Sako M-591 barrel thread: 1" 16 tpi

[TRECustom]

Don't know if it'll help anyone, but I just finished a barrel for a 591. I couldn't find any info on tenon dia, thread pitch or length. Measuring the receiver was all I had. .995" dia, 16 tpi made a very nice fit. Headspace would vary from another receiver, but this one was 1.121". Also, the barrel shank had to be turned down to 1.160" to match the receiver thrust face diameter.

Tom

 

[Robert]

Don't know if it'll help anyone, but I just finished a barrel for a 591. I couldn't find any info on tenon dia, thread pitch or length. Measuring the receiver was all I had. .995" dia, 16 tpi made a very nice fit. Headspace would vary from another receiver, but this one was 1.121". Also, the barrel shank had to be turned down to 1.160" to match the receiver thrust face diameter.

Tom

Tom,

Are you sure it was not a M25 x 1,50mm thread?.

16tpi = 1,5875mm...


R.G.C

 

[TRECustom]

Robert, good question. One that I asked myself several times before committing. Some model Sako's had inch pattern and some had metric pattern threads, but none that I could find had a combination that matched the dimensions I was able to measure on this receiver.

I'm uncertain about the math, but I believe the difference from a 1" major O.D. and a 25mm O.D. is about .020". At any rate, I don't think that a .995" major (barrel) thread diameter would ever fit into a 25mm receiver thread. Also, the tenon for this receiver needed to be 1.010" long, and the accumulated error from a 16 tpi male thread to a 1.5 mm female thread would prevent it from advancing more than a couple of turns (I think). It would certainly bind in less than 16 turns. This one ran smoothly all the way with no wobble.

Because of the difficulty I had in trying to determine the exact diameter and pitch, when this combination worked perfectly, I decided to post it. It just worked too well not to be right.

I was hoping to get some feedback, and your question is a good one.

Thanks, Tom

 

[Robert]

Robert, good question. One that I asked myself several times before committing. Some model Sako's had inch pattern and some had metric pattern threads, but none that I could find had a combination that matched the dimensions I was able to measure on this receiver.

I'm uncertain about the math, but I believe the difference from a 1" major O.D. and a 25mm O.D. is about .020". At any rate, I don't think that a .995" major (barrel) thread diameter would ever fit into a 25mm receiver thread. Also, the tenon for this receiver needed to be 1.010" long, and the accumulated error from a 16 tpi male thread to a 1.5 mm female thread would prevent it from advancing more than a couple of turns (I think). It would certainly bind in less than 16 turns. This one ran smoothly all the way with no wobble.

Because of the difficulty I had in trying to determine the exact diameter and pitch, when this combination worked perfectly, I decided to post it. It just worked too well not to be right.

I was hoping to get some feedback, and your question is a good one.

Thanks, Tom

Tom,

The reason of my question was because I rebarelled long ago Sako"s who were metrics. I think I remeember the diameter was 25mm, quite small for a medium (.473 base) calibre barrel shank , but the pitch was defnitely 1.50mm.

You are correct about the fit of course. I presume you checked with a thread gauge.


Except the UN and ISO thrads, I only know the Whitworth at 55° as a possible, but his is not very common those days..
R.G.C

 

[shortgrass]

Robert, I just did some threading work on a 70's era Tikka (imported by Ithica into the USA). Didn't have a barrel so I did the best I could measuring (with a thread gage) on the tenon threads in the action, not the best of circumstances, to say the least. My lathe was already set up for metric threads, and I had figured that tenon thread was 25.5x1.5mm. Guess what, got real close, but the thread would 'load-up' when test fitting the action, telling me the thread wasn't correct. Found an old Win. 70 barrel here in the shop (1"x 16tpi) and it screwed right into the action, no bind, just like it was made for it. The strange part is, the action screws are metric, the bolt body O.D. is metric, a direction I hadn't anticipated, to say the least. (before we go crazy,,,, I had a piece of bar stock in the lathe chuck, not a barrel!) I saved the metric stub I had made and stamped the size on it, just for future referance.

 

[TRECustom]

Ted, I couldn't help but think about coincidence when I read your reply. Up until about a month ago, I had hung on to a Winchester M-70 take-off barrel that had a wrecked bore and chamber. Decided I would make a truing mandrel for small ring Mausers out of it, so completely recut everything. The mandrel worked fine, but I thought about my timing. If I had waited one more month, I would have had that barrel to try!

