MA6 clutch flywheel

[Jack O.]

Does anyone know anything about flywheel casting # 3973457N. This is the flywheel that is on my '69 L71 with the MA6 clutch. The flywheel has a series of drilled holes so it appears to be externally balanced. This is very odd since the block is a correct 427 block. Is it possible that my 427 has a 454 crankshaft? Wouldn't that also require a different harmonic balancer than the 427?

Thanks.

[Craig S.]

Re: MA6 clutch flywheel

Jack - the series of holes are for balancing the flywheel dynamically, they don't mean the engine is externally balanced. With externally balanced flywheels and harmonic balancers, there is an offset weight near the outboard circumference of the flywheel to add additional balance offset weight besides what is inside on the counterthrows of the crankshaft. While Joe will chime in about this part number, I am certain it is an internally balanced flywheel...Craig

[Clem Z.]

Re: MA6 clutch flywheel

the 454 flywheel ID number is 3993457 and the 427 flywheel ID number is 3856579 so chech your flywheel number closer as the "7" may be "9"

[Joe L.]

Re: MA6 clutch flywheel

clem-----

Yes, one of the 454 flywheel casting numbers is 3993457. So, if the third digit is actually a "9" rather than a "7", then the flywheel is definitely a 454 unit.

However, there were several casting numbers used for 396/427 flywheels. One of these numbers was 3973458 which is suspiciously close to the number reported.

The MA-6 clutch applications used a special flywheel of GM PART #3955151 (later GM #3992094). Unfortunately, I don't know the casting number used for these units, but it could well have been 3973458.

[Clem Z.]

Re: MA6 clutch flywheel

just check the back side of the flywheel for the cast in offset weight in one spot on the flywheel.

[Jack O.]

Re: MA6 clutch flywheel

I take it this cast on weight is obvious. Would the harmonic balancer also have to be replaced to use a 454 crankshaft in a 427?

Thanks.

[Clem Z.]

Re: MA6 clutch flywheel

yes the damper would also have a cast in offset weight on the back side.

[Joe L.]

Re: MA6 clutch flywheel

Jack-----

The weight will be very easy to see on the flywheel, if it's there. Simply mark the flywheel (so you know where you started) and turn it around 1 revolution observing the rear perimeter. The weight will appear as a more-or-less rectangular extension of the perimeter "flange" of the flywheel. It's very easy to see and if you turn the flywheel around, if it's there, you will not miss it. I'd be very interested in what you find. If it's an original MA-6 flywheel, as I expect, we'll then know the casting number for one of the original ones used for this application.

[G B.]

Here's one dual-disc GM flywheel i.d.

It is cast P-8999 GM2NF J17. I don't know if this is an over-the-counter flywheel or if it began life on an car assembly line. It is for an internally balanced engine only.

The real tip-off for this clutch application is the odd bolt pattern for the pressure plate. The six bolt holes are spaced in 3 groups of two. In other words, the 6 holes aren't evenly spaced around the perimeter.

I would be most interested in hearing about the pressure plate bolt spacing on your flywheel. You would be able to see this on the back side of the flywheel.

[Jack O.]

Re: MA6 clutch flywheel

Okay, here's the poop on the flywheel. I double checked the casting number and it's definitly casting # 3973457N with another casting number of C283 which I suppose is a date. It is for a MA6 clutch so it does of course have the 3 groups of two bolt pattern. The flywheel also has the extra cast area with a series of holes drilled in this area as well as arounf the perimeter in other places. I suppose this means it's externally balanced? If so, this is disappointing news. I wanted to switch to a standard clutch which would require another flywheel. But if the one on the car is externally balanced, can I switch it and somehow keep the engine balanced?

[Joe L.]

Re: MA6 clutch flywheel

Jack-----

Is the "extra cast area", basically, a raised, rectangular extension of the perimeter machined area? If so, then this is an externally balanced flywheel. At this point, double check your harmonic balancer to confirm that it's also counterweighted on the rear of the hub section.

This clutch, if it's for sure an MA-6, could be the 71 LS-6 unit. 1971 LS-6s used an MA-6-type clutch assembly as "standard equipment".

If the engine is an externally balanced 454, you can easily replace the flywheel with a single plate style unit. The part you need is GM #3993827.