72 Flywheel/ Pressure Plate balance

[Jeff B.]

Did GM originally balance the flywheel and pressure plate as one unit? If so, I would assume the drilled holes in the back of the flywheel would be 180 degrees out from the welded weighted plates on the front (transmission side) of the pressure plate. I lost painted the factory markings on both the flywheel and pressure plate.
I am reassembling now- let me know if this is wrong to assume.

Jeff

[Joe L.]

Re: 72 Flywheel/ Pressure Plate balance

Did GM originally balance the flywheel and pressure plate as one unit? If so, I would assume the drilled holes in the back of the flywheel would be 180 degrees out from the welded weighted plates on the front (transmission side) of the pressure plate. I lost painted the factory markings on both the flywheel and pressure plate.
I am reassembling now- let me know if this is wrong to assume.

Jeff

Jeff------


One way or another they did. I don't know if they did it with the components (flywheel and pressure plate assembly) off the engine or if it was done at final engine balance. In any event, originally installed flywheels and pressure plates usually have an "X" mark on the flywheel and on the pressure plate cover to indicate the originally installed relationship between both.

Of course, if either the flywheel or the pressure plate is replaced, the mark on the non-replaced part is then meaningless.

Whenever a clutch is replaced, I always recommend that the flywheel be removed and precision balanced with the pressure plate assembly by a competent balancing shop.

[John H.]

Re: 72 Flywheel/ Pressure Plate balance

Did GM originally balance the flywheel and pressure plate as one unit?

Jeff -

No, the flywheel was balanced by itself at the engine plant (where it was machined), and the clutch was balanced by itself by the clutch supplier.

[Jeff B.]

Re: 72 Flywheel/ Pressure Plate balance

Joe and John,

Thanks for your responses! If the flywheel and pressure plate were individually balanced then it should'nt matter how they are re-assembled.

[Domenic T.]

Re: 72 Flywheel/ Pressure Plate balance

Jeff,
I say no,that would create problems when replacing. As far as I know nothing was ballanced as a unit.
I just balanced my fly wheel then bolted the new preasure plate to it and did them as a unit and they were about 4 grams off so I added weight and they were right on. Good balance shops want it all to do it right but face it we all have to replace things later so that's why I say no.

I weighed FACTORY pistons that were flat top against dome and they were the same to the gram.
I got my steel cranks out of trucks back then and weighed the truck piston to the high perf piston and they were the same. I win a lot of bets that way.
piston sets today from the speed shops all weigh different, set to set. It means a balance job each time. The youngsters selling them make up stories that would insult a seasoned rebuilder.

In production it made sence to have cranks all balanced the same with all rods and pistons being the same. That saved a lot of room and time.
That was more than you asked but I hope it helps.

DOM

[Terry M.]

Re: 72 Flywheel/ Pressure Plate balance

Jeff -

No, the flywheel was balanced by itself at the engine plant (where it was machined), and the clutch was balanced by itself by the clutch supplier.

John,

Then what was the balancing operation that was done at the hot test station? Seems to me that would have been a balance of the entire rotating assembly -- including the flywheel clutch plate and pressure disk.

[Joe L.]

Re: 72 Flywheel/ Pressure Plate balance

Joe and John,

Thanks for your responses! If the flywheel and pressure plate were individually balanced then it should'nt matter how they are re-assembled.

Jeff-----

The flywheel and pressure plate were definitely individually neutral-balanced. You will note balance holes drilled on the rear of the flywheel and, sometimes, weights welded to the pressure plate cover. This was part of the neutral balancing operation performed at their point of manufacture.

However, at some later time I believe the assembly was balanced together, perhaps on the final engine balance at the run test as I mentioned previously. That's the only reason I can see for the "X" marks on the flywheel and pressure plate and the instructions in the service manual to re-align these marks if the pressure plate assembly is removed and re-installed. If there was only a neutral balance of the individual components (i.e. flywheel and pressure plate assembly), there would have been no reason to "X" mark the flywheel and pressure plate assembly for indexing purposes.

Attached are photos of the original flywheel from my original owner 1969. You will note the "X" mark. This was aligned with a similar "X" mark on the pressure plate assembly (which I could show you if I was willing to dig out the original assembly which I removed from the car years ago and replaced).

As I previously mentioned, if a pressure plate assembly or flywheel is replaced, the marks have no meaning or use, whatsoever. In that case, the flywheel and pressure plate need to be taken to a balancing shop. The balancing shop will neutral balance the flywheel and pressure plate assembly separately. Then, they will usually final check the balance of the bolted-together assembly and mark the flywheel and pressure plate for indexing purposes, just like the factory did. If they simply neutral balanced the components separately and that's it, there would be no reason to index mark the flywheel and pressure plate. Usually the shops will do this with a stamped character, chisel mark, or, more often these days, paint.

By the way, the original flywheel I have pictured has 200,000 miles on it. It has some light surface rust on it from sitting around now for quite awhile but the surface is not heat cracked or excessively heat-blued. The circumferential original machine marks are not even worn smooth. This flywheel would be fine for re-installation as-is and I'm saving it for that eventuality.

