03/27/07
Transmission Swap
Project
This is a description of my effort to put a different manual transmission into my 70 Chevrolet, 2 wheel drive, ¾ ton pickup, with a 4.10:1 rear end ratio. The main problem was the reverse gear ratio in the stock Muncie SM-465 four speed transmission. The pickup is often used to pull and maneuver a 10,000GVW trailer. Maneuvering, often involved backing the trailer up a hill along a winding little road. The 6.05:1 reverse ratio in the SM-465 proved to be woefully inadequate. I was constantly having to slip the clutch. (Why did they think I would need to go 25 or 30 MPH in reverse?!!) Going forward in the 6.55:1 first gear wasn’t as bad, but it could also be a bit of a problem. Note that all this would not be as much of a problem in a 4 wheel drive rig because deeper gear ratios would be available with the transfer case.
As I wondered about a solution, I remembered the 62 Chevrolet, Model 60, truck we used to use on our farm in the early 60’s. That truck had a 5 speed transmission hanging off of a small block 327. In my efforts to find out what that transmission might have been, I discovered the “GMC Guy” web site where a number of transmissions and gear ratios are listed. This is where I discovered the NP-540GL. The gear ratios are as follows:
1st: 7.41, 2nd: 4.05, 3rd: 2.40, 4th: 1.48, 5th: 1.00, Rev: 7.85.
This is as close to perfect as I could ask for. The only thing else I could wish for would be a sixth gear as an overdrive.
After asking around at wrecking yards, I found that these transmissions used to be put into pickups fairly often. In fact, two different guys told me they used to do this all the time and that, except for the driveline, the thing would bolt right in. Except for one little surprise, this turned out to be mostly true.
The following is a description of what was considered, the advantages and disadvantages of the different options, and what was involved in installing the NP-540GL into my pickup.
The Considerations
There were a few other options I considered before deciding to replace my SM-465 with the NP-540GL. One was using the NV-4500, the NP-542, and the NP-435GL. (I should point out that there is some disagreement about the ratios for the SM-465. I think I have them correct, but I am working on finding out more.)
NV-4500
The references I found indicated the ratios for the NV-4500 transmissions to be as follows:
Transmission: NV-4500 NV-4500 NP-540GL SM-465
Early GM Current GM
(4.88 rear end (4.88 rear end
Equivalent) Equivalent)
Gear
1st 6.34 (7.55) 5.61 (6.68) 7.41 6.54
2nd 3.44 (4.09) 3.04 (3.62) 4.05 3.57
3rd 1.71 (2.04) 1.67 (1.99) 2.40 1.70
4th 1.00 (1.19) 1.00 (1.19) 1.48 1.00
5th 0.73 (0.87) 0.73 (0.87) 1.00 -----
Reverse 6.34 (7.55) 5.61 (6.68) 7.85 6.05
It is obvious that the Early GM version is an improvement with reverse when compared to the SM-465, but makes my problem with 1st a little worse. To actually get the deep 1st and reverse I am after, I would have to swap out my 4.10 rear end with a 4.56. This would give me an equivalent reverse and 1st gear of (4.56/4.10)x6.34=7.05, which is not as much as I want but not too bad. If I went to a 4.88 ratio rear end, I would get (4.88/4.10)x6.34=7.55. This is even better.
The same rear end swap with the Current GM version gives me an equivalent reverse and 1st gear of (4.56/4.10)x5.61=6.24. This is a slight improvement for reverse and a problem for 1st. A 4.88 ratio rear end ratio would yield (4.88/4.10)x5.61=6.68. This is at least a useful improvement for reverse and a slight improvement for 1st.
Another thing to consider is what my equivalent rear end ratio would be when I throw the early GM or the Current GM version into 5th gear. Both versions have a 0.73 5th gear. For the 4.56: 4.56x0.73=3.33. For the 4.88: 4.88x0.73=3.56. The 4.56 ratio rear end gives me an equivalent rear end ratio, 3.33, when in 5th gear that is a little taller than I think I would be happy with. Either way, 5th gear wouldn’t be of much use when loaded except on very level roads.
My conclusion was that I would need to find a 4.88:1 rear end and the Early GM Version in order to get the deep 1st and reverse gears I was looking for. It may not be easy to find the 4.88 ratio rear ends. Also, look at the equivalent ratios for all the gears with both NV-4500’s and compare to the current SM-465 and the NP-540GL. Another thing that is a bit of a pain with my SM-465 is the big jump when shifting from 4th back to 3rd before starting down a hill, etc. (This is actually kind of handy when driving around town unloaded. Except for stopping, you can go from 4th to 3rd and almost drive all over town with out any more shifting.) The large jump from 4th to 3rd is about the same in both NV-4500’s. Also, working through the gears in the NP-540GL from 2nd to 5th is almost like driving a close ratio 4-speed.