Cutting a trial tenon on barstock would have made sense, but I had plenty of shank on this barrel, and reasoned that if I was right, I would save several hours. If not, cut an inch off and start over.

I'm rounding up stuff now to go and test fire the rifle. It finished out very nicely. How can you go wrong with a Sako action, Lilja barrel, and McMillan stock? Should be a dandy.

Robert, Shortgrass summed up the confusion. A weird mix of the metric and inch systems. I probably didn't help with the .995" major diameter, but I like to leave a tiny flat on top of the barrel threads, so I normally will turn the major diameter .005" smaller than the nominal thread size.

Thanks to both of you for jumping in.

Tom

 

[shortgrass]

I'd say that pretty much sums it up. Another good reason to gage 'um yourself, before doing any cutting. Takes more time, saves making mistakes. I'd have sworn the threads in my lill' Tikka would have been metric, not the case this time. Thanks for posting the results you found on that Sako, Tom.

 

[TRECustom]

Robert and Ted, while we're on the subject of metric threads, I picked up a like new barrel a couple of years ago that came out of a Sako M-995, which I believe is a hunting version of the TRG-S or 42. The barrel is a like new factory .30 x .378 Wby. The thread diameter is 1.055", tenon length is .998", and the thread pitch is between 16 and 18 tpi. A (cheap) metric dial caliper gives a diameter of 26,75 mm, a length of 25,2 mm, and an apparent pitch of 1,5 mm. I don't have a metric pitch gage, so I'm not positive. I had to use the dial caliper to gage the pitch, but it's pretty close, I think. My lathe is a Romi, made in Brazil and has some unusual settings, like 16.5 tpi, and 17.25 tpi between the 16 and 18 tpi settings. I'm wondering if one of those might match the 1,5 pitch metric thread (if that is in fact what it is)? I don't trust my metric conversions at my best, and when I'm tired, I don't do anything that requires precision. Today's been a long day.

Internal threads are harder for me (I have to remember to crank the crossfeed and compound backwards), but unless one of you know what the inch equivalent of 1,5 mm is, it would make sense to cut a set in a piece of pipe bored to about .975" and see if one of the oddball pitches work with the barrel. I'll attempt the conversion tomorrow, when my one brain cell might be working.

I did get the .270 WSM test-fired. The dimensions on the fired case were alarmingly larger than the unfired dimensions of the Winchester Supreme ammo. However, when I checked the SAAMI chamber dimensions, I felt a lot better. Fired dimensions are within .001" - .003" of the chamber size. A couple longer, the diameters smaller. The factory ammo is just undersize. I did notice that it is very hot. Flattened primer, but acceptable extraction.

Later, Tom

 

[Robert]

I'd say that pretty much sums it up. Another good reason to gage 'um yourself, before doing any cutting. Takes more time, saves making mistakes. I'd have sworn the threads in my lill' Tikka would have been metric, not the case this time. Thanks for posting the results you found on that Sako, Tom.

Tom,
I have in front of me the specifications of the Sellers (UN) threads system.

‘D’ (OD) is nominal diameter. It is not unwise to have the male diameter a little smaller in order to ensure the bearing on threads sides.

This ‘D’ diameter however already includes a flat of .10825 x Pitch.

Threads heights (both female and male), to include the same .10825 flat at the bottom is .64962 x Pitch.

Particularity of the Sellers is that, in theory, a perfectly realized threads bears on all surfaces, pitch angles and top and bottom flats. In principle, then, a perfectly realised Sellers profile would perfectly centers the barrel.

ISO profile not truly same, as, in theory, it only bears on the angles. They then could be less self-centering.

Whitworth profile (Mausers 98 for instance is 55° with rounded apexes in the profile…just good enough for water pipes IMHO!!!!..

