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Outer cam wheels smashed up
Posted: Fri Mar 29, 2019 1:07 am
I’ve lurked for a while but haven’t needed to post until now.
Some time ago when I had more money than sense I bought a beautiful all matching DOT MG-34 from TNW. It functioned flawlessly apart from some minor ejection issues solved by an 11mm booster cone and the ejector tuneup kit from Brian at BRP.
I made a mistake and decided to trade it for a scoped K43 and SVT-40.
A few years ago I got the urge for another belt fed and after some pleading with the wife got another TNW rifle. This one non-matching with an alphabet soup of manufacturer codes.
Anyways, I cleaned it up. Greased it up. Converted over to .308 with the SA (Finnish) kit. Took it out and got off about 5 rounds before it seized up. I pulled it apart and the cam wheels were completely smashed apart. I talked with Brian at BRP and he mentioned that the booster cone they ship with the .308 is more for guns with cycling issues and that maybe I need to try a large cone. So I replaced the cam wheels, put in an 11mm booster cone and tried again. The gun ran better this time. I got through about 65 rounds, but had two things happen:
The cam wheels smashed again.
The gun had some double and triple shots.
I pulled it apart (attached photos are before I cleaned it up) and noticed that the “ramps” at the front of the receiver seem to have a ~1mm bevel and I am curious if the cam wheels are striking this bevel?
Secondarily, the sear level has completely lost grip of the sear ring. Attached photo shows position of sear lever when I began to disassemble the bolt. (Side note, I called Lance at TNW and he said to just shave the sear lever down a little bit to give it a small “lip” so it can grip the ring better, is this good advice or should I get a new sear lever?)
So, basically, what do I need to fix here to get this gun up to par again?
I confirmed that the recuperator spring is incredibly strong and the barrel slides freely and effortlessly in the jacket.
Re: Outer cam wheels smashed up
Posted: Wed Apr 10, 2019 1:20 pm
I am not a gunsmith, but I know the TNW semi pretty well. I had to work through lots of issues with mine.
Unless there is an "illusion" in the photo of the receiver cam, it looks like at least one of the cams is grossly short. That results in the bolt rollers on that side being dumped into a steel wall rather than "gently" guided back to the receiver channel. The ramp on the cam should slope all the way back to the receiver channel. When I built out my TNW receiver, there was a very slight lip where the cam ramp ended just shy of the receiver channel. I beveled that very slightly to improve performance. That is not what you have--if the picture is showing what I think it is, one of your cams is WAY WAY too short. You would not bevel that. You would replace the cam. (Can you confirm that the rollers that broke were on the same side as the "short" cam?)
I attached two pictures of an original MG34 receiver with the cams intact. You can see the stark difference between the cams in it and yours. Also, comparing it to the original receiver, it may even look like a portion of the receiver is broken off next to that section of the cam. How many rounds have gone through it? Is it possible that operation broke bits of the cam and receiver off in that area? (Again, hard to say from the photos.) If you need a picture of a functioning TNW receiver showing the cams, let me know and I can take one.
Was your gun sold as a complete rifle by TNW or was it built by an individual using one of TNW's receiver kits? If TNW built this gun and installed the cam, I would talk to Lance about it. TNW has sent some heinous stuff out the door in years past but Lance is a decent guy. Also, as I re-read your post, it's been modified to 308 so it's possible something happened there that beat the crap out of the gun. (Maybe someone else more knowledgeable about that conversion can chime in.)
You should also do a a search here on the forum about the orifice size of the booster insert. There were many different size orifices out there. Some will result in too much pressure and will beat up your gun. Too little and you may have cycling problems. From what I recall, there were some inserts that raised pressure due to the cold conditions on the Eastern front making the guns freeze up. That's not necessary on a well oiled semiauto in regular warm temps (we're not shooting in -40F weather). Based on a conversation I had with Lance a couple years ago,my recollection is that TNW likes to send them out a little hot to make sure they cycle. That's something you would need to experiment with if having problems.
Re: Outer cam wheels smashed up
Posted: Wed Apr 10, 2019 1:52 pm
I am familiar with sear problems in the TNW MG34 and several things could be the cause. You said that "the sear lever has completely lost grip of the sear ring". Are you saying that the sear is releasing during the cycle--the sear arm is not staying "caught" on the sear ring as it cycles into battery? If so, that is a VERY common problem with these. It can result from several things. You need to cycle the gun by hand and observe exactly what is happening. Is the sear lever dropping into position, but then it pops out when the bolt slams into battery? Or is the lever a little long and it's not reliably dropping down to engage the sear ring fully? Keep in mind that cycling slowly by hand may give you a different sear result than racking the bolt and letting it close naturally. I have encountered that so you need to observe both (I had one that engaged fine while slowly hand cycling, but if I racked it and let it close under spring pressure, it released more than 50% of the time.)
I cannot tell from your picture which generation sear lever you have. A side shot showing where the sear arm is pinned would show that (they change the pivot point, which provides more leverage and presumably lightens the trigger pull). At one point years ago, TNW sent out batches of a newer design sear that were not heat treated properly. The inevitable result of using one of those is that the tip of the sear arm mushrooms and develops a lip that prevents it from staying caught on the sear ring. This happened to me within 100 rounds. Inspect the front tip of the sear arm.
I got replacement sears from TNW to replace the defective ones with the bad heat treat. However, the new ones were not reliably dropping into position on the ring. I spoke with Lance and he said that they had to be slightly trimmed. I have not tackled that problem yet. You don't want to do that with a power tool and take off too much material. Ask Lance for advice on how to do it. At $140 a pop, that would be an expensive mistake.
Another problem I ran into was compatibility of sears and rings. When TNW changed the sear arm designs, they also changed the profiles of the sear rings. The oldest design had a wide and relatively flat-faced sear tip--that mated with a relatively flat sear ring. The later versions of the sear had "jeweled" tips on the sears and TNW mated them with a more dished sear ring that also had a slight lip on the profile. If you accidentally use a newere sear with and older ring, it will result in slippage. I accidentally mixed up my versions and it did exactly that. I spoke to Lance and he confirmed it.
Anyway, I hope some of this helps you narrow it down.
Re: Outer cam wheels smashed up
Posted: Sun Apr 21, 2019 1:45 am
Thanks for the very thorough reply! Sorry I didn’t get back sooner, I have been out on vacation.
I can confirm there is no illusion on the cams. If we are facing the picture the cam on the left is much worse than the one on the right. Both get damaged, but the left roller has definitely taken the brunt of the damage. That absolutely makes sense to me now.
I did buy this direct from TNW back in 2015, and it was explained to me at the time they were all out of matching parts kits, so it’s all kind of different parts. I am out of warranty, but I will give Lance a call and see if there is anything we can work out.
Regarding the sear I had problems when I first bought the gun. The sear lever grips the sear until you charge it, and then about 50% of the time it will let go and fall away from the sear ring. (See side profile photos). When I first had problems Lance mentioned shaving a “lip” onto the sear lever, I gave that a try with a dremel tool, but it didn’t seem to help so they sent me this new one.
I have the 11.5mm booster cone on right now. I am fairly confident that is the right one to go with as I am using.308 and that isn’t as hot as 8mm as I understand. My previous 34 had ejection problems with the 9mm cone that TNW shipped with, but switching to the 11.5mm solved that, and that gun was only ever 8mm.
The only other weird thing I can think of is the bolt sticks sometimes just before the trigger group. I think the receiver is still pretty tight and might need further breaking in? It never sticks during operation, only when I am trying to clean it and put the bolt back in.