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December 2011 Techline Tech Questions

Ford Bronco
Willie Worthy | Writer
Posted December 1, 2011

Burnt Clutch, Bronco Gears, Chevy Stroker, and More!

Ratio Right
I have an ’86 Ford Bronco. This past year I put a 4-inch lift and 35s on it, but it is still running the stock gear ratio. What would be a good ratio to get back the lost torque?
Brandon Hansen
Redlands, CA

Yep, your Bronco has to be pretty much a lame dog with 35s and a stock axle gear ratio. In 1986 the Bronco had either a 3.07:1 or a 3.50:1 axle ratio. I can’t even imagine driving it with the 35s and 3.07 gears. As to the proper gear ratio, well that depends on what transmission you have, and how much highway driving you plan to do. If you have the T-18 four-speed or the C6 automatic, then I think I would not go any lower than 4.11 if you spend much time at the maximum speed limit. If the Bronco sees lots of trail usage, I say go with some 4.56s. Now if you have one of the AOD four-speed automatics, then it has a 0.67 Overdrive in Fourth gear and I think 4.56s would be ideal for the road. Same goes for the four-speed manual Overdrive trans that has a 0.71 Overdrive gear. The biggest problem with this manual trans is its relatively high 3.25:1 First gear ratio, so again 4.56s are the ratio of choice.

Clutch Burner
I have an ’82 Chevrolet K20 ¾-ton. It has a 4-inch block lift in the rear and lift springs in the front. It’s powered by a well-built 350 V-8 mated to a four-speed and an NP205. All this spins Thornbird 33x12.50 tires on PIA 16.5-inch wheels. With most of the specs out of the way I am having quite the issue deciding what clutch to put in it. I’m not sure what the specs of the clutch were when it was purchased. However, after the build I was out driving around (being a good little boy) and the clutch was destroyed within a few minutes. I replaced it with what I was told was an awesome mudding clutch (Perfection Hy Test CA 1909). Well this thing is a turd. I can’t even drive on the road at 50 mph and step on it without this thing just smoking. So I need a good clutch that will not slip. Do you have any suggestions?
Super_probie
Via fourwheeler.com

Pulling an SM465 trans and an NP205 transfer case is not my idea of a fun weekend— and probably not yours either. I suggest that you sit back and rethink your installation process and take a look at a couple of different aspects. First off, what is the condition of the flywheel? Did you put a straight edge across the face and check to make sure it was not warped? Does it have hard spots on it indicated by dark blue marks? Lots of people think that one can just machine the flywheel and get rid of these marks but in reality you cannot. The grain pattern in the steel has been altered and they will come back. It’s always a good idea to re-machine the flywheel just to make sure that it is flat and true and has no circular scratch marks. These scratch marks indicate high and low spots on the flywheel and prevent equal surface contact with the disc.

Somehow I get the impression that you are just replacing the clutch disc and not the pressure plate. One should always replace the disc, throwout bearing, and pressure plate as a matched set. You can install the world’s best disc but using a pressure plate with weak or broken springs will not allow the clutch to hold properly. Sometimes it is difficult without a very close inspection to see if a spring is broken.

Something that a lot of people don’t think about is the fact that the transmission input shaft may not be centered to the crankshaft. The bellhousing is what does this and the two alignment pins must be in place otherwise there is enough slop in the bellhousing bolt holes to exceed the 0.006-inch allowed misalignment factor. Even with the pins in place it’s not uncommon for this to be as much as 0.015- to 0.020-inch off. If the input shaft runs at an angle, hard shifting and early clutch and pilot bushing wear can take place. There are several websites available that show how to check this. One is www.powertraintech.com.

One should loosen the bolts an equal amount around the pressure plate as they are removed and tighten them in progressive equal amounts when reinstalling. Otherwise you can damage the mounting surface and actually warp the assembly.

I have to assume that you are using a clutch alignment tool when installing the disc. When reinstalling the transmission, it’s important that the transmission’s input shaft does not put excessive loads on the disc’s center hub. To prevent this I like to take a couple of long bolts and cut the heads off and taper them to a rounded point. With a file, saw or grinder, cut a screwdriver slot in the end. These are then screwed into the bellhousing. This way you can slide the transmission’s mounting holes over these bolts to support the weight as the input shaft slides into the disc. When the other two transmission mounting bolts are in place you can then remove the long bolts and replace them with the proper bolts.

Even if you did replace the pressure plate, could it be that the throwout bearing is not adjusted properly? When the clutch pedal is totally out, that is with the disc engaged; there should not be any contact between the fingers on the pressure plate and the throwout bearing’s face. I believe that 0.125-inch is pretty much the standard clearance. Could it be that the throwout bearing is actually pushing in on the pressure plate’s fingers and releasing some of the contact pressure?

Oh, one other thing while I am thinking of it—you did install the disc facing the proper direction, right? It’s always a good idea to also inspect the pilot bearing in the crankshaft for excessive wear. Here is a trick to remove it if it is necessary to replace it. Pack the inside of the pilot bushing with grease, even wet toilet paper will work. Then take a punch the same diameter as the hole and drive it in. The idea is to force the grease or toilet paper between the bearing and the crankshaft and in turn push the bearing out.

One other thing that is really important is that you keep all grease and oil off the clutch contact surfaces. This includes greasy fingerprints. A spray can of brake parts cleaner and a clean white rag works great.

As to a replacement clutch, well there are a lot of great clutches out there in the aftermarket including the brand that you used. Actually, a factory clutch should work just fine considering the size tire you’re using.

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