Grade 300 bolt?

[Jeff B.]

Dr. Rebuild advertises a grade 300 bolt as correct for certain applications such as the differential attachment bolts and cushin bolts. I can't find any reference to a grade 300 on the internet or the NCRS archives. Is this a GM standard, SAE or ASTM standard? What are the head markings for this grade?

[Larry M.]

Re: Grade 300 bolt?

Jeff:

I believe that it is an ISO grade standard. Google "ISO bolt strength and grade" for more info.


Larry

[Jeff S.]

Re: Grade 300 bolt?

Jeff

The '300' refers to a GM specification number for the chemistry and metallurgical charactertistics of the steel in the fastener (bolt, in this case).

Recall that the well-known SAE Grades (2, 5, 8 most common) with their characteristic radial-line head markings (none, 3, and 6) represent hardware that meets certain minimum strength (yield, tensile) requirements under tensile loading. There are no strict chemical limits or ranges that must be met under this system -- rather, the fastener's chemistry is bound only by phrases like "medium carbon alloy steel, quenched and tempered", to use the Grade 8 example.

My understanding of GM300 is that it imposes chemistry tolerances on the material that their Grade 8 bolt suppliers must use. I've heard that GM300 is a Ni, Cr, Mo alloy with about .40% C which is VERY similar to AISI 4340 steel. This makes sense, since the minimum strength requirements of Grade 8 can be met with a very wide range of steels -- even certain carbon steels. By imposing the chemistry, GM can not only meet the minimum strengths, but ensure that ductility, shear, grain size, hardenablilty, and other such properties are consistent no matter who the supplier or what the bolt diameter. A 4140 steel (Cr, Mo + same C) would have identical strength and hardness, for example. But, without Ni, toughness, impact strength & corossion resistance would be less.

Maybe Doc has developed some private source of GM300 -- if so, it's likely a Tier 1 supplier to GM that provides him one-sy and two-sy size orders, rather than a gazzilion pieces like a GM purchace order. Myself, I'd have no qualms about just going with a quality Grade 8.

[Michael G.]

Re: Grade 300 bolt?

GM300 and other such specifications do, in fact, specify permissible materials, but more importantly they also specify minimum performance requirements, such as minimum tensile strength, which must be tested on every lot. If you don't have a copy of the GM spec, go by the Grade 8 minimums and you'll be fine. Such parts should be marked with six radial lines on the head. If they were made recently, make sure they have a manufacturers mark on the head, as such mark is required by U.S. law.

Mike