My F150 has a np435 4 speed and I've put a replacement clutch in it from Rockauto. It was an 11 inch half ceramic/ half organic disc. Within two weeks the "ceramic" material gave out and slips like crazy now. You can clearly hear when the clutch is touching the flywheel because it sounds like paper rubbing against a wall. the organic side is still good however and its still very drivable, just doesn't grip like it did when it was new. I could chirp 35 inch tires with my 300 in second gear, now it doesn't even think about it. Just lugs. Anyways im looking for clutch recommendations because buying another one from rockauto just aint gonna cut it. Truck is lifted 6 inches with 35 inch tires, Im about to swap a zf5 in it too so whatever I get needs to work with it. Thanks.

 

Does your clutch pedal have any FREE PLAY? Free play is when you step on the clutch pedal and the pedal pushes rather easy until the point that the throwout bearing starts pushing on the pressure plate.

In other words without FREE PLAY it is like you are pushing on the clutch pedal all the time and the clutch can't fully grab. If you have no free play you may be able to adjust it down by the clutch slave cylinder. Just a thought.

 

Unless you changed the gearing, 35 inch tires are going to be hard on any clutch. I had 35 inch tires on my 89, and had to use 1st gear which is a partial granny to get it going from the stop. And it has 4.10 gears in it. Your 4 speed has a full blown granny gear, so you probably can't even use it, so you are most likely starting out in 2nd, and I am wondering how much you have to slip the clutch to get those tires going. What gear ratios are you running?

A gas zf is going to have a tall granny gear like your 4 speed. Only the diesels had the higher speed granny 1st in the zf 5 speeds.

P.S. A zf will give you overdrive. I wonder if you can even use it if you have a low numbered gear in it.

 

the clutch feels firm until it “breaks over”. But it is not firm right at the top of the pedal. You can definitely feel when it starts to move the diaphragm when you first push it down.

 

So I actually did have 3.08 gears a while back but I had swapped them out for 3.55’s. I wanted something lower than what I had but still tall for easy highway going. I do start out in second quite a bit, granny gear is most definitely unusable on the street lol. But it really doesn’t slip all too bad starting off in second, the fact that it’s the 11 inch clutch and not the 10 helped tremendously when I first swapped it out. The 10 inch I felt like I was at the end of second by the time I completely let off the pedal.

 

I swapped to 31's when I had a f150 and it had 3.50 gears in it. It definitely killed it's get up and go with 31 inch tires. I would imagine 35's are really bad. I know it's expensive, but a gear change will really help. I finally wore out the 35's I have on my diesel now and went to a 31 inch tire on it. It's a different truck. I tow a lot with it, it tows better and runs down the road better with the smaller tires.

 

Did you have the flywheel re-surfaced? Is there clearance between the throw out bearing and the pressure plate?

 

Well I’m actually about to put a 9 inch under it that I’m rebuilding but I already got 3.55 gears for it. When these 35s wear down I’m probably bringing the lift down to about 3 1/2 inches of lift. And definitely smaller tires.

 

Actually I got a new one, it was so cheap I had to. About 70 dollars.

And there is no clearance, the spring pressure of the slave holds the fork against the bearing against the flywheel. Light pressure though, not enough to cause an issue.

 

On the hydraulic style clutch systems, the throwout bearing rides against the clutch at all times.

 

Does the rear main bearing seal on your engine by chance leak? That would certainly cause the clutch to slip. If your rear seal does not leak an 11 inch clutch should certainly not slip once you are in a higher gear and moving and accelerate.

 

But big tires and a high rear gear you have to slip the clutch more than smaller tires and a lower gear just to get going.
That builds heat on the clutch system and kills it.
I think till smaller tires and / or lower gear he will have the same issues.
Dave ----