Request for LS-6 Information

[Joe L.]

Does anyone out there have or have access to a known original 1971 LS-6 with 4 speed transmission? I'd like to confirm what the bellhousing casting number is for such a car. It should be either GM #3899621 or GM #3858403, but I'd like to confirm which. Thanks.

[Bill Clupper]

Re: Request for LS-6 Information

Joe, although I don't have a car to look at, I have a '71 parts book that indicates the HD 454 engine used the clutch housing cover that is only compatable with the 403 Bellhousing. That would be consistant with the use of the 12 1/2 inch flywheel and the dual disc clutch. Not an absolute, but at least some verification.

[Joe L.]

Re: Request for LS-6 Information

Bill------

As a matter of fact, I tried to see if any of this "peripheral" parts information might shed some light on the LS-6 bellhousing. The various annual editions of the parts catalogs that I have seem to present conflicting or contradictory information on bellhousing "related" components (e.g. flywheel, bellhousing, clutch, cover, and starter motor). My editions of the manuals indicate "67-71 with HD 427, 454" use the cover plate for the traditional 10.4" clutch bellhousing (GM #3858403). For 67-69 the HD 427s were, of course, the L-88s and ZL-1s. For 1970, the LS-7 would have been the HD 454 if any were built. Many of the parts manuals contain parts information for the LS-7, even though it was never installed in a Corvette. However, I have never seen GM otherwise refer to the 71 LS-6 as "HD". Usually, the term "special high performance" is the designation used for the 71 LS-6.

[Stan F.]

Re: Request for LS-6 Information

Joe,

A couple of items for you to think about...

My understanding is that both the M-21 or M-22 could be specified as a manual transmission choice for the 1971 LS-6. The M-21 version almost certainly mirrored the other high performance big blocks by using a single plate 11.0" inch clutch and therefore the 621 bellhousing. The M-22 (non ZR-2) version received the dual plate clutch and while I am unsure as to what the proper belhousing is in this case, it probably was the same as the 1969 MA-6 option bellhousing which was also a 621. The ZR-2 would have used an M-22 with the smaller 10.4" clutch and as a result, the 403 bellhousing.

Regards,

Stan Falenski

[Joe L.]

Re: Request for LS-6 Information

Stan-----

Actually, I believe (but I'm not certain) that all 1971 LS-6s used the same dual disc clutch regardless of whether equipped with M-21 or M-22. Also, the dual disc clutch assembly was the same as that used for 1969 L-88 with the MA-6 HD clutch, save for one difference. The 69 clutch discs were 10 spline, while the 71 disc were 26 spline. However, the pressure plate assembly was the same.

The other factor is the flywheel. I believe that it is the flywheel size that most influences the size of the bellhousing. Also important, although an 11" clutch will not fit in a bellhousing designed for a 10.4 clutch, a 10.4" clutch will fit within a bellhousing designed for an 11" clutch. For the 1969 L-88 with dual disc clutch, the flywheel used was GM #3866735. For SERVICE, this later became GM #3991406 and is currently GM #14085720. All of these flywheels are 12.75" nodular iron flywheels for use with 10.4" clutches and were used with the GM #3858403 bellhousing. Of course, none of these flywheels could be used for a 454 engine like the LS-6 since these do not have the counterweights necessary for an internally balanced engine like the 454.

Here's where it gets interesting (and confusing): there is an externally balanced equivalent of the 3866735/3991406/14085720 flywheel. This flywheel is GM #3963537 (casting number 3935411). This flywheel was first issued in 1969 and continues to be available to this day. Like its internally balanced equivalents, it is a 12.75" OD nodular iron flywheel of approximately 15 pounds weight. However, the LS-6 DID NOT USE THIS FLYWHEEL. The LS-6 used flywheel GM #3992094. In fact, as far as I can tell, it was the only application for this piece. Unfortunately, try as I may, I cannot get any spec information on this particular flywheel. So, I am theorizing that the 3992094 flywheel may have been a specially configured, externally balanced flywheel of 14" diameter, but designed for use with the 10.4" dual disc clutch set-up. In that case, the bellhousing used would be the GM #3899621. As a matter of fact, my interest in the LS-6 bellhousing is partially a surrogate for my interest in the GM #3992094 flywheel. If the LS-6 bellhousing is a '621, then it's likely that the 3992094 was a 14" flywheel; if the LS-6 bellhousing was a '403, then it's likely that the 3992094 was a 12.75" flywheel. All this assumes, of course, that the 10.4" dual disc clutch was used on ALL LS-6s, but I'm reasonably sure that was the case. In any event, I can find NO information that an 11" clutch was used on any LS-6.

