action, It will keep gas out of your face. Easy to mount a scope. Has some resale value. Better triggers, safety, trigger guards available..
 
Speer is using the same source of info as the NRA Gunsmithing Guide which is DeHaas; he is credited as author of the Guide articles. We just talked about this subject over here. The conclusion is that the pitch is 1.75mm.

Assuming you can cut metric threads, I would make a trial piece to check thread dimensions. Start with M27x1.75 and see if that fits. 27mm is 1.063. 26mm is 1.024. Maybe 27mm with the proper thread form.
 
Speer is using the same source of info as the NRA Gunsmithing Guide which is DeHaas; he is credited as author of the Guide articles. We just talked about this subject over here. The conclusion is that the pitch is 1.75mm.

thanx for the clarification. I am not home so cannot measure an example. the particular rifle I am looking to upgrade has a very nicely figured sporter stock and a mauser floorplate/magazine conversion, pix of which have been posted on the castboolits forum under same user name, so is worth the effort of caliber change. I guess we are back to barrel stubbing...
 
If the thread is really 1.75mm and you have a lathe like a Southbend 9c you could play with the change gears and get 14.5 TPI and be close enough.

I used to make screws pillar screws for commercial Mausers. I was sure the screws were metric so I ended up cutting them 1.15mm and it turned out they should have been 22 TPI.

Anyway if you do not have metric gears real close is usually good enough.
 
If you aren't going to or can't cut the proper pitch, calculate the error per thread and multiply by the number of engaged threads. This will tell you the error at the last thread. It probably should be less than 10% of the thread pitch so you aren't moving too much metal in the later threads.
 
Radical idea, but why not measure the thread pitch instead of relying on "about 14 tpi" BS?
What is really odd here is this rifle has been around for 125 years and no one knows what the threads are. Speerchucker said he thought they were 26 X 1.75. The metric system was new then and the meter standards were not reasonably perfected yet.
I have tried to identify other threads from that era and they are sometimes originally very imperfect and now they are worn and damaged. Even with an optical comparator it is hard to know for sure, even with averaging the spacing over 3 or more threads.
The OP on this one having an aftermarket stock and a converted magazine already, has one of the few that is really worth working on, maybe he will let us know what he finds out.
 
Nothing odd. DeHaas was the only one, interested enough to take measurements of dozens of obscure rifle barrel threads. No one else bothered. He wrote several books and articles, so he made money doing this...

When he did this, (years ago) all he had was a standard inch thread gage.

This bothered nobody, due to the fact many gun nuts only had inch lathes, and were going to cut closest Inch thread anyway...

There was no internet, ordering tools took a week. Getting US lathe metric change gear info or parts, was even slower
 
My math must be faulty. 125 years ago was 1890'ish by my calc's, and the Metric System was 'invented' in 1799.

Metric system - Wikipedia, the free encyclopedia

BTW, sometime in that 125 years the inch was changed to become exactly 25.4mm.

Inch - Wikipedia, the free encyclopedia

Changed by only 2 millionths. No wonder you have trouble getting reliable results with your comparator.

We were discussing an 1891 Caracno rifle for from Italy. About 125 years old. The best international meter standards were made in the 1880s. How soon were the Italian lathe lead screws compared to those standards? Who knows? And were the threads in question even metric or some other dying standard? The taps of that time were lathe cut and wore quickly and resharpened often. Tapped holes from a tap that had been resharpened many times were tapered and will "pull" the threads of whatever is screwed in to them. Chasers even today will cut "drunken" threads, how much more so in the 1890s. On top of that 60 degree angle threads of that time period had exaggerated radii, often hand filed. Checking these on a comparator today is hard because the pitch varies from peak to peak.
KragScrewDetail.jpg
The OP might find that his is 1.75 mm or it might be 1.8mm or 14.5 tpi or some other since standard threads back then were not "standard". I have made copies of screws from that time in 1.15mm (should have been 22 TPI)25 and 30 TPI.
 
They did produce the model 1891 rifle right up into the 40's so there not all hit or miss rifles. You can say it's the most famous rifle in the world.
Wowsers! With that long a history and all the advances in machining during those years, I would guess the threading may have changed and improved during the run. Maybe that contributes to how hard it is to pin down the thread pitch on the barrel. Speerchucker might have been right when he said 26 X 1.75 but it sounded like he was calling back from memory, even if he is remembering correctly there may be others out there with slightly different pitches.
 
He was simply using the same old DeHass info.

No magic, No guru...

I have dealt with Oddball threads on a Chauchat LMG. It had inch, metric, and some oddball thread form metric French.

Since the barrels were modified Lebel rifle barrels and were old French, newer parts were a mix of metric and inch. Extra threads were cut on OD of barrel to retain aluminum cooling shroud.

Chauchat - Wikipedia, the free encyclopedia

Using a thread gage as posted several times in this thread, will quickly show what size it is...
 








 
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