Lets talk cam pins. I hear that bolts break from worn cam pins due to the extra slack between the two parts. I like to replace mine every 4000 rounds or so. Would this be within normal parameters or excessive?

Cam pins are relatively cheap and if it works for you, it works for you (confidence in your gear).

The bolt is much more likely to lose a lug (adjacent to the extractor) and should be replaced at 6,000 rounds so you could do both at the same time.

Bcm's Filthy 14 went much further than 6000 rounds before a lug sheared. Isnt that number more related to the carbine gas system? I was thinking I would replace the bolt around 10,000. If I wanted to keep one barrel just to see how many rounds I could put through it over the years, could I just keep replacing bolts as they break, as long as headspace checks out, or will the barrel extension eventually wear enough to where headspace would be too great? Always wondered this. Filth 14 made it somewhere around 60,000 rounds I think on 2 bolts and still shot 2 inch groups at 50 yards.

If you have a micrometer handy you can answer this question yourself.  No sarcasm intended.  I am curious to know the result you obtain.  I have far more rounds (over 20k) on my most used pin - there is no park left on it - but have not seen a dimensional change.  IIRC, cam pins are seriously surface hardened.  I thought bolts were, too.  But my guess is the ID on the bolt will change first.  I never thought to measure the pin hole ID, but will do so when I get a chance (using small hole gauge set).  Sam

I have measured the cam pin hole in the bolt in the past and never noticed any change, but have measured worn cam pins and there was a few times where a .001 wore off leaving extra slack in the bolt during lock up.

Thanks.  In your initial post you mentioned having heard that extra slack at the cam pin interface can break bolts.  Can you provide a reference?  

I don't know that there is a right or wrong to a 4k cam pin change.  I recollect some advice to consider bolt longevity as double that of a barrel service accuracy.  I interpreted that to mean around 40k.  I have run bolts and pins pretty hard but never broke a bolt at lug or cam level.  And I have moved bolts to new uppers with no problem a few times now.  So I have some pretty high mileage Colt and FN bolts.  

Which brings us to another observation, one made in the context of MP discussions.  Seems most bolt failures either occur pretty early on (defect), or late/not at all.  I do remember that being discussed here with some frequency.

Sam

ETA:  Just took some measurements of cam pins across bearing surface (front to back as when installed w/bolt locked).  All parts nominally Colt.  Brand new cam pin, 100% park:  .3103".  Pin with about 6k:  .3094".  Pin with estimated 30k:  .3100".

As to the bolts, you are right about the IDs.  From my most abused to brand new, both Colt and FN, all were silly close to .314".  To be fair, I had to use a small hole gauge set.  Probably a bit less precise vs direct mic of pin OD.  But no detectable differences to .001", w/measurements taken along the same axis as with the cam pins.    

What I cannot tell (wish I'd kept track of measurements done years back) is whether the dimensions on that high mileage pin actually changed over time.  Bottom line:  I am reassured by the lack of change over 30k.    

There is such a wide range for when parts are going to show problems it is probably best to pull regular TI’s and replace items when they are excessively worn or showing cracks.
For a SHTF weapon that you want to be GTG at all times then a more proactive replacement schedule may be useful.
AFAIK bolt service life will be in the 12k-15k range for a lot of people.