BB Front Stab Bar, 7/8th or 15/16th

[Dale C.]

Need some help here. Joe Lucia is sticking to his guns on the 7/8th fnt. (and not the 15/16th) stab bar being on the 68 BBs without the F41 from his post on the minute 1/16th inch difference as being part of the confusion. If he is correct not only are the parts books wrong but the JM, page 98, states (implies all???) 427s had the 15/16 inch diameter fnt. bar.
Dale

[John C.]

Re: BB Front Stab Bar, 7/8th or 15/16th

Dale

I measured my front bar on my 68 BB with the standard suspension with a dial caliper and I got .94 inches (15/16). I have no reason to doubt it is not original.

John

[Michael F.]

Re: BB Front Stab Bar, 7/8th or 15/16th

my original 70 454 BB bought from 1 owner had 15/16"....car 100% original never had anything replaced except h20, oil, filters etc....I bought in 73. I documented every part I could see or touch, was anal back then and a stickler for originality.

[Duke W.]

Re: BB Front Stab Bar, 7/8th or 15/16th

I agree with Joe. My 9-77 rev. of the P&A catalog,, Gr. 7.241 lists the 3871318 for all big block base suspensions. It also lists this bar for all 63-69 w/ Sp. Per. HD Susp.!

The Chevrolet Power Manuals from the seventies list all available Corvette bars along with their diameters The ...318 is 7/8", and the 3831972 is 15/16", but this bar doesn't even show up in Gr. 7.241 even though its utilization dates back to the '63 Z06.

Looking at my 2-77 rev. of the P&A adds even more confusion. The listing for the ...318 has a note: (repl 3831972).

So was the ...972 discontinued at some point prior to 1977?

One-sixteenth of an inch may not seem like much, but an anti-roll bar's contribution to roll stiffness varies with the fourth power of diameter, so a 1/16" difference will make a noticeable difference in dynamic response.

Measuring bar diameter can be tricky. It should always be measured at a point where the bar is straight, and its best to measure at multiple spots. Wire bush off the areas to be measured to knock off any exfoliation corrosion, undercoating, or other foreign matter.

Also, check your AIMs for the part number listed for base big block and F41 bars, and the AMA specs for the bar diameter listings. The '63 AIM lists the ...972 for Z-06 and the AMA specs say .94".

Duke

[Dale C.]

Re: BB Front Stab Bar, 7/8th or 15/16th

Duke, the 9-71 P&A (closer to the 68 and 69 model year) and the 1-76 P&A both reference the 3831971 (3/4th) bar as applicable to "65-69 - Corvette - (427,454) (exc. spec. perf. susp., H.D. susp.)". The parts books then go on to reference the 3831972 (15/16th) as "63-69 with spec perf H.D. Susp." and then no mention of any 68 or 69 as using the 3871318 (7/8th) front Bar.
Anybody having trouble typing and letters being omitted???? My cursor is kind of flashing. Jst me??? See, and not a typo. I checked Wrd and no problem there.
Dale

[Duke W.]

Re: BB Front Stab Bar, 7/8th or 15/16th

There appear to be a lot of discrepencies between the various releases of the P&A catalog, and I am also finding descrepencies in the AMA specs. ...971 is the 3/4" bar used on base suspension small blocks, and I'm sure everyone will agree that all big blocks whether base or F-41 do not have a 3/4" front bar.

IMO the AIM is the most accurate information, so if anyone can contribute the AIM part number listings, that may help sort out this issue.

The "base" vehicle (base engine and suspension) front bar will be listed in Sect. 3, but also check engine option sections and the F-41 section for front bar substitutions with those options. Also check the revision boxes on all applicable pages to see if there were any changes to the front bar part number.

Here is the list from the Chev. Power Manual of available front bars along with my understand of utilization.

