An FN oddity

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81police
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An FN oddity

Post by 81police » Sat Dec 03, 2011 12:57 am

I took apart an F.N. 1900 tonight to do a little research. Found some interesting things but what was unusual was when taking the barrel assembly apart.

The barrel nut and jacket bushing were surprisingly loose. And as I removed the other components, strangely enough the recoil spring case was acting as the barrel nut washer, in fact it had been shot enough there were imprinted serrations from the barrel nut's recoiling force against it! And the actual nut washer wasn't a typical part, it did not have any serrations/teeth just a flat smooth washer and it was used as a spacer between the recoil spring case and the buffer spring. Very odd I thought to myself, so I pulled out an old F.N. manual thinking perhaps the Belgians did it differently. Nope the F.N. parts are arranged identical to the 8/81. Made me wonder how long this rifle had been reassembled incorrectly! It must have worked anyways. I'll post some pics in a few days.

I'll definitely put in a new nut washer and reassemble it correctly this time ;)
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Re: An FN oddity

Post by DWalt » Sat Dec 03, 2011 11:06 pm

I'm a little confused - what is a recoil spring case? Mine have only the washer between the spring end and the nut.

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Re: An FN oddity

Post by 81police » Sun Dec 04, 2011 8:08 am

It's part number "35" in the list below. Typically this case rests against the buffer spring and upon full recoil the recoil spring itself is compressed within this case as the barrel nut washer and nut drive down. With this rifle however those parts were flipped and part number "35" was acting as the barrel nut washer. Pictures to come for a better explanation.

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Re: An FN oddity

Post by DWalt » Sun Dec 04, 2011 9:59 am

Very interesting. The reason for my confusion is that neither of my 81s have that part in place, and the recoil spring just sits directly on the buffer spring. Is there anything critical about the presence or absence of it? I guess I can't see what function it is intended to perform, but there must be one or it wouldn't be there. I'm sure I could make a pair easily from sheet metal if it's something necessary, if I knew the dimensions.

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Re: An FN oddity

Post by 81police » Mon Dec 05, 2011 4:10 pm

Well the recoil spring case prevents over compression of the recoil spring, and probably extends the life of the spring. The recoil spring is compressed to a total length of about 2-2.5" inside the case and when the barrel nut washer hits the recoil spring case, then the buffer spring is compressed. If there was no case and the spring was completely compressed/mashed against the buffer spring, I'd imagine that would definitely shorten the life of the spring. Replacement spring cases can be found.

The picture at top was how this FN's barrel assembly was put together (incorrectly). The picture below it is the correct order.
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Re: An FN oddity

Post by DWalt » Tue Dec 06, 2011 8:00 pm

It seems the case diameter is about the same as the buffer spring. I guess I could scale it from the picture, but how long is the case, and how thick is the case wall? I may try to make up one. I still don't see its presence or absence as making much difference, as the spring is limited in its movement by the barrel, but John Browning was probably smarter than I on such things. The Model 1911 .45 has a recoil spring cap that does sort of the same thing, but there in nothing inside the .45 recoil spring to confine it.

It doesn't appear that the barrel travel is long enough for the barrel nut washer to hit the recoil spring case at the end of the stroke.

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Re: An FN oddity

Post by 81police » Wed Dec 07, 2011 9:37 am

It doesn't appear that the barrel travel is long enough for the barrel nut washer to hit the recoil spring case at the end of the stroke.
That's because this photograph isnt showing how everything truly sits inside the jacket. All those components are actually crammed in the jacket to about half the length of what you see. The barrel nut washer and the recoil spring case are definitely coming in contact with each shot (given everything is working properly). I'd imagine the recoil spring wasn't designed to be completely compressed against the nut washer and the buffer spring, probably not good on it.

I know Numrich has one in stock for the 25, 30, and 32 rifles.
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Re: An FN oddity

Post by DWalt » Wed Dec 07, 2011 12:24 pm

The presence of the spring case makes a little more sense if the barrel nut and washer contacts the spring case, in that recoil energy near the end of the stroke would be directly transferred to the buffer spring via the case rather than through the compressed recoil spring column, which would ease the stress on the recoil spring a little as it would then not be fully compressed at each shot.

I would think that the spring case would likely be the same length for all calibers, unless there was a significant difference in the length of the buffer spring for the heavier vs. lighter calibers. What's the story on that?

