C1 '58 Flywheel

[Ted S.]

I have two flywheels of which neither appear to be correct for my car. One is casting 3973456N and the other is casting 3789733 with a ink stamp 3889694. My understanding is the '58 used casting 3729004 which was drilled fo a 10 1/2" clutch/pressure plate, correct? I believe all three are 14" od, correct? Will either of the ones I have work for my '58? Thanks, Ted

[Ted S.]

Re: C1 '58 Flywheel

One interesting point is the 3973456N was installed in the car with an 11" borg & beck style clutch/pressure plate. This was all under a correct "365" bellhousing. After reading through the archives my understanding is theoretically an 11" clutch shouldn't fit under this bellhousing.

[William C.]

Re: C1 '58 Flywheel

Generally the problem is in getting the starter to work given the difference in the outside diameter of the flywheel is different, thus the number of teeth on tyhe flywheel is different and the location of the gear on the starter relative to the centerline of the crankshaft moves.

[Ted S.]

Re: C1 '58 Flywheel

Bill thaks for the response, from Joe Lucia's posts in the archives it looks like the original was a 14" flywheel and it used a 10" clutch assy. From a couple of the posts it sounds like the 10" original and the later 10 1/2" (actually 10.4") used the same bolt pattern, so they could interchange.

The posts also indicated that an 11" clutch assy wouldn't physically fit under the original "365" bellhousing without interference. I assume because of the curvature from the mounting flange to the transmission mounting surface. The borg and beck pressure plate is rather tall and curves back pretty quickly. Maybe it's providing enough clearance. Also since the original and the ones I have are both 14" flywheels, the starter mount should be ok. Admittedly I never operated the car with this set up since the car wasn't running when I bought it.

[William C.]

Re: C1 '58 Flywheel

If you have a 14 inch flywheel, with the proper tooth count on the ring gear you should be ok. I have put away all my books in preperation for installing new flooring in my office, so I can't reference the tooth count from the parts book right now, but I'm sure someione will pop up with the correct count.

[Joe L.]

Re: C1 '58 Flywheel

Bill and Ted-----

There were only 2 tooth count flywheel ring gears ever used for any 57-96 Corvette that I am aware of----- 153 tooth and 168 tooth. As far as I know, all 57-62 used 168 tooth count.

[Joe L.]

Re: C1 '58 Flywheel

Ted------

Both of the flywheels you have are incorrect for any 1957-62 Corvette. I have heard varying reports regarding their usability on a C1 and having no experience, at all, with C1 clutches in about 40 years, I can't say one way or the other. It may be that they'll work IF the right combination of other parts are also used. Certainly, there should be no problem with them bolting up to the crankshaft flange.

Generally, the above flywheels are drilled for 11" clutches. Some are dual-drilled for both 11" and 10-1/2" (actually 10.4") clutches. The one that you have which is ink-stamped "3889694" should be drilled only for a 14" clutch. The other one may be dual drilled or drilled only for a 14" clutch. Both of the flywheels should be 14" units with 168 tooth ring gears.

1957-62 Corvettes did originally use flywheel casting #3729004 and it was a 14" OD, 168 tooth ring gear flywheel. This flywheel was originally available under GM PART #3729003 and several supercessive part numbers. The last one, GM #3986390, was discontinued without supercession in January, 1994.

The original clutch for your application was a 10" clutch with a 12-3/8" clutch COVER and of the coil spring type ("3 finger"). This clutch configuration was unique to 1957-62 Corvettes and some 1957 Chevrolet passenger cars with 283/4 barrel engines.

Later 10-1/2" (actual 10.4") clutches were of the diaphragm spring type. Their clutch COVER was just slightly larger than the 57-62 Corvette units and it is these clutches that the flywheels you have, when they are dual drilled, are designed to accommodate. Whether the bolt pattern is close enough that the 57-62 Corvette clutches will also bolt up, I do not know.

By the way, the use of the terms "10 inch", "10-1/2inch", "10.4 inch" for Corvette clutches can be confusing. In general, the "10 inch" refers to the 57-62 clutches. The 10-1/2 inch" or "10.4 inch" are the same thing and usually refer to the 63-68 small block, some 65-69 big block, and 70-72 ZR-1 clutches. Sometimes, even the 57-62 clutch is described as "10-1/2 inches". However, I believe that it's closer to 10" than it is to 10-1/2". I have several NOS examples of this clutch disc around here somewhere, so maybe I should dig one out sometime and measure to settle it.

[Tom P.]

Re: C1 '58 Flywheel

For ALL 55-85 Chevrolet V8 engines, there were only TWO sizes of flywheels:
14in diameter with 168 teeth
13in (actually about 12 3/4in) with 153 teeth (introduced in 63). Period
ANY 55-85 Chevy V8 flywheel is physically interchangeable on ANY crankshaft.
ALL 55-62 Chevrolet V8 engines ONLY used a 14in, 168 teeth flywheel. Thus, a 1985 14in, 168 teeth flywheel is a 100% bolt-on interchange with a 55-62 V8 with a 14in 168 teeth flywheel.
Within these years (55-85), there were ONLY TWO bolt patterns for the clutch pressure plate (some people like to refer to the pressure plate by the parts book term: clutch cover. But VERY FEW people use or know this term. Pressure plate is the "common" term used by engine building, knuckle bustin' car people). MOST of the early flywheels (as already mentioned) had a bolt pattern for a 10in clutch (later replaced by a 10 1/2in or 10.4in clutch). I said most, some early flywheels, such as for trucks) got a bolt pattern for an 11in clutch. Since the 55-62 Corvettes ONLY had the larger, 168 teeth flywheel, then of course, ANY 168 teeth flywheel up through 1985 is a pure, 100% bolt-on swap. Most later flywheels got a bolt pattern for an 11in clutch, thus, you can easily install a flywheel with the 11in clutch. Any 11in diaphragm clutch fits VERY EASILY in a 55-62 bell housing. That's all I run in my cars, including my 56 Vette with a 60 alum bell housing. As I mentioned, all of the above flywheels are a 100% bolt-on fit, but, the SB400 and 454 engines require flywheels with additional on one side for balance purposes. Since the early cars had flywheels with bolt patterns for a 10in clutch, a later flywheel with a bolt pattern for the bigger 11in clutch can be used and there is no way to tell which flywheel/clutch is used because it is totally concealed by the bell housing--------------------------OH, I forgot, that doesn't matter, this is the NCRS site.

