66 327/350 clutch ... odd size finding?

[Bob S.]

I have a late 66 327/350hp L79 Roadster ... build date 6/29/1966 ... mileage 57k

After disassembly of trans and clutch components as part of my body-off resto, here's what I found ...

Flywheel ... 12 3/4" (measured ... looks original w/orange paint, special GM runs and all)
Clutch disc ... 11" (measured outer diameter)
Pressure plate ... 11" (measured plate diameter ... 3 prong style ... according to resto reference book, original style)
Bellhousing ##3858403

After reading archive posts/comments from others (i.e. Joe Lucia) on mid-year clutches, these measurements don't look like they should have worked together ... but they did. And it worked great for the years (6) that I've owned my baby, er I mean my vette.

I'm curious, How could that be? Is there something I'm not comprehending?

In the reference material I have, and according to the archives here, 66 327's should have 10.4" clutches ... Was that different for late 66 327/350hp L79's? (FWIW, my 66 has the flawed aluminum valve covers, like 67's)

Was there a change in the 67 clutch size that GM "may" have implemented in late 66 production vettes? (Ecklers lists 11" clutches for 67-68 327's)

When ordering a replacement clutch disc and pressure plate, would using my old clutch outer dimension as a size guide be correct?

Any advice/experience would be appreciated ... many thanks in advance.

[John H.]

Re: 66 327/350 clutch ... odd size finding?

Flywheel ... 12 3/4" (measured ... looks original w/orange paint, special GM runs and all)
Clutch disc ... 11" (measured outer diameter)
Pressure plate ... 11" (measured plate diameter ... 3 prong style ... according to resto reference book, original style)
Bellhousing ##3858403

Was there a change in the 67 clutch size that GM "may" have implemented in late 66 production vettes? (Ecklers lists 11" clutches for 67-68 327's)

When ordering a replacement clutch disc and pressure plate, would using my old clutch outer dimension as a size guide be correct?

The old 3-finger Borg & Beck-style coil-spring pressure plate says someone's been in there before - GM quit using those and went to the diaphragm clutch in 1963, and never went back; your '66 was "born" with a 10.5" diaphragm clutch.

What's the casting number on the flywheel, and is it drilled for two different clutch cover diameters?

[Bob S.]

Re: 66 327/350 clutch ... odd size finding?

Thanks for your reply John,

I'll look at the casting number this evening. As I recall, only one set of pressure plate bolt holes, but I'll verify.

[Stuart F.]

Re: 66 327/350 clutch ... odd size finding?

Many Hot Rodders in the 60's went back to the 11" B & B clutches as a carry over from the late 50's early 60's as they were commonly used then for drag racing. They offered the user the ability to taylor their clutch pressure to their needs as well as a simple design that increased pressure at higher RPM's through the use of roller weights. You could go up to using white springs and suffer bent and/or broken clutch linkages, or even watch your dash "torque" in passenger car applications. The added pressure made for difficult power shifting unless you had a real strong leg and few brains.

I learned early on the benefits of a good diaphagm clutch and ran a full season wearing out 4-speed syncros with just my toe. I looked through the local dealer's stock for discs with a lot of metal and found me a real nice truck 11" pressure plate and matching flywheel. Beyond that, we used an "O" sanding disc to apply a swirl pattern on the flywheel and disc.
Worked like a champ.

I've noticed a lot of parts lists with part numbers have been posted about various components used on Corvettes and passenger cars, but seldom see any that include the truck parts and numbers. Back in the day we used a lot of truck parts - some worked great, some not so great - specially those for engines as usually their "duty cycles" were quite different and therefore were not compatible on performance applications.

Stu Fox

[Bob S.]

Re: 66 327/350 clutch ... odd size finding?

I just checked the casting number of the flywheel, it's 3791021 . Along with F146 and GM8 in the casting. It has only one set of pressure plate bolts holes.

John, I think you're right ... someone has been in there before.

Stu, your explanation sounds very plausible. Wouldn't surprise me if that were the case.

Tomorrow, I'll order a 10.4" clutch kit and put my baby, er my vette, back to her original configuration.

I sure appreciate the help. I'll go to sleep a lot less confused tonight.

[Joe L.]

Re: 66 327/350 clutch ... odd size finding?

I just checked the casting number of the flywheel, it's 3791021 . Along with F146 and GM8 in the casting. It has only one set of pressure plate bolts holes.

John, I think you're right ... someone has been in there before.

Stu, your explanation sounds very plausible. Wouldn't surprise me if that were the case.

Tomorrow, I'll order a 10.4" clutch kit and put my baby, er my vette, back to her original configuration.

I sure appreciate the help. I'll go to sleep a lot less confused tonight.

Bob-----


The flywheel is original. However, the thing I don't understand is how someone bolted up an 11" pressure plate assembly to it. I presume there must have been some sort of aftermarket, 11", Borg&Beck style clutch that used the same bolt pattern as the GM 10-1/2" clutch. You can be CERTAIN, though, that clutch was NEVER in there when the car was born.

[Bob S.]

Re: 66 327/350 clutch ... odd size finding?

Thanks Joe ...

Yeah, I didn't quite understand it either.

I'd research it, and everything said 10.4" clutch ... then I'd measure the parts I took off and they were 11".

Apparently Borg&Beck style 11" clutches must have been available to fit the 12 3/4" flywheels, since my original flywheel shows no signs of modification and the casting is 1 week prior to engine build (6/21/66), and 2 weeks prior to my car assembly date.

Funny what these old vettes do to us.

When I was younger, everything was about making our cars better than stock.

Now, here I am spending countless hours (and $$$) chasing down bolts with correct headmarks, parts with correct casting numbers and doing everything I can to save sloppy production flaws while being careful to not "over restore" her.

Yeah, this car has to be female ... empties my bank account and keeps me up at night ... all for a fun ride now and then ... but in the end I know she's worth it.

[Joe L.]

Re: 66 327/350 clutch ... odd size finding?

Thanks Joe ...

Yeah, I didn't quite understand it either.

I'd research it, and everything said 10.4" clutch ... then I'd measure the parts I took off and they were 11".

Apparently Borg&Beck style 11" clutches must have been available to fit the 12 3/4" flywheels, since my original flywheel shows no signs of modification and the casting is 1 week prior to engine build (6/21/66), and 2 weeks prior to my car assembly date.

Funny what these old vettes do to us.

When I was younger, everything was about making our cars better than stock.

Now, here I am spending countless hours (and $$$) chasing down bolts with correct headmarks, parts with correct casting numbers and doing everything I can to save sloppy production flaws while being careful to not "over restore" her.

Yeah, this car has to be female ... empties my bank account and keeps me up at night ... all for a fun ride now and then ... but in the end I know she's worth it.

Bob-----


I have no concern that the flywheel was modified (i.e. drilling a new bolt pattern). That would be very tough to do. The bolt holes would have to be drilled at precisely the right location and I don't see that any machine shop would know where. Get it "off", even by a little, and there's going to be BIG problems. Plus, the bolt holes for the pressure plate are not a simple "drill and tap" job. There is an unthreaded area preceding the threads into which the shanks of the special bolts fit with little clearance. This is to further ensure the exact positioning of the pressure plate assembly.