The thing that I cannot reconcile is this: if the 3992094 flywheel is a 12.75" flywheel, why didn't GM just use the 3963537 flywheel which is, as I mentioned, the 454 equivalent of the flywheel used for the dual disc L-88? As a matter of fact, I have also confirmed that the 3963537 flywheel was the one destined for use on the 1970 LS-7, if it had been produced, and intended for use with the same dual disc, 10.4" clutch as used on the 69 L-88 and subsequently used (with 26 spline discs) on the LS-6.

[Wayne M.]

Re: LS-6 Clutch Information

Joe --- Here's my take on this clutch/flywheel/bellhousing topic, based on the '70 GM Corvette Parts Manual, and a '71 edition of "How to Hot Rod Big Block Chevys". First, the 3992094 and 3963539 flywheels are both used with the 454 HD crankshaft 3963524, but they are both 12.75" diameter, as I interpret it. The "539" is lighweight nodular iron --- not sure about the "094". For a dual disc clutch set-up on a 454, you need the 14" dia. flywheel 3963541, also balanced for use with the "524" crankshaft. Of course, this large a flywheel calls for the "621" bellhousing, and the "309" starter nose. I don't believe there is any dual disc set-up for 12.75" (153 tooth) flywheels. In spite of the dual disc(s) friction surface diameters (outer/inner) of 10.0 x 6.0", giving a per-disc friction area of 100.55 in² (less than the single disc 10.5 x 6.5" (123.7 in²), the bolt pattern (11.625" dia.?) of the "dual" pressure plate cover requires the larger flywheel, (IMO).

For the same dual disc clutch on a 396/427, you require 14" dia. flywheel 3955151 (and for the pre-71, 10-spline trannys) the 3959175 pressure plate assy + forward driven disc 3959176. For the 26 spline trans input, the P-plate assy is 231254, and the forward disc 3993815.

I don't believe that MA6 was only available with L88; it could be ordered with either L71 or L89 435 hp versions. In fact, I believe that if you ordered L88 with dual disc, you automatically got the heavier 14" flywheel set-up., but without MA6, it came with the 12.75" 3866735 and the "403" bellhousing, as you've pointed out.

As for the 454 flywheel 3963537, (lightweight nodular iron), my GM book says it was intended for '70 LS7s EXCEPT w/HD clutch.

[Joe L.]

Re: LS-6 Clutch Information

Wayne-----

Yes, I think that I finally got this one figured out just about the time that you were apparently posting your response.

There were a few things that were confusing me and a few incorrect assumptions that I had made. The big problem was that I somehow missed the 69 GM #3955151 flywheel (and I cannot imagine how I missed that). That would have "re-oriented" my thinking on the whole thing. Instead, I assumed that the 12-3/4" flywheel was used for the 69 with dual disc clutch.

I have since confirmed that both the GM #3955151 and the 3992094 flywheels are, indeed, 14" flywheels. The former designed for use with externally balanced big blocks and the latter for externally balanced big blocks.

In the case of the 3992094 what I've confirmed is exactly what I had earlier theorized----that the 3992094 flywheel is a 14" flywheel configured for use with a 10" clutch. Of course, the "10 inch clutch" is the dual disc 10" clutch. But, that's exactly what I had theorized even though, over the years, others had indicated to me that the dual disc clutch was used with a 12-3/4" flywheel. While I incorrectly accepted the notion that the 69 dual disc application used the 12-3/4" flywheel, I just could not see how the 71 LS-6 dual disc application could have used anything but a 14" flywheel.

What I've learned here is that 69 427s with the dual disc clutch also used a 14" flywheel. So, that's a new piece of knowledge to file away in the old 'nogin.

What's also, apparently, come out of all this is that for the period of 1969-72, the only applications that probably used the 403 bellhousing were 69 L-88 w/o dual disc clutch and 70-72 ZR-1.

[Stan F.]

Re: LS-6 Clutch Information

Joe,

You mention that the only use of the 403 bellhousing would be in 1970-72 ZR-1s and 1969 L-88s without the dual disc clutch. I was unaware of any L-88s that actually received the MA-6 clutch, but judging from your post, you have information that they did. I'd be very interested to hear what you have discovered regarding the L-88/MA-6 combination.

By the way, great info on the LS-6 clutches - I was unaware that all LS-6s received the dual disc clutch, regardless of M-21 or M-22.

Regards,

Stan Falenski