As I mentioned above, if I went with the NV-4500, I think I would have to have the Early GM version. I think these are available, but a little harder to find. There are bell housings and other stuff available. A cross member would be necessary, some messing around with clutch linkage and maybe the clutch, electronic speedometer output, etc. All this is in addition to the 4.88 rear end. An advantage to the NV-4500 could be the overdrive.
NP-542
The following table shows the ratios for the NP-542, NP-540GL, and the SM-465.
Transmission: NP-542 NP-540GL SM-465
Gear
1st 7.24 7.41 6.54
2nd 4.33 4.05 3.57
3rd 2.61 2.40 1.70
4th 1.59 1.48 1.00
5th 1.00 1.00 -----
Reverse 7.22 7.85 6.05
First and reverse are not quite as deep for the NP-542 as with the NP-540GL, but they are OK. Second, 3rd, and 4th are a little deeper than with the NP-540GL, but that could be an advantage. The jump from 5th to 4th is a little wider than I would like, but it is probably also OK.
A disadvantage to the NP-542 is that I would have had to replace the clutch. However, there is a huge advantage to this transmission that I decided to risk doing without. It is still possible to find hard parts for the NP-542. Apparently, hard parts are not available for the NP-540GL.
NP-435GL
Transmission: NP-435GL NP-540GL SM-465
GM version
Gear
1st 6.68 7.41 6.54
2nd 3.34 4.05 3.57
3rd 1.66 2.40 1.70
4th 1.00 1.48 1.00
5th ----- 1.00 -----
Reverse 8.26 7.85 6.05
The NP-435GL provides a huge improvement in reverse over the SM-465. First gear is only slightly improved. Second gear is a bit of a problem because it is a taller gear than 2nd in the SM-465. This would be a pain when trying to start out in 2nd.
I didn’t look at this option any further. If I am going to go to all this trouble, why not go for a 5-speed?
Doing It
One of the reasons I choose the NP-540GL is that I was running out of time and it looked like it was going to be as close to a slam-dunk as I would ever get. Also, due to time restraints, I decided to have a local transmission shop install the thing. The first thing they brought up was that the SM-465 front retainer flange is 5 1/8 inches. This looked to be larger than the NP-540GL front bearing retainer flange of about 4.7 inches. I suspect that the slam-dunk idea came from the fact that the old SM-420 transmissions had a front bearing retainer flange that was the same size as with the NP-540GL. I am still checking on that. **
The solution was to find another bell housing. The one I found had been on a 63 Chevrolet Model 30, one ton pickup, with the 292 six cylinder, a 13 inch clutch, and the NP-542 transmission on it. The front bearing retainer flange and all the bolt holes lined up perfectly. The starter and clutch parts also fit just right. (It is important to point out that the bell housing needs to be large enough to accept the 10 or 11 inch clutch that is in my pickup. I don’t know if just any bell housing off of a 60’s vintage pickup with the SM-420 transmission would do the trick. This is worth checking out.)
The backup light switch in the NP-540GL didn’t work, but the one from the SM-465 fit and works just fine.
The speedometer cable worked OK, but it had to be stretched a little which caused an occasional nasty noise. It eventually had to be replaced. The ratio was way off and needed to be adjusted.
The drive line was almost a fit. But, it had to be cut shorter by a small amount.
The transmission floor cover was fine and the hole for the stick did not have to be moved. Of course, the stick had to be bent in a couple of places to keep from hitting the dashboard.
The Speedometer Speed Ratio Adjustment
Since the NP-540GL was originally in a truck of some size, the speedometer gears were set for a rear end ratio of 5.29, 5.38, 5.86, 6.17, or even 7.17. Sure enough, the speedometer indicates a speed much slower than it should be.