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Re: An FN oddity

Post by 81police » Wed Dec 07, 2011 12:53 pm

I would think that the spring case would likely be the same length for all calibers, unless there was a significant difference in the length of the buffer spring for the heavier vs. lighter calibers. What's the story on that?
I haven't examined a whole lot of recoil spring cases, but I imagine it's not the length that's different but the inside diameter where the barrel goes through. Model 8's (and 81's) had differing barrel diameters for the 35/300 and the 25/30/32. Surely you could open up the diameter and make one work for any caliber. I imagine outside diameters would be the same to correspond w/ the barrel jacket.

Hope that helps.
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Re: An FN oddity

Post by imfuncity » Wed Dec 07, 2011 5:36 pm

I only have a few 8-81s but the mic indicates that the springs and recoil buffer also have different dimensions.
Though defensive violence will always be “a sad necessity” in the eyes of men of principle, it would be still more unfortunate if wrongdoers should dominate just men. - St. Augustine

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Re: An FN oddity

Post by 81police » Thu Dec 08, 2011 8:00 am

Well there you go, question answered. Once again Mitch comes through!

Side note: in the photograph I posted the barrel nut is not the correct part. I've observed this part in two other FN's and I think it is a spacer between the buffer spring and jacket reinforcement. This is a part the Model 8 does not have. Anyways whomever assembled the jacket last left out the barrel nut washer. So I'll have to get a replacement to make it right. The "Correct" way for the FN's to be assembled would have that washer behind the buffer spring which I did not picture! Ok I"m done. :D
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Re: An FN oddity

Post by DWalt » Thu Dec 08, 2011 1:00 pm

Might be interesting to explore the dimensional differences (with measurements) and interchangeability of those parts which are different in M8/81 rifles of different calibers. Would probably be valuable in answering questions like "Can I use a (part name) from a (caliber) rifle in my (caliber) rifle?" I am assuming that most if not all parts of the .35 Rem and .300 Savage rifles would interchange with each other, but not necessarily with the .25s, .30s, and .32s. Also M8 vs. M81 parts. Has anyone done that?

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Re: An FN oddity

Post by DWalt » Fri Dec 09, 2011 12:00 pm

Amazingly, I found a spring case for my 1949 M81 in .300 Savage locally and installed it. I took the opportunity to measure some barrel assembly dimensions as I had to take it apart. Here they are:

Recoil spring:
Free Length - 10-3/4" (of course, this one has been in place a long time, so a new spring could well be longer)
OD - 0.732"
Turns - 30 (including one end, see below)
Wire diameter - 0.055"
On mine, there are two close turns on one end, none on the other.

Buffer spring:
Free length - 2.165"
OD - 0.870"
ID - 0.626"

Spring case:
Length - 1.729"
OD - 0.883"
Hole in rear end - 0.612" dia
Wall thickness - 0.064"

Barrel:
OD is 0.597" - 0.598" from behind muzzle threads to a point 10" from the muzzle. It then tapers outward to the chamber, which is about 1.75" long, measured from the joint with the barrel extension. Chamber OD is 0.882"

An observation - as 30 turns of the spring (30 X 0.055") is 1.65", the spring case is approximately 0.07" longer than a fully compressed recoil spring, accounting for the spring case base thickness.

If anyone else has similar dimensional data for other calibers, it might be helpful to someone if you posted it.

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Re: An FN oddity

Post by DWalt » Fri Dec 09, 2011 1:34 pm

One more thing about a missing barrel nut washer. I was also missing one, and made one up, fairly simple to do. I used a bench grinder (to reduce OD) and a Dremel tool (to increase ID), starting with a washer for a 1/2" bolt. Only took a few minutes to do. Anyway, it's not like the factory washer as it has no anti-turn protrusion for the thread slot, and it does not have all of the little notches. I originally drilled and threaded a hole in the barrel nut for a setscrew. That worked for awhile, but it kept backing out. Next step was to wrap a few turns of Teflon plumber's thread tape around the barrel threads, then screw on the barrel nut tightly. That seems to work fine, tightens the fit, and no indication yet it is coming loose with firing. It's good to do that even if you have the correct washer, as it ensures the nut and barrel threads will never freeze up in the future.

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Re: An FN oddity

Post by imfuncity » Fri Dec 09, 2011 2:03 pm

Love the Teflon tape - excellent idea. Several have reported the freezing up issue! (And others have reported rifles that have had various attempts to keep the nut on, including welding it on!!) Good on you - think this should be posted on the maintenance thread too.

Also, my measurements agree with yours – 35Rem and 300Sav parts are for the most part interchangeable, as are the 32, 30, 25Rem.
Though defensive violence will always be “a sad necessity” in the eyes of men of principle, it would be still more unfortunate if wrongdoers should dominate just men. - St. Augustine

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