Gunco Forums banner

Barrel Pin Interference Fit???

3420 Views 13 Replies 6 Participants Last post by  loner42
What size interference fit should the barrel pin to hole be?? .0015"-.002"???

I have to oversize one that I have headspaced and I have a .302" barrel pin, but I am not sure what size to drill/ream the hole. I am hoping that this size will clean it up properly without having to go up to the .312". Either way though, I am not sure how much under the barrel pin size to drill/ream the hole. TIA,

1 - 14 of 14 Posts
I don't know where I got my figures, but my notes say .001-.0015.

Theoretically even .0001" would be enough, except the trunnion is harder than the barrel and the hole winds up out-of-round, so you need a little more interference to take up any slack.
That's about what I figured. Thanks TRX!!!
Drill smaller first, measure and then drill a little larger....repeat till you get 2 or 3 thousandths interference. If you go for the exact size to start off with, the slight drill wobble will take up .001 or 2 and you'll be able to push the pin in by hand.
IMO, using a reamer for the final cut will minimize the out-of-round condition due to differing hardness. A drill is made for making holes; a reamer is made for making circles/cylinders.
Thanks for the advice 555th.

I'm going the way of Sjohnson and drill to a .297" then final ream it to .3005" for a .302" pin.
It's usually cheaper and faster to turn an oversize pin to the desired size than to order a reamer.
Standard machine shop practice is to drill the hole .010-.015 under the reamer size. A .0005- .001 press fit is all you would need for the barrel pin and everything else on an AK.
Anyone who thinks they can drill a perfectly round hole and control the size to within .005- .001 is just not being realistic, especially with a cheap Chinese drill press . That is what reamers are for.
what makes you guys think that the hole needs to be perfectly round?? as long as the pin fits tight its not going to make a bit of differance. any thing from .0003- .0001 will work just fine. I simply use a 7mm drill bit that fits easly through a staandard trunion hole and drill the hole after I flat spot it with a small end mill. I have yet to have a barrel move or a pin not go in tight. I seriously doubt there reamed when new.

BTW .015 is awfull tight for a hole that small IMOO
A .0001-.0003 fit is not enough IMHO, or did you mean .001-.003 ?

I would also seriously bet that they ARE reamed from the factory because that is the only way to control the hole size in a production process.

If you have good luck in just drilling your pin holes with a drill then have at it. All I'm sayin is you cannot control the hole size consistently with just a drill. Try drilling 10 holes in a piece of scrap bar stock about 3/4 in. thick and then check the fit on all of them with the same pin. I can pretty much guarantee there will be some tight and some not so tight or tighter than others.

The .010- .015 is a reference of how much smaller to drill the hole before reaming it, not drilling it .015 smaller for the pin. :headspin:
.0003 to .001 is what I meant. a .002 press fit on a 3" hole on hardend steel is a tight fit let alone on a little 7mm hole

trying to shove a softer pin into a hardend part with will only distort the pin more than nessacary.

I guarentee that if i take a bit that that drills a hole .0005 smaller than my pin and use it in a mill or any good drill press all the pins will be tight. if there not then then its time to buy better equipment or change your drilling meathod. unless your using a floating reamer holder the equipment is going to still cause run out. I can tink of thousands off applications were a dowel pin is press fit into a hole that is drilled not reamed. take any engine apart and you will see some. a press fit in hardend steel hole that small does not requre any were near .002" . .002 is tight on a 23mm barrel bore let alone a 7mm hole.

personaly I dont want that much metal distortion on the softer barrel above a perscion cut chamber. its the barrel that will compress to allow the pin to go in and and it will aonly induce stress on the barrel and affect accuracy and as well as slightly disort the chamber. the pin need to only stay in securaly

the main thing is the barrel is correctly head spaced before you drill and that the bit is not removing metal from the trunion. any hole in fact if the barrel portion of the hole was .000 the pin is still going to be tight in the original factory drilled or reamed trunion hole. the soft barrel is not what holds the pin in its the trunion. measure a virgine trunion and a new pin if you want a rough Idea of what the factory used as interferance. I come up with aprox .0005" as a average but check it your self. there are industry recomedations for press fits of bearing races and pins for differant metals per bore size find them and you will see nothing that requres a two thousands press fit n a 7mm hardend steel hole.
See less See more
I seriously doubt there reamed when new.
None of mine were reamed. I've been a machinist long enough, I figure I can tell...
If I was really concerend that a .0003 press would come out ( and im not) Id simply stake the pin on on both sides or peen it over. as long as its not loose and moving causing wear it will be all good.
:deadhorse Whatever................................Looks like the OP will have some sorting out to do as far as what are the best numbers to use.
I offered up my advise to the OP and others who do not know what sizes or methods to use based on doing press fits and all other aspects of precision machining every day , 40-60 hrs a week as a Manual Machinist for the last 44 yrs.
If drilling only, instead of drilling and reaming works good for you ,then knock yourselves out. If you want to take the chance of drilling the hole oversize and having a barrel pin that you can push in with your fingers to save 15-20 bucks on a reamer that you can use over and over on multiple builds, then be my guest.
Happy building......................................
1 - 14 of 14 Posts
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.