R.G.C

 

[shortgrass]

Multiply metric thread pitch (1.5mm in this case) by .0394 for conversion to inches. example , 1.5 x .0394 = .0591"= thread pitch in inches. Threads per inch for distance from crest to crest, root to root. 1 divided by number of t.p.i. example, 1 divided by 16 t.p.i. = .0625" A general "rule of thumb" formula for depth of thread (crest to root). .866 divided by number of threads per inch, multiply the result by percentage of thread (say 75% which is common) = depth of thread. example using a thread with 16 t.p.i. , .866 divided by 16 = .0541 , .0541 x .75 = .04059 not a 'perfect' formula for thread depth, but one often used on the 'shop floor' that is 'close enough'! Many of those Whitworth 'water pipe' threads have worked well for over 100 years. I've built many Mausers, using 55* Whitworth threads, that 'drive nails'. We could 'over engineer' everything to death, but lately, especially on the forums, we neglect the biggest variable of all, The Nut Behind the Trigger! The human involvement of shooting a rifle. The threads for my Tikka and for Toms Sako wil be at least as good as those 'water pipe' threads. Stocks won't be wrecked, barrels won't be 'launched' down range, and they'll be more accurate than the person shooting that rifle. Although, I do turn the major diameter on the 'strong' side before threading.

 

[TRECustom]

Robert. thanks for the info. I've never studied threads or their theory, but usually when things mechanical seem right, they are right. Logic works. I don't think it was on this site, but several of us got into a very complicated discussion about thread fits, and in particular, "perfect" thread fits. The consensus was that you just can't get there, but by cutting threads to the closest fit possible, many benefits are realized. Uniform load distribution, minimal distortion, and minimal movement under shock. Like I said, it all makes sense.

I got my brain cell kick-started this morning, and came up with these figures.
1,5 mm = .059055"

Set to cut 16 tpi, unity would be .944", accumulating a pitch error of .056" in a 1" long tenon. 56 mils would quickly overpower a well-fitted thread. No wonder Ted's Tikka bound up.

Set to cut 16.5 tpi, (available on my lathe), unity would be .9744075", accumulating a pitch error of .0256" in a 1" long tenon. 25.6 mils is still way too much error for a finely fitted thread.

Set to cut 17 tpi, (not available on my lathe), unity would be 1.003935", accumulating a pitch error of .004" in a 1" long tenon. 4 mils of error in an inch probably would be O.K.

Finally, set to cut 17.25 tpi, (which is available on my lathe), unity would be 1.01869875", accumulating an error of .0187" in a 1" long tenon. 19 mils is too much error for a properly fitted thread.

Bottom line for me is that I can only cut this thread on my lathe by switching to the metric table. A 1,5 mm setting is available, but I've been told that because I have an inch thread (4 tpi) lead screw, I'd have to leave the carriage engaged, crank the cross slide in and out while simultaneously stopping, reversing, and forwarding the lathe. If that's true, I'm not sure I have the dexterity to do it. I've considered giving it a try, but would probably turn the threading tool upside down, run in reverse, and feed away from the shoulder. At least, I wouldn't risk chewing up the shoulder and breaking a tool. Maybe.

Whew, time to give it a rest!

Ted, do you have to go through these gyrations to cut a metric thread?

Tom

On edit: Ted, by the time I finished this novel, you had posted what I was struggling with. I agree about the Mauser thread. I may talk the perfection talk, but when I've done a Mauser, I just used my 60 deg threading tool. So far, like you, they've been very accurate too. My .220 Swift is on a Yugo (Charles Daly marked), with a reworked military bolt, and parts out of the junk box, a #7 fluted Lilja and is well under 1/2 moa at 100 yds when I am. It easily outshoots me. How 'bout if I send you a pitcher? It's my gaudy gun.

2nd (dumbass) edit: changed 1' tenon to 1".

 

[shortgrass]

When using a 60* threading tool to cut 55* threads, using your threading tool centering gage, set the tool, now swing the compound to 27.5*. You will cut a 55* thread.

 

[Robert]

When using a 60* threading tool to cut 55* threads, using your threading tool centering gage, set the tool, now swing the compound to 27.5*. You will cut a 55* thread.

Shirtgrass,

Whatever you turn the compoud slide, with a 60° tool, you will always cut a 60° profile....

And the Whithworth has wide rounded crests and bottoms.

R.G.C

 

[TRECustom]

Robert, point well taken, but if you offset the compound from 30 deg in the right direction, you could get a 55 deg thrust face, but you would have more clearance off the back side I think, sort of like a slight buttress thread.

This is getting pretty complicated for an old fat man!

