Bolt Grades

[David H.]

Re: Bolt Grades

CF thread (link below) has a link to pdf with Group 8.9 fastener information not typically in Parts Catalog. Seems like mostly older cars, but some part numbers may work for later. Don't see any Grade information other than generic lines.

Response #10 on that thread has pdf link.

https://www.corvetteforum.com/forums...ly-manual.html

Dave

Parts-Gp8.9.jpg

[Michael G.]

Re: Bolt Grades

The lines are references to specific strength classes, as defined in federal standards and described in GM Standards GM-260 (Grade 2, no lines), GM-280 (Grade 5, three lines), and GM-300 (Grade 8, six lines). All the bolts described on these pages could be ordered in any of the three strength classes that I Listed. Each combination of physical dimensions, strength class, and surface coating was given a specific part number. Many of those part numbers were never real parts, as no one ever ordered that combination.

When you see a part with no radial lines, assume its un-heatreated, low-carbon steel, the weakest part. There is a dramatic difference in tensile strength between these classes, you should not interchange them.

[Dan D.]

Re: Bolt Grades

What I’ve observed during the restoration of my ‘66 is that for the chassis and engine, where grade is important, the vast majority of bolts (and I mean something like 95% or more) have both TMs and grade lines.

Have you really found more than half of your bolts have no grade lines? Can you tell us where those bolts came from on the car? I’m very curious.

Gary

Thank You Gary, and all others that have replied to my thread. Let me offer the following observations and history of what I have.

a. I have a bolt (including nuts and washers) 'inventory' of probably 1000 or more bolts -1/4, 5/16, 3/8, >3/8. These date back to the late 1950s when I first started messing around with mechanical stuff. The bolt collection started then and grew over the years, so some are non auto, some are GM, some are Corvette, others I have no idea.

b. The collection originally was one large bin. But over the years the collection grew and some effort was made to try and organize them, but it really became unmanageable so a few years ago I did organize them all. See the photo of my collection, which is quite good.



c. Some of the collection has period correct TMs; some have grade lines, some have both, and some have no markings at all. My question was for the fasteners that have TMs, but no grade markings, am I to assume that these are grade 2? I would think so, but I was hoping to get confirmation on this is all.

d. I think from reading everyone's reply's that grade 2 is probably correct. I am going to assume that it is.

e. What prompted me to question this is I recently re-installed the front end cross member in by 57. There are qty.16 3/8 bolts holding this thing in. All 16 were TR markings, but no lines. The new bolts I bought from CC were TR and grade 5 (3 lines). I thought that the front end would require stronger than grade 2, but with 16 of them, perhaps not.

f. This photo is a snap shot of some of my 5/16 -18 3/4" long bolts. Others are similar. The photo is not too good, but you can kind of make out the markings on some of them.

20220511_113733.jpg

L WB TR EP
1C (3 lines)
EWF
(6 lines)

UR TR 0 --
(4 lines)



g. Gary, I can't tell you what percentage of bolts have grade lines or where they came from, but I don't think grade lines are anywhere near 95%. Its possible that C2s have a higher percentage of grade marked bolts than C1s do.

h. The correct torque value for these fasteners is variable, and questionable. I am going to start a new thread on this subject as soon as I get some time.


Dannn

[Michael G.]

Re: Bolt Grades

Hi Dan,

It's likely that the heads with no lines are Grade 2. It cannot hurt to use the grade 5 bolts, but I would not use a higher torque range, as the failure mode may become nut strippage rather than the preferred bolt breakage. The only real difference will be in the amount that the stronger bolts stretch, in essence, they will stretch slightly less at the same load. This won't make any difference for your purposes. You can compensate for this stretch by tightening to the upper end of the torque spec range, which isn't risky with the stronger bolts.

I wouldn't vary the torque specs from those listed in the AIM. The same bolt can have different specs due to the testing that was done at the time, wherein the actual parts were tightened to develop the spec - a difference in component surfaces or constructions can require different specs for the same bolt, depending on the use.

[David B.]

Re: Bolt Grades

Dan,
The 16 bolts used to attach the front cross member on a '57 are plain GM 260-M grade 2 Town-Robinson 3/8-16 x 11/4 bolts. If your car was being judged, since you replaced with grade 5 obvious 3 line fake TR bolts there should be points deducted.
As shown in the AIM the '56 Corv. front end was attached with a slightly different bolt 3/8-24 x 11/4 grade 2 TR bolt.
You are correct plain head bolts are grade 2

[Dan D.]

Re: Bolt Grades

Thanks Dave. I appreciate your info.

My car is a driver and will never be judged, and I know these bolts would never pass muster. But some of the original bolts twisted off trying to remove them after 65 years of being dormant, and I would not trust any of the ones that did come out okay - they were in bad shape.

I think 57 and up bolts may have been slightly longer to accommodate the tapered caster shim that Zora added to improve high speed handling. Why the different thread pitch, I don't know. Fine thread will accommodate higher torque.

Next I am going to discuss torque in a separate thread, very soon.

Dannnn

[Dan D.]

Re: Bolt Grades

Hi Dan,

It's likely that the heads with no lines are Grade 2. It cannot hurt to use the grade 5 bolts, but I would not use a higher torque range, as the failure mode may become nut strippage rather than the preferred bolt breakage. The only real difference will be in the amount that the stronger bolts stretch, in essence, they will stretch slightly less at the same load. This won't make any difference for your purposes. You can compensate for this stretch by tightening to the upper end of the torque spec range, which isn't risky with the stronger bolts.

I wouldn't vary the torque specs from those listed in the AIM. The same bolt can have different specs due to the testing that was done at the time, wherein the actual parts were tightened to develop the spec - a difference in component surfaces or constructions can require different specs for the same bolt, depending on the use.

Hi Mike,

The grade 5 bolts came with L' washers and nuts. So one would think that the nuts would be the same steel alloy as the bolts. Of course "one would think" is the magic phase here. They did torque okay, although I used a lower torque setting.


Dannnn

[David B.]

Re: Bolt Grades

If you want to save yourself a lot of time researching torque specs look no further than June 1958 Chevrolet Service News page 4. It has listed a printed Standard Torque Table Chart. "recommended for all applications where specific requirements are not stated in the Chevrolet Shop Manual".
Re: the current discussion a 3/8-16 grade 2 bolt should be torqued 15 ft. lbs. The same size bolt in Grade 5 recommended torque is 27 ft. lbs.
The '56 3/8-24 grade 2 bolt should be torqued to 18 ft. lbs. The same size bolt in Grade 5 recommended torque is 33 ft. lbs..

[Michael G.]

[Gary C.]

Re: Bolt Grades

1964 GM Fasteners Info attached - it's in 2 sections because of the large file size

[Mike B.]

Re: Bolt Grades

Gary,

Thank you for posting the 1964 GM Fastener Info here. This is the most comprehensive listing I've seen that covers the bolt head marks on my C2 Corvettes.

Much appreciated,
Mike

[Dan D.]

Re: Bolt Grades

Dan, The nuts should also have grade markings. There are a few different ways of doing that, so google it for the options. If you use a weaker strength class nut, it won’t matter at those torque levels.

Hi Michael, can you briefly tell me how the grade classes for nuts are marked? I can't seem to find it, and I see no markings on any kind on any of my nuts.

Thanks,
Dannn

[Michael G.]