C158 Bolts vs. MPI

[Wake27]

So C158 is nothing special, i.e., not a material "better than milspec?"

It's all hype. PSA is just proud that they do what everybody should do in the first place.

Correct, it is the Mil-Spec material.


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[Todd.K]

There are a lot of "not C158" bolts out there.

BCM and a bunch of others do it because it's the spec. It's the spec because the military had always done it that way and required it.

[Doc Safari]

Understanding that magnetic particle inspection is part of the process, how important is it really?

What percentage of bolts made of the proper material get rejected during the MPI?

[Sry0fcr]

As mentioned C158 is TDP as is HPT/MPI. However, I find that HPT/MPI to be an overblown issue and could easily justify batch testing or dispensing with it altogether depending on the rejection rate. It's done that way because that's what the TDP and contracts say to do, most other are just following suit.

[Sry0fcr]

Understanding that magnetic particle inspection is part of the process, how important is it really?

What percentage of bolts made of the proper material get rejected during the MPI?

I dunno. Usually companies are pretty tight lipped about internal non-conformances.

[markm]

I dunno. Usually companies are pretty tight lipped about internal non-conformances.

Yep. And I'm sure it varies between batches/runs. Could be high for one batch, and almost non-existent in another.

[Doc Safari]

I have a PSA AR pistol that the bolt is marked C158 MPI.

A friend's is only marked C158.

I wonder what the service life, on average, is for one of the PSA bolts.

[VIP3R 237]

HP can fatigue the bolt, but mpi should be done on every bolt, and won’t fatigue it. Of course this takes time (not much, we do mpi testing on our parts) and time = money and the budget companies will only batch test.

As mentioned C158 is the spec, although 9310 is considered a substitute. However most of the recent bolt failures are because of improperly heat treated 9310 bolts. C158 generally doesn’t have this issue.

[alx01]

Bolt needs to be Carpenter 158 with MPI/HPT. Period.

Depending on the manufacturer they might or might not mark it as such - Aero is a good example. Their BCGs (both nitride and phosphate finish) are MPI/HPT, but only marked MPI. This has been confirmed by them multiple times.

As indicated above some manufacturers, who feel confident in their processes and manufacturing, do not HPT their bolts (KAC and LMT mentioned). Others might do it as a cost cutting measure.

I prefer Carpenter 158 with MPI/HPT, there isn't much money to be saved on buying 9310 bolts or without MPI or HPT if you take into account a lifecycle of a quality bolt.

[Todd.K]

You can do MPI without HP, so a bolt marked MPI may or may not have been HP'd before the MPI.

If you stick to known quality, C158, phosphate bolts I would not worry about the marking or testing.