I am starting to have problems with my sear and the sear surface on the bolt of my Sten peening causing them to slip and the gun to run away or double and triple in semi. I have refaced both sear and the bolt before and it will hold for a few thousand rounds and then need to be redone again.

The issue is I am basically eyeballing the angle of the surface using a vice and a file to cut the metal back.

I am wondering what the optimal angle is for a sear on an open bolt weapon so that way I can go over to a buddies house and mill the sear and bolt. Or am I at the point now where I just need to buy a new bolt in Sear?

I get the impression that these parts originally were hardened and then I may have now gotten below the hardened surface of the metal.

At this point wouldn't it be easier to store away the original bolt and sear, and replace them?

It certainly will and that's what I plan on doing but I have a hard time putting the originals in the scrap metal bucket.

I wouldn't throw them away.  Just coat them with some oil or grease and store them away.  Good to keep all the original parts of the gun.

Are you using the trigger correctly?
Open bolt machine gun sears will be damaged by the shooter using a traditional slow pull like one would do during accuracy shooting. This causes the sear to only partially engage and thus a large load is placed on a small area. This will peen and damage the sear faces.

When shooting an open bolt gun the trigger should be on or off basically being jerked into position. It makes for crap trigger finger memory but will keep the gun from being damaged.

When ever I let a new comer shoot one of my machine guns I make sure they know to pull the trigger immediately all the way to the rear when firing and let off completely when not.

The other issue maybe poor heat treatment during manufacture. It was a war after all and they were pushing them out fast. The bolt or sear might ahve been case hardened and the sear contact surfaces may have been dressed enough to be through the case hardening and are into the soft material.

I suggest getting spares and using them.

I do treat the trigger more like and "on" and "off" switch for the exact reason you mention and when we have MG ranges at work I instruct firers to do the same thing.

The only time I stray from that is when shooting semi auto.

I believe, as you mentioned, that the sear surfaces I have refaced are now below the case hardened surface. I have refaced them two other times over the last seven years I have owned the gun and this is the first point in time where I have felt they are unsalvageable at this point. In fact I have already replaced the parts with my spares and ordered more. I plan on saving the originals to have recasehardened at some point if the opportunity presents. That is why I was asking about what the ideal sear angle is in another thread so I can set that angle and put them away until I can have the parts re treated. Sten parts are not getting any cheaper. A parts kit used to cost what a bolt does today.

One other concerning issue which is probably unavoidable is because the sear engages the bolt on the right side (due to the necessity for space in the center of the receiver for the semi auto tripping lever) there is asymmetric stress on the sear pin which means the right side sear pin hole has become slightly more elongated on the right side causing the sear to sit slightly crooked and resulting in the right edge of the sear and bolt sear surface to peen first.

This is frustrating but what do you really expect in a 31 + year old gun which I have put at least 15-20k rounds through? It was a gun that was originally designed to last for the duration of a single war which was cheap enough that they were probably thought of as fairly expendable.  

Isn't this just a great example of one of the frustrations of being an MG owner? Seeing the cost of MG's and the exclusiveness of their ownership I guess most other gun owners would consider these issues to be "gold plated problems" as a friend of mine calls frustrations that don't really matter to life's big picture.

this, My seared MP5 still is not running right. Most will say the HK family is the Gold Standard for mgs, but when they don't run. it is not that fun.