3831971 ...... 3/4" ... all base suspension SB through early seventies
334930 ...... 13/16" ... mid-seventies up C3 base suspension (not sure of the changeover year)
3871318 ..... 7/8" .... 396 and all base suspension 427, 454
3831972 ..... 15/16" ... Z-06, F-40, F-41
351596 ....... 1 1/8" ... '75-up FE7

Note that the base small block C2 and Z-06/F-40/F-41 bars are sequential part numbers, which would indicate that they were designed at the same time - probably the same design engineer on the authority of the same engineering work order.

Also interesting is that the '63 AMA specs list the base front bar diameter as .6875 (11/16"). The Sect. 3 Sheet 5 revision box lists a revision, dated 9-14-62, that specifies the 3817572 (probably 11/16") was replaced with the ...971. That's pretty late in design/test cycle, but I'm sure they finally realized that a bigger front bar was needed to tame oversteer. Even with the 3/4" bar my SWC is twitchy at the limit.

Duke

[Joe L.]

Re: BB Front Stab Bar, 7/8th or 15/16th

I agree with Joe. My 9-77 rev. of the P&A catalog,, Gr. 7.241 lists the 3871318 for all big block base suspensions. It also lists this bar for all 63-69 w/ Sp. Per. HD Susp.!

The Chevrolet Power Manuals from the seventies list all available Corvette bars along with their diameters The ...318 is 7/8", and the 3831972 is 15/16", but this bar doesn't even show up in Gr. 7.241 even though its utilization dates back to the '63 Z06.

Looking at my 2-77 rev. of the P&A adds even more confusion. The listing for the ...318 has a note: (repl 3831972).

So was the ...972 discontinued at some point prior to 1977?

One-sixteenth of an inch may not seem like much, but an anti-roll bar's contribution to roll stiffness varies with the fourth power of diameter, so a 1/16" difference will make a noticeable difference in dynamic response.

Measuring bar diameter can be tricky. It should always be measured at a point where the bar is straight, and its best to measure at multiple spots. Wire bush off the areas to be measured to knock off any exfoliation corrosion, undercoating, or other foreign matter.

Also, check your AIMs for the part number listed for base big block and F41 bars, and the AMA specs for the bar diameter listings. The '63 AIM lists the ...972 for Z-06 and the AMA specs say .94".

Duke

Duke------



The GM #3831971 was discontinued in September, 1983 and replaced by the GM #3871318.

The GM #3831972 was discontinued in June, 1975 and replaced by the GM #3871318.

The GM #3871318 was discontinued without supercession in October, 1992.

I'm quite sure that each of the above-referenced supercessions involved a "suspension compromise" since, in these cases, the supercessive part number was definitely not equivalent to the original part.

[Joe L.]

Re: BB Front Stab Bar, 7/8th or 15/16th

Anybody out there with a known original 63-72 with F-40/F-41 that can measure their front bar?

[Duke W.]

Re: BB Front Stab Bar, 7/8th or 15/16th

I'd really like to get this issue resolved. Some of you out there must have a '65 to '74 AIM. Can you lend a hand by checking the big block and F-41 option sections for the front anti-roll bar part number.

Duke

[Joe L.]

Re: BB Front Stab Bar, 7/8th or 15/16th

I'd really like to get this issue resolved. Some of you out there must have a '65 to '74 AIM. Can you lend a hand by checking the big block and F-41 option sections for the front anti-roll bar part number.

Duke

Duke------


I already thought of that avenue. Unfortunately, there is no F-40/F-41 section in any 1963-72 AIM. Components for these options were specified only in the "Bill of Materials" which generally we do not have.

For 1965-72 there is no information regarding the part number for the front stabilizer bar for big block RPO's. Once again, it's a "Bill of Materials" item.

For 1973+, the format of the AIM's changed. Gone were separate sections for RPO's. The RPO's were incorporated into the regular pages of the AIM with alternate part numbers for the different parts required for RPO's. I sure wish they had done this from the beginning.

Unfortunately again, for 1973 the only front bar shown is the one for base configuration and this bar changed for 1973 to GM #334930. No parts are shown for LS-4, F-41 or Z-07 (the only way to get F-41 for 1973).

