Bolt size and thread pitch for action screws

[Flowjoe]

I’m finally putting together the 1916 WOK I picked up here as a project.

It only had the rear action screw so I picked up another original set from another member.

Cleaned and lubed all the threads first. Rear screw threads in fine. Front screw gets tighter and tighter as it goes in (not cross threaded) but will not go in full depth.

Visual inspection shows that the threads look a little off but not goobered up. Of the three action screws I have loose all thread into the rear hole and all struggle with the front hole. I’d like to chase that front hole.

A simple bolt gauge shows 6mm to be a good size and thread pitch to be metric 1.5…but…wanting to double check myself I did some googling and see that 1/4-22 is supposedly the size (confirmed on this site by a member who did some pretty accurate measurements).

Can anyone confirm that 1/4-22 is actually the best choice for this issue? If I run a tap down into the thread hole to clean it up will I need to chase the apparently good threads on the screw with 1/4-22 also or will the factory threads work in the 1/4-22 chased hole?

I have tried generously lubing the screw threads and then slowly working it into the hole past the point of resistance then backing it out and repeating. Made some headway but it ultimately bound up too hard to force.

 

[Muncher 1953]

1/4-22 is the size. ( not supposedly) The metric system was coming on line in Europe mid-late 1800s, but expense of converting machinery, compatibility w/existing products & the endless feast-or-famine cycles of the firearms industry worked against its acceptance & use in the model 98 design. There are still weird/odd/unusual thread pitches used by the firearms industry today.

If you must, run a ‘bottoming tap’ into the front action post, then holding the action in normal firing orientation, spray aerosol gun cleaner up into the hole several times to flush out any metal fragments or crud. DON’T run a die over the front screw, unless it’s numbered & matching AND the threads are visibly buggered. If it threads smoothly into the rear tang, LEAVE IT ALONE. Running it through a die will make the threads fit looser in the front post, possibly leading to pull out when tightened. Also taps & dies for cutting new threads where there weren’t any are different from thread CHASING taps & dies which are only made to ‘clean up’ existing threads. (they cut less stock) Finding thread chasing dies for an odd pitch like 1/4-22 will be like looking for hen’s teeth.

In the machine tool repair business sometimes we “fixed” rough acme threads by using 1200 grit lapping compound on the threads & nut & runing them back & forth against each other, but it’s easy to ruin stuff when you lack experience.

 

[Flowjoe]

1/4-22 is the size. ( not supposedly) The metric system was coming on line in Europe mid-late 1800s, but expense of converting machinery, compatibility w/existing products & the endless feast-or-famine cycles of the firearms industry worked against its acceptance & use in the model 98 design. There are still weird/odd/unusual thread pitches used by the firearms industry today.

If you must, run a ‘bottoming tap’ into the front action post, then holding the action in normal firing orientation, spray aerosol gun cleaner up into the hole several times to flush out any metal fragments or crud. DON’T run a die over the front screw, unless it’s numbered & matching AND the threads are visibly buggered. If it threads smoothly into the rear tang, LEAVE IT ALONE. Running it through a die will make the threads fit looser in the front post, possibly leading to pull out when tightened. Also taps & dies for cutting new threads where there weren’t any are different from thread CHASING taps & dies which are only made to ‘clean up’ existing threads. (they cut less stock) Finding thread chasing dies for an odd pitch like 1/4-22 will be like looking for hen’s teeth.

In the machine tool repair business sometimes we “fixed” rough acme threads by using 1200 grit lapping compound on the threads & nut & runing them back & forth against each other, but it’s easy to ruin stuff when you lack experience.

Thanks for the feedback.

I’ve been restoring old cars since the mid 80s so, while not a machinist, I do have some experience dealing with less than perfect nuts, bolts and blind threaded holes.

I said supposedly because I couldn’t find any definitive, official designation that stated that was THE size. Mostly a lot of arguments about size and thread pitch. The most authoritative source was the thread on this site (from years ago https://www.k98kforum.com/threads/trigger-guard-screw-thread-size.7831/) which normally I would take as gospel but there was quite a bit of back and forth in that post so I wanted to confirm.

Since some sources argued that 1/4-22 was “close enough” I thought it might not be the exact size which means the screw would now be off from the receiver hole I just chased (tapped) creating a new problem. I think I might have worded my initial query poorly.

And I agree, would prefer to chase the threads since they don’t seem that far off (to the eye) but so far a 1/4-22 chase has been, as you say, a hen’s tooth. So it looks like a tap will have to do.

Thanks also for all the tips. I’ve used plenty of lube while attempting to gently work the screw into the existing threads and cleaned the threads on both screw and blind hole repeatedly. Will do the same with the tap when I have it. Slow and steady on something like this to be sure.

 

[Peter S]

The thread for the action screws is 1/4-22 British Standard Whitworth. British Std threads have a different form than what is used today.

