Posted by Charlie on September 30, 1999 at 02:52:37:
In Reply to: Re: CHARLIE:QUESTION ON NP540 posted by Gary Robertson on September 28, 1999 at 17:05:29:
Gordon: Clark makes it easy to change, the front two gears on the countershaft are changeable, they are removable so you don't need a whole new countershaft, just the different 4-5 cluster, new input shaft, and new 5th (?4th). The NP540 has a 1 piece countershaft, so changing a direct 540 to overdrive requires an input gear, the gear behind it on the mainshaft, and a new countershaft. In either case, the ratios change in 1st thru 3rd, down by a factor of 1.25 or 1.22 because of the different #s of teeth on the input gear and its' mate on the countershaft.
Gary. 104 mph? Is that for real, have you gone that fast? With 9.00-16s, bias ply type, 590 rev/mile, that's 4200 rpm. So it sounds like you want a 5th direct 5 spd. That should be easy to find. Start looking. The internet places have 540s and Clarks (I even found a 280VO overdrive for $1250 allegedly rebuilt on one site). The direct versions are about 10% cheaper rebuilt but should be dirt cheap "as is". The big problems are making sure what you're getting and also making sure it isn't junk. For the latter, you want to turn it in all gears (you can get ratios that way), and then ,if you think it's what you want, take the top off and inspect. According to the 1978 Advance Adapters catalog, a Chrysler NP435 has a 8.50" long input shaft (stickout from front face), 0.750" pilot diameter, 5.125" input bearing retainer. Measured distance between bolt holes is 8.5" on top, 8.75" bottom, 2-7/16" up and down left side, 3-1/16" right side.
Most differences can be worked around but not always. Different bolt pattern may be work-around-able by drilling and tapping bellhousing. Sometimes it isn't possible, there will be no metal where you need a hole. You can make an "adapter ring" for a smaller bearing retainer, or mill the bellhousing for a bigger one. You can use a different pilot bushing or machine the tip down sometimes. As far as length, usually they stick into the crank 1.0", down to 0.5" is tolerable. Advance Adapters talks about "extended pilot bushings". Clutch disks can be made with whatever hubs. But there must be splines and no interference with the bearing retainer snout. Clutch disks can be flipped over. Bearing retainer snouts can be shortened. The throwout bearing must match the snout. The t/o bearing must compress the clutch pressure plate fingers sufficiently.
All in all it's easier if the transmission looks identical to yours on the front.
If you have LOTS of room, look for an Eaton/Fuller FS-5406A or FS-6406A. Different torque ratings (520 and 660 ft-lb), same weight (340), length (21.5", about 10.5" longer than your 435 - is your jackshaft THAT long?). Really nice ratios: 9.01, 5.72, 3.22, 2.04, 1.36, 1.00. I believe some of the direct NP540/1/2 are 4th 1.22, 3rd 2.13; Clarks (Eaton Fuller FS-4205A/B/C) are 1.17, 1.28, 1.48 4th; 2.13, 2.13, 2.45 3rd, 3.78, 3.78, 4.35 2nd; and 6.99, 6.99, 8.05 1st.
Anyways, you are in an enviable position, the ratio splits are much smoother with the direct high gear boxes, and the transmission is less likely to break since the power flow in high gear (highest input loads are in high gear) is straight through. You can fine tune high speed rpms with tire sizes: Michelin 9.00R16 XYs 35.5", XZLs same size 36.9", 11.00R16 XLs or XZLs 38.7".
Charlie