For 1974, though, we get a break. All of the front bar part numbers are shown and are as follows:

base--------------GM #334930

LS-4--------------GM #3871318

Z-07 or FE-7-----GM #3831972

FE-7 was the successor to F-41. However, most components changed for the 1975 model year.

[Wayne M.]

Re: BB Front Stab Bar, 7/8th or 15/16th

.....Unfortunately, there is no F-40/F-41 section in any 1963-72 AIM. Components for these options were specified only in the "Bill of Materials" which generally we do not have.

For 1965-72 there is no information regarding the part number for the front stabilizer bar for big block RPO's. Once again, it's a "Bill of Materials" item......

Duke --- as an example of Joe's paragraphs (above), checked my '65 AIM and F40 appears in the RPO option list with an asterisk * that says "denotes option indexed only"

Same under UPC L78 (396 option) "non-illustrated parts", sheet A2, UPC 3 Front Suspension * (assembled ) same as UPC F40 [for which there are no separate sheets]

BUT -- UPC 3 has a front spring chart which lists "production" 3851100, and F40 3832518. Also, both part #s for the shocks are shown. No front sway bar, however.

[Duke W.]

Re: BB Front Stab Bar, 7/8th or 15/16th

Thanks for checking, Wayne.

Joe - you must have every year AIM. I should have known! In the '63 AIM different parts that are installed the "same as regular production" are listed in the option section, like the ...972 bar for Z-06. I wasn't aware these parts substitutions were removed from the AIM to a BOM in later years.

You stated that the 334930 13/16" front bar went into production for the base suspensions in 1973. Is it still available?

I've recommended this bar on base suspension C2s to tame oversteer, especially if alignment is set to my "sport" recommendations, which include negative front camber.

GM dialed in more understeer as the years passed. The new '68 strut rod bracket lowered the rear roll center, which biased dynamic response to understeer. Likewise, the larger base suspension front anit-roll bar further biased response to understeer.

Duke

[Bill M.]

Re: BB Front Stab Bar, 7/8th or 15/16th

Anybody out there with a known original 63-72 with F-40/F-41 that can measure their front bar?

[Joe L.]

Re: BB Front Stab Bar, 7/8th or 15/16th

Thanks for checking, Wayne.

Joe - you must have every year AIM. I should have known! In the '63 AIM different parts that are installed the "same as regular production" are listed in the option section, like the ...972 bar for Z-06. I wasn't aware these parts substitutions were removed from the AIM to a BOM in later years.

You stated that the 334930 13/16" front bar went into production for the base suspensions in 1973. Is it still available?

I've recommended this bar on base suspension C2s to tame oversteer, especially if alignment is set to my "sport" recommendations, which include negative front camber.

GM dialed in more understeer as the years passed. The new '68 strut rod bracket lowered the rear roll center, which biased dynamic response to understeer. Likewise, the larger base suspension front anit-roll bar further biased response to understeer.

Duke

Duke------


Yes, I see now that it is included in the RPO Z-06 section for 1963, although not in the diagrams (which is what I was looking for on a quick look-through earlier). It's also shown in the 1964 Z-06 section. After 1964 Z-06 was, of course, dropped. There is no UPC RPO F-40/F-41 section in the 1965-74 AIM's. As has been mentioned, the alternative springs and shocks used for RPO F-40/F-41 are shown in the regular UPC 5 and 6 front and rear suspension sections of the AIM, but not the difference in the front sway bars.

The GM #334930 sway bar was discontinued without supercession in October, 1982.

[Ray K.]

Re: BB Front Stab Bar, 7/8th or 15/16th

After all the above discussion, I decided to go measure the front stabilizer bar diameter on my original '65 396. It does not have the F40 /41 suspension. The bar measures 7 / 8" (.875) which makes it the #3871318. All of my parts catalogs verify this as the correct shaft .The other numbers #3831971 ( 3/4 ) and #3831972 ( 15/16 ) were the standard shaft and H. D. Suspension shaft as Joe & Duke have referenced . The 3831972 was listed in my 1972 P&A catalog but was not listed in my 1978 P&A catalog and was discontinued in the 1975 time frame.Ray