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| Ideas on dealing with bolt kick up and worn bolts http://www.yesterdaysweapons.com/phpBB3/viewtopic.php?f=23&t=8696 |
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| Author: | Gun Nut [ Sat Jan 02, 2010 1:38 am ] |
| Post subject: | Ideas on dealing with bolt kick up and worn bolts |
While fitting my replacement bolt body several things became apparent that I'd not been able to figure out till now. First off is Bolt kick up on firing. This is usually not going to be a problem since when the charge ignites chamber pressure would prevent the bolt from moving. But when old Milsurp ammo is used the well known tendency to hang fire can allow the bolt handle to kick up before the charge ingites, reducing the effective locking lug contact before pressure is applied. I've traced the phenomena to its energy source, the vibration of the striker hitting the rear opening of the bolthead is transfered to the bolthandle, the handle when in contact with the action strap is bounced upwards by the vibration. I discovered this when I found that if the bolt handle root is just a hair above contact with the strap, no kick up occurs even if bolt head fit is very loose. If the bolthead is a good fit without sideplay and the extractor spring pressure in its cut out strong enough then the bolt body is pressed againts the inside of the bolt track which gives a slight but adequate resistence to the turning motion. This nomally prevents noticable bolt handle kick up. But even then theres usually a few hundredths of an inch movement. Just enough to see daylight. In very bad cases I've seen the bolt raise enough to reduce locking surface contact by 1/3 or more. This I found is due to the cam of the cocking piece being too far from the matching surface on the bolt body when fully forward on firing. The length of the rear end of the striker shaft extendin from the collar while in contact with the rear of the Bolthead and how far it is threaded into the cocking piece governs how far forward the cocking piece cam will be on firing. I found that by threading the shaft in one full turn the distance the bolt handle could rise in kick up was greatly reduced. When I installed the same firing pin and cocking piece in the new Bolt Body and Bolthead, I found the new head allowed the pin to extend further from the face (the tip of the pin will need to be shortened slightly now), and the distance between camming surfaces was reduced even more, allowing only a tiny amount of turning motion before there was contact. So in the end I figure that if one has an otherwise seviceable yet somewhat worn bolt and bolthead that displays un nerving amount of bolt handle kick up, reducing the distance between the cocking piece cam and mating bolt surface should greatly reduce kick up. A better fitting bolt head without side play can also greatly reduce kickup. So long as the extractor spring does its job, replacing it would be only a temporary gain, not addressing the actual causes. Excessive extractor spring pressure accelerates wear anyway. |
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| Author: | A square 10 [ Mon Jan 04, 2010 6:53 pm ] |
| Post subject: | Re: Ideas on dealing with bolt kick up and worn bolts |
this is not an uncommon thing in general on a minor scale but im not the one who knows that much about this kind of thing but neil may be able to help |
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| Author: | Gun Nut [ Tue Jan 05, 2010 8:29 pm ] |
| Post subject: | Re: Ideas on dealing with bolt kick up and worn bolts |
Later on I may send my numbered to the receiver bolt body off to a gunsmith that has the resources to fit it with a bolt head that has larger than normal threads. With the variations in tolerances of wartime production I'm pretty sure there are some like that to be found. As is I'm pretty well pleased with my replacement bolt body and head. I managed to knock a couple of thou off the headgap, so reloading should prove fruitful. The cases didn't form annular rings with the old bolt, I think it would measure about .068-.069, the new bolt probably has .067 or very close to that. Under .070 seems good enough for reloading, so long as the chamber diameters aren't loose and are in the round. A very slick chamber wall also reduces stretching, or more precisely smooths out the stretching to reduce or prevent case wall thinning and cracking. |
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