virgin romy barrels and G kit spec question

guys,

have a 86 and 78 G kits and 2 virgin romy barrels.

78 G kit is stamped 2 and one barrel is 2-2-2-2

i mic'd everything twice and confirmed with a 2nd set of calipers.

the 78 kit and 2-2-2-2 barrel

fsb -.005
gb- -.004
rsb -.004
barrel od -.004

the 86 kit (not nearly as nice as the 78 kit)

fsb -.0015
gb -.003
rsb -.0045
barrel od -.0075

SOOO.. looks good? looks like i will have to turn down the barrel on the 86 kit?

so the question is... what are acceptable tolerances for a press fit for an AK?

I find .0015 +.0005  to be the minimum that gives a snug fit.  I usually start at .0020 minimum or larger, and try an install.  Usually 1/3 of the way is enough to know if it is good, or if it is too tight and needs to come off again and spin the barrels with some 320#.  It is as much feel as anything else.

That the numbers on the barrel may match the numbers on the parts, but some parts press off easy and others sound like a gunshot, suggest that even at the factory it isn't an exact science.

thanks for the reply.. theres no spinning.. its all by hand lol..

got all of the 86 kit to .003.. trying to figure if i should proceed.. it really seems (my opinion) that .002 is a sweet spot.  my concern is im doing a lot of hand work.. and its not near as precise as machine work..

im thinking to error on the large side.

"It is as much feel as anything else"

i think you hit the nail on the head

thanks for the help

I use steel pipe with a (replaceable)  brass reducer to the barrrel diameter screwed on the end.  An air hammer fitting on the other, but int he pre-air hammer days I put a cap on the other end to smack with a hammer.   You get a lot easier movement with the vibration the air hammer induces, so I can have a tighter fit without galling than the same deegree of interference with a hammer.

Presses - I won't do a vertical press as I think it has too much risk of bending the barrel.  Just can't control force.  I do use to remove stuff, but I get really uncomortable with the RSB and a tight fit.  Seen too many barrels slightly bent.

Presses - I won't do a vertical press as I think it has too much risk of bending the barrel. Just can't control force. I do use to remove stuff, but I get really uncomortable with the RSB and a tight fit. Seen too many barrels slightly bent.

bingo.. JUST bought a press.. welded it together.. hated the bolts.. ive been leary of presses since the 9th grade when i bent cut to length pushrods..

ive got a compressor.. may do the air hammer route

just mocked up the barrel assby

GP.. your spot on.. i turned everything to .0002.. the fsp was .0015 already.. using engine assbly lube everything went on SMOOTHLY.. on my 86 i will try .003

the air hammer is a MUST have.. along with a brass punch..

thx for the help

It is an exact science Mark as far as interference fits go.

http://www.mitcalc.com/doc/tolerances/help/en/tolerancestxt.htm

What you may see is incompatibility of selection groups between manufacturers or even within the same manufacturer from different time periods. Selection group guarantees proper interference fit for parts manufactured to less tighter tolerances. Statistically with enough pieces made that will all even up. You get me right? That is me Polish Guy btw.

I did the allthread method on my build. For me it was far easier to press the barrel in too far, then tap back out with a 3lb hammer, bolt and a brass nut while checking headspace.

The 12T press definitely has its place.

GP- I found your AK notes invalueable. Too bad I started my build before discovering your site. Thank you.

Posted Via AR15.Com Mobile

[b]
That the numbers on the barrel may match the numbers on the parts, but some parts press off easy and others sound like a gunshot, suggest that even at the factory it isn't an exact science.[/quote]

It is an exact science Mark as far as interference fits go.

http://www.mitcalc.com/doc/tolerances/help/en/tolerancestxt.htm

What you may see is incompatibility of selection groups between manufacturers or even within the same manufacturer from different time periods. Selection group guarantees proper interference fit for parts manufactured to less tighter tolerances. Statistically with enough pieces made that will all even up. You get me right? That is me Polish Guy btw.

[/quote]

Hey WJ.

what I am referring to is the number codes on the barrel and the parts on the barrel don't seem to match - a 3-2-3-3 barrel with a 3-2-3-3 set of components, may have the front sight glide off with relative ease and a 3x air hammer,  and the gas block release after heavy pressure in a 12 ton press, sounding like a gunshot.

So maybe it isn't so much the interference fit being an exact science, but the arsenal dimensioning of the components that isn't an exact science.