[Clem Z.]

Re: 72 Flywheel/ Pressure Plate balance

NEVER balance the pressure plate and the flywheel as a unit because if you ever have to replace the pressure plate you will also have to rebalance it using the flywheel. the "X" was used by GM because they did balance them as a unit and that is what caused people vibration problem if they replaced the pressure plate with out using the flywheel when rebalancing. been there done that. GM put the "X" there if you just replaced the clutch disk and reused the pressure plate

[Duke W.]

Re: 72 Flywheel/ Pressure Plate balance

The "X" marks are similar to tire/wheel match marks, but instead of matching the high/low spots the clutch/flywheel X-marks match the heavy/light spots.

Balancing always involves some tolerance. Once the balance was within tolerance the the heavy/light spots on the clutch and flywheel were marked, so any OE clutch matched with any OE flywheel will have assembly balance at least as good as the individual components and likely better.

Duke

[John H.]

Re: 72 Flywheel/ Pressure Plate balance

John,

Then what was the balancing operation that was done at the hot test station? Seems to me that would have been a balance of the entire rotating assembly -- including the flywheel clutch plate and pressure disk.

Terry -

There was no final balance operation at hot-test in the 60's, although I understand that was incorporated at some point later in the 70's.

I used to have some contacts with what used to be GM Balance Engineering, but I haven't been able to find them yet; that's who supplied the production high-speed balancing equipment for all GM component plants.

I'm thinking that in the 60's, the matching X's on the flywheel and clutch cover were stamped on the engine assembly line, after clutch installation and before the bellhousing, so the clutch cover could be re-installed in its original position if the disc was changed later in the car's life. It's interesting to note that in the '67 Chassis Service Manual, it says during disassembly of the clutch, "Look for "X" mark on flywheel and clutch cover. If "X" mark is not evident, *****-punch marks on flywheel and clutch cover for indexing purposes during installation." The 1969 and 1971 CSM's say the same thing.

[Terry M.]

Re: 72 Flywheel/ Pressure Plate balance

Terry -

There was no final balance operation at hot-test in the 60's, although I understand that was incorporated at some point later in the 70's.

I used to have some contacts with what used to be GM Balance Engineering, but I haven't been able to find them yet; that's who supplied the production high-speed balancing equipment for all GM component plants.

I'm thinking that in the 60's, the matching X's on the flywheel and clutch cover were stamped on the engine assembly line, after clutch installation and before the bellhousing, so the clutch cover could be re-installed in its original position if the disc was changed later in the car's life. It's interesting to note that in the '67 Chassis Service Manual, it says during disassembly of the clutch, "Look for "X" mark on flywheel and clutch cover. If "X" mark is not evident, *****-punch marks on flywheel and clutch cover for indexing purposes during installation." The 1969 and 1971 CSM's say the same thing.

When I had the 1970 apart to replace the release bearing I decided to replace the driven disk "while I was there." My best recollection is there were no index marks between the flywheel and the pressure plate. I remember having to stamp some marks to index the pieces so I could get them back together in the same orientation.

I will have to look closely at the vibration damper next time I have the car in a position to do that to see if there are weights (pins) added to the damper as an indicator of the balance at hot test. If we can get enough owners of original Corvettes to look for those pins (I will have to get a photo) we may be able to identify when the dynamic balance at hot test begn.

[Domenic T.]

Re: 72 Flywheel/ Pressure Plate balance

I posted earlier that I balanced my flywheel, then both flywheel and preasure plate together. I trust the balance we get when we buy a preasure plate from Napa or who ever but there is nothing like bolting it on where it will live and do it as a unit.
I use what we use to center and balance 210 MPH airccaft tires as Some tires are smaller in diameter than the flywheel, but it does a fine job.
It is a static wheel balancer and is not expensive. I usually find that there is a limit as to where people stop when balancing a PRODUCTION part.
In the late 60's I was told by the chevy rep that the engines were production balanced within 4 grams. That is about what I see on the heavy rotable parts when I double check what I buy or install.
A stick shift will vibrate at some RPM because you use combustion as part of the total mass weight after balancing the large and small end of the rod to get the rotating weight. I do my motorcycles at about 65% using the combustion for 35% of total mass weight. This will balance them at certain RPM's and not so good at others.
If you notice the automatic engines run smoother than a manual shift engine balanced the same way. I have put a torque converter on a engine that was rough with a flywheel and the converter smoothened it out. The reason is that the fluid is forced to the light side of the converter EVERY revolution to rebalance it as it runs.
I've experimented with a few engines years ago with a fluid balancer I machined and it works. When the heavy side swings out it slings the fluid to the light side. SIMPLE, of coarse the fluid can't be 100% full as the torque converters don't run at 100% unless they are leaking at the out side weld or gasket for the older converters.
Any way I was off the subject but thought it might be interesting to know why some engines vibrate at different RPM's. Of course there will be some one that says their engine (manual shift) is smooth all thru it's range but when you put an instrument on it you can see their not.

DOM