Double Clutch: the second time around

It’s fixed--the truck, in any case. Just for kicks I’ll chronicle some of the more exciting moments (sarcasm)! My buddy came over and kept me company for a while. He also found my camera. The tool pictured here was the end result of a three-hour struggle with a single mounting bolt located at the very top of the bell housing. Can anybody say "monkey boy?"

So the problem turned out to be nothing more than a worn clutch. The last one only lasted some 30k miles for some weird reason. Why? I have a theory which I explain at the end of this post*. Don’t know how a clutch works? Here’s a great link.

Everything came out:

The transmission. The sucker weighs about 100 lbs. It’s not too bad coming out. The transmission has to be slid away from the engine about 4-5 inches parallel to the crankshaft before it can be lowered down. Unfortunately the foot well sits right behind the bell housing (the flared portion of the transmission that mounts to the back of the engine) preventing the transmission from moving more than a couple inches back. No problem; I just lifted the front end of the engine with a jack. With the engine titled the transmission can be pulled away from the engine at a slightly downward angle (from horizontal) allowing it to clear the foot well. The hard part was getting it back in. You have to lift the sucker and “jiggle” it in order to get the transmission input shaft to go into the corresponding hole in the crankshaft. You’ve got to do this without cocking the transmission at all or else the shaft binds in the hole and you ain’t going nowhere! Good thing I’ve been going to the gym!

The clutch cover and pressure plate. This is really a three piece assembly that also includes the diaphragm spring, but it comes out as a single piece. Pictured here is the old one. The rivets that hold the clutch material had just barely begun to rub against the pressure plate so there were no grooves worn into the friction surface.




This is the new pressure plate. What a beautifully freshly-ground surface!







Next came the old clutch. How do I know it’s worn out? Enough of the friction material has worn away that the rivets have begun to rub against the mating pressure plate. You can see this as shiny spots on the rivet heads here. Also, some of the radial grooves on the friction material are no longer visible because they’ve been worn away. A particularly alarming thing I noted was that the clutch wore out more quickly near the hub than at the outer edge.


Contrast the new clutch. The radial grooves are perfect.








Finally, the old flywheel came out. The rivets on the side of the clutch that abut the flywheel had not begun to rub on the flywheel yet. No grooves were evident, though the surface was a little bit wavy. This looks like ripples in the shiny surface in the picture. The new one looks the same, but with a perfectly uniform, fresh ground surface.



* Ok, so here’s my theory of why the first clutch gave up the ghost so quickly. In a nutshell, I didn’t resurface the flywheel when I installed it. Anyone who does their own brakes knows that rotors have to be turned and resurfaced when the pads have become so worn that their metal backings gouge the rotor surface. Now, my friend (another mechanical engineer) and I or of the opinion that there’s not really any need to turn the rotors unless they’re really gouged because the new pads will eventually wear in and conform to the rotor surfaces. This is true, but the price is faster wear of the brake pads because, before the pads conform, they touch the rotor only along the high spots. To get the same stopping power from this reduced contact area the pads need to apply more pressure. More pressure means the pad material wears away faster, at least until they conform to the rotor surface. Since brake pads are pretty thick and the unevenness in the rotor surface is usually pretty small, the faster wear during break-in doesn’t matter too much.

Not so for the clutch. The clutch is super-thin; only about .040” on each side! So, even slight surface imperfections on either the pressure plate or the flywheel are likely to eat up a significant portion of the clutch before it wears in and conforms to those imperfections. This time, I installed a new flywheel with a perfectly flat friction surface. Ditto for the pressure plate.