Tom

 

[shortgrass]

Only the 'leading' cutting edge of a single point tool cuts, watch closely the next time you thread. I dress the 'point' of my threading tool with a slight rounded profile by hand with a medium diamond lapp (I use old fashioned brazed carbide). A good 'polishing' of the finished thread with a soft polishing block 'rounds' over the crest. Not precise, but it works. I have a 55* thread setting gage, the 'point' fits perfectly in a finished thread. Advance the tool with the compound, not the cross slide.

 

[Robert]

Only the 'leading' cutting edge of a single point tool cuts, watch closely the next time you thread. I dress the 'point' of my threading tool with a slight rounded profile by hand with a medium diamond lapp (I use old fashioned brazed carbide). A good 'polishing' of the finished thread with a soft polishing block 'rounds' over the crest. Not precise, but it works. I have a 55* thread setting gage, the 'point' fits perfectly in a finished thread. Advance the tool with the compound, not the cross slide.

Shortgrss,

I lift you my hat if you are able to cut a 55° angle with a 60° cutter!!!.

As for inspecting threads I have inspected eough of them of all kinds, including GROUND ones with profile projectors, and still have access to this instrument if needed to know all what cam happen and result from forming threads.. and I use "forming' intentioally

R.G.C

 

[shortgrass]

Walk to your lathe, Robert. Set it up like you were getting ready to cut a 60* thread. Move the compound 2 1/2* , then come back and tell me the angle didn't change in relationship of cutting edge to work piece. I'm used to working with a lathe that the compound is set on 30* to cut a 60* thread, on some imports, the compound will say 60*. If you are cutting deeper with each pass by moving the cross slide further into the work (not they way I'd do it!) you get the whole profile of the tool. If you return the cross slide to a pre-set "zero" and advance the tool with the compound you are only cutting with the edge on the chuck side of the tool (when cutting a right hand thread). Before you declare me wrong, you need to try it! Cut a 'sample' and take it to your 'shadow-graph'. If we were standing in front of a lathe it would take less than 10 minutes to show you. You might inspect 'um, I've been cuttin' 'um for 35+ years,,,,,,, and they pass inspection.

 

[therifler]

Shortgrass , this was an interesting post took me back to my school days . also makes me glad i can cut just about any metric thread there is by just changing a gear . T.R.

 

[Robert]

Robert, point well taken, but if you offset the compound from 30 deg in the right direction, you could get a 55 deg thrust face, but you would have more clearance off the back side I think, sort of like a slight buttress thread.

This is getting pretty complicated for an old fat man!

Tom

TRC,

I understood that from the beginningn of the discussion but, for me, a threads must be upright in profile, not leaning one side or rhe other...unless it is an artilery thread, and I do not think this is commonly realised those days except in specific puroses..

If I had to cut a 60° thread, I use a 60° tool, and a 55 for a 55, or a trapezoidal for a trapezoidal, square for qsuare whatever.

I must add I do not like the use of the compound slide. It is the easy way BUT does not allow a close control of the threads depth, an important factor as well to respect the profile...

R.G.C

 

[Robert]

Walk to your lathe, Robert. Set it up like you were getting ready to cut a 60* thread. Move the compound 2 1/2* , then come back and tell me the angle didn't change in relationship of cutting edge to work piece. I'm used to working with a lathe that the compound is set on 30* to cut a 60* thread, on some imports, the compound will say 60*. If you are cutting deeper with each pass by moving the cross slide further into the work (not they way I'd do it!) you get the whole profile of the tool. If you return the cross slide to a pre-set "zero" and advance the tool with the compound you are only cutting with the edge on the chuck side of the tool (when cutting a right hand thread). Before you declare me wrong, you need to try it! Cut a 'sample' and take it to your 'shadow-graph'. If we were standing in front of a lathe it would take less than 10 minutes to show you. You might inspect 'um, I've been cuttin' 'um for 35+ years,,,,,,, and they pass inspection.

Shortgrass,

If you base knowlegdge by numbers of years of practise, bad or good in experience, then, I beat your 35 years by a large margin. It might be unfortunate, but I am still learning every day and enjoyed every peirot of this large span...

Now, as to 'go to the lathe'...he is iddlel since 14 years now, but in excellent condition, thank you......

R.G.C