These are drawings that had been previously posted on this forum.

 

[Muncher 1953]

The thread for the action screws is 1/4-22 British Standard Whitworth. British Std threads have a different form than what is used today.

These are drawings that had been previously posted on this forum.


View attachment 452711View attachment 452712

BINGO! Thank you, I knew I was forgetting something.
IIRC, the difference is in the angle of the thread (steepness of the sides of the ‘pyramid’) SAE & metric are 60 degrees, BS-W & Italian threads are 55 degrees, but also Whitworth threads come to a finer, thinner point than other standard thread profiles, like the use-one-time NPTF profile for fuel fittings. A truncated peak to the thread profile (nip off the mountaintop) improves initial assembly & subsequent assembly/disassembly cycles.

 

[Flowjoe]

The thread for the action screws is 1/4-22 British Standard Whitworth. British Std threads have a different form than what is used today.

These are drawings that had been previously posted on this forum.


View attachment 452711View attachment 452712

I saw those in the thread I linked. I’d also run across references to the whitworth thread pitch versus SAE.

Any of the taps available today would be 1/4-22 SAE which would be a different pitch than the original whitworth pitch. Would it still be advisable to chase the receiver’s blind hole with the 1/4-22 SAE tap?

I would think that running the SAE tap into the hole would make it interface poorly with an original whitworth threaded action screw. Or is the difference slight enough that it could still be threaded together with minimal effort?

 

[biggymu]

No you can purchase 1/4-22 BSW taps online. Do a search and include Mauser
Just a for instance

Mauser trigger guard screws, tap & die set 1/4 inch x 22 TPI Whitworth | OZZISTRAIGHTSHOOTER

Ozzistraightshooter Tap and Die set for Mauser trigger guard screws Thread is 1/4 in x 22 Threads per inch. 55 degree Whitworth. Top quality High speed steel. Mauser trigger guard screws are sometimes stripped or damaged. You can make new screws or clean up the threads on...

www.ozzistraightshooter.com

 

[Flowjoe]

No you can purchase 1/4-22 BSW taps online. Do a search and include Mauser
Just a for instance

Mauser trigger guard screws, tap & die set 1/4 inch x 22 TPI Whitworth | OZZISTRAIGHTSHOOTER

Ozzistraightshooter Tap and Die set for Mauser trigger guard screws Thread is 1/4 in x 22 Threads per inch. 55 degree Whitworth. Top quality High speed steel. Mauser trigger guard screws are sometimes stripped or damaged. You can make new screws or clean up the threads on...

www.ozzistraightshooter.com

Will do.
Thanks

 

[Flowjoe]

Plenty of 1/4-22 taps out there but very few - including Brownell’s - call out if they are BSW threads. Also lots of 1/4-20 & 1/4-26 BSW taps (mostly aimed at British motorcycle enthusiasts).

Ozzi Straight Shooter seems to be the only place that carries the 1/4-22 size in BSW.

Looks like I’m ordering a tap from Australia

 

[biggymu]

I was just perusing their catalog. Quite a jackpot of very obscure gun smithing taps and dies. Never ran across them before.

 

[Flowjoe]

I was just perusing their catalog. Quite a jackpot of very obscure gun smithing taps and dies. Never ran across them before.

I’d run across their website during my initial googling but it didn’t seem like I really needed to order this tool from Australia - there had to be alternatives. Little did I know, eh?

They do have a lot of stuff. As you say, jackpot. I’m a little surprised that that no one else carries this sort of thing. At least that’s how it seems.

 

[Muncher 1953]

It might be worth contacting both Brownells & the tap maker for clarification. Its clearly not marked BS W, a bit surprised at Brownells, but the “cut & paste disease” is rampant across the internet. When I select the 1/4-22 tap, some of the product photos are of an 8-32NC tap……

 

[Fal Grunt]

1/4-22 taps are literally rarer than hens teeth. I had a batch made about 10 years ago because they simply did not exist. I have never found a source in the US or the UK.

Brownells taps are garbage, their bolt shroud taps are completely wrong geometry. I wouldn't buy or use any of their taps. They have really gone downhill in the last 20 years.

Send me a message with your address I'll send you a loaner. Throw some cash in on the return to cover shipping.

 

[Flowjoe]

1/4-22 taps are literally rarer than hens teeth. I had a batch made about 10 years ago because they simply did not exist. I have never found a source in the US or the UK.

Brownells taps are garbage, their bolt shroud taps are completely wrong geometry. I wouldn't buy or use any of their taps. They have really gone downhill in the last 20 years.

Send me a message with your address I'll send you a loaner. Throw some cash in on the return to cover shipping.

Thanks man. That’s really decent of you.

And I appreciate the feedback on Brownells - I was going to reach out to them, as suggested above, to see if I could gain gain clarity on whether or not the pitch was BSW or not. Won’t bother now.

PM inbound.