I couldn’t clock my speed very well with traffic, snow, and construction. So, with the diameter of the rear wheels, measured from the top to the street, and the rear end ratio, 4.10, you can calculate the engine RPM for different speeds. I did this in a spread sheet. ** Spread Sheet Link** The result is that the tachometer should read about 2000 RPM at 45 MPH and 2545 RPM at 55 MPH. The speedometer indicated 34 MPH instead of 45 MPH at 2000 RPM and 43 MPH instead of 55 MPH at 2500 RPM. At 2445 RPM, the speedometer would have read 56.24 MPH (Please note that all these decimal places of accuracy are absurd for this calculation, but it helps me keep my numbers straight. Also, note that there is some error associated with measuring tire diameter and with reading the tachometer.) So:
56.24/43 = 1.31 and 45/34 = 1.32. The available speed ratio adaptors that are the closest to what I need will increase the speedometer by 1.28 or 1.38. So:
43x1.38 = 59.34 MPH 59.34-56.24 = 3.1 MPH fast
34x1.38 = 46.92 MPH 46.92-45.00 = 1.9 MPH fast
43x1.28 = 55.05 MPH 55.05-56.24 = 1.2 MPH slow
34x1.28 = 43.52 MPH 43.52-45.00 = 1.5 MPH slow
The 1.28 adjustment seems to be the closest. However, I hate having the speedometer read slow, so I ordered the 1.38 speed ratio adaptor. I will try it out and see what happens. If I can’t stand it, I’ll break down and order the 1.28 adaptor. I also ordered a 20 inch speedometer cable extension in order to straighten out the cable and remove the squeaks. This was before I realized I would have to replace the original cable.
When I went to install the ratio adaptor, I discovered that it was a good thing I also ordered the 20 inch speedometer extension. It turns out that even if my existing cable was long enough, the ratio adaptor comes so close to the floor of the cab that the cable would have a hard time making the corner with out binding inside, or something. So, the solution was to attach the 20 inch cable extension to the transmission, make the corner, attach to the ratio adaptor, and then to the existing cable.
I found that the existing cable was still making noise, so I replaced it with a new slightly longer one. I also found that the 20 inch cable didn’t have the opening properly formed, so I had to file the stem that fits into the extension from the adaptor into the correct shape. What a pain! I talked to the vendor about it and was told that you just have to do the grinding.
NOW, after having an opportunity to drive on dry roads and clock the truck at 60 mph, I found that the 1.38 adaptor puts the speedometer about 6 mph fast. So much for all the calculations!! One problem is that speedometer error is a percentage. The result is that you can read the speedometer more accurately at higher speeds when the actual mph error is greater. I was reading the speedometer when it indicated 34 and 43 mph in order to make my calculations. I should have waited until I could drive a lot faster. I should have found an opportunity to drive about 75 mph without an adaptor to get a more accurate speedometer reading along with the tachometer reading. When I was finally able to clock the truck at 60 mph with the 1.38 adaptor, the speedometer was reading 66 mph. Therefore, (66/1.38)x1.28=61.22 mph with the 1.28 adaptor. So, if I install the 1.28 speed ratio adaptor, the speedometer should be just over one mph fast at 60 mph.
At least the calculations got me into the ball park.
The Screw Ups
The reason I had the initial job done by the transmission shop was that there wasn’t time to take the truck 500 miles to my brother’s place, do the job, and get the driveline work done over a weekend, etc. I needed to get the new transmission installed and tested during the warrantee period. Since I am planning on rebuilding the engine during the summer when there is more time, I decided to have the transmission shop leave the clutch alone. The clutch was working OK, so I thought it could wait until the engine rebuild. Also, the rear main was leaking a little bit and I didn’t want to get a new clutch wet. Soon after the transmission was installed, the pilot bushing started messing up on the new transmission shaft. Not one of my brighter moves!! It would have been well worth while to have the transmission shop just swap out the clutch stuff and put up with any leaking from the rear main. Anyway, I took the rig down to my brother’s place and we swapped out the clutch stuff over a weekend. See Fig. 56 to Fig. 76, or so.
I should have had the driveline shop put one of those “easy to take apart” U-Joints on the drive line at the transmission. Without this type of U-Joint at the transmission, removing the driveline is a pain. See the explanation for Fig. 059.
The other thing I should have done would have been to drive a little faster while getting speedometer and tachometer readings in an effort to figure out which ratio adaptor to order.
The Results
The NP-540GL ratios are great. Shifting through 2nd, 3rd, 4th, and 5th seems just like driving a close ratio 4-speed. When going around a corner in town, unloaded, most of the time I can get away with shifting back from 5th to 4th and continue without having to go back to 3rd. I have yet to drive it with the heavy trailer. That will be the real test.
The transmission shifts a little clunky, but not bad. It’s obviously not a sports car. The only real problem I have is getting it from 3rd into 4th. For some reason, it is easy to miss this shift and have to hunt around for 4th, and I found reverse a couple of times. I have to push the stick forward, deliberately move it the correct distance to the right, and push it into 4th. I am getting used to it, but occasionally I have to mess with it. We will see. At this point, I am optimistic.
** Here is a link to some pictures and more explanations**
**Back to Beth's Backyard home page**