[Joe L.]

Re: C1 '58 Flywheel

Tom-----

I stated in a previous post that there were only 2 different size ring gears ever used for any 1957-96 Corvette---153 tooth and 168 tooth. So, I really don't see what point that you're trying to emphasize here or correct me on. As a matter of fact, I believe that the ring gear size actually extends right up through 2008 since I think all C5 and C6 Corvettes use a 168 tooth ring gear.

I said in a previous post that there would be no problem bolting up later Chevrolet flywheels to any 1957-62 Corvette crankshaft flange. So, while the tone of your post implies that you are somehow "correcting me", I fail to see what the correction is. I do agree with you that this only applies to 1985 or earlier flywheels (i.e ones designed for use with 2 piece rear main seal engines).

I also stated earlier that I did not know if there would be a problem using a later clutch in a C1. I'm glad to know that, based on your experience, this can be done with no problems. However, I have heard other reports from folks that said it didn't work for them. I've never done it (like I said) so I can't say from personal experience. I do know, for sure, that GM NEVER replaced the C1 flywheel or clutch assemblies for SERVICE with any later-style units. The C1 flywheels and clutches were discontinued without supercession. This, of course, IMPLIES that GM never considered the later style flywheels or clutches as suitable replacements for a C1. Otherwise, they would normally replace the earlier style parts with the later style when the earlier style was discontinued. They do this all the time. But, for some reason, they didn't do it in this case.

The "parts book term" for a "pressure plate" is not "clutch cover". The "parts book term" for a "pressure plate" is "Cover, with Plate". The components of the assembly are, of course, a clutch cover, a plate, a spring or springs, attaching fasteners, and, in the case of Borg and Beck type clutches, release levers. In any event, it is the clutch cover COMPONENT OF THE ASSEMBLY that actually bolts to the flywheel. That is why I used that term in my previous post; in order to emphasize the fact that the flywheel bolt pattern has to match the bolt pattern of the clutch cover. I am well aware of the fact that the term "pressure plate" is used in the vernacular; I use it all the time, just not in this case for the reasons stated. In any event, the size of the clutch cover for the C1 applications is slightly different than the the clutch cover used for later 10-1/2" clutch applications. That's why I said I didn't know if the bolt pattern for the later 10-1/2" diaphragm clutches was the same as the bolt pattern for the C1 10" clutches. I DID NOT SAY THAT THEY WERE NOT THE SAME; I SAID THAT I DIDN'T KNOW IF THEY WERE THE SAME. Based upon what you've stated, it seems they are the same. I'm glad to know that.

As far as 63-85 applications are concerned, the 12-3/4", 153 tooth flywheels are drilled and tapped for 10-1/2" diaphragm-type clutches only. The 14", 168 tooth flywheels are drilled and tapped for 11" diaphragm-type clutches OR they are DUAL DRILLED for both of the forgoing clutch sizes. I don't know if any PRODUCTION flywheel was ever dual drilled but some GM SERVICE flywheels were definitely so-manufactured.

By the way, there were a few exceptions to the 14" flywheel clutch bolt pattern. These were 1969 L-71 with MA-6 dual disc clutch and all 1971 with LS-6 and 4 speed transmission (i.e with dual disc clutch). They used a unique bolt pattern.

[Tom P.]

Re: C1 '58 Flywheel

No, No, No, I'm not trying to correct, I was only trying to emphasize that a later, 14in flywheel, up through 85 could be bolted on and used with an 11in clutch all the way back to 55. Of course, this assumes that the correct corrosponding bell housing is also used. AND, I am aware of the "oddball" flywheels, such as dual pattern and LS6 type flywheel bolt pattern, but I suspected that it is extremely unlikely that anyone is going to have one of those readily available. Although, a dual bolt pattern service flywheel is going to be more common than the LS6 (dual disc) type flywheel.
I suppose it is possible that there may be some style of Chevy bell housing for use with a 14in flywheel that doesn't provide adequate clearance for an 11in clutch. But in 45+yrs of jacking with old Chevys, I've never run across a bell housing/14in flywheel/11in diaphragm clutch that had a clearance problem. This includes 55-62 iron, 60-62 alum and the later enclosed housings such as 444, 621 and 697 versions. Now, admittedly, getting an 11in clutch in/out the bottom of a 55-62 iron (or iron through 70 for pickups) housing requires a little "manuvering", but it is much easier with a 60-62 alum housing (I don't know why it's easier to get an 11in in/out of the alum housing). But my point was to emphasize that any 55-85 flywheel is a bolt on fit for early cranks and a later 14in flywheel/11in clutch WILL fit within the 55-62 bell housings and will work fine.