As many of you may know from following along, we started a project to replace the ring and pinion gear set in Ole Charlie, the Stovebolt Flagship, to improve cruising speed, lower engine RPM's, etc.
To start with, we had a mostly original and unrestored (but very good condition) 1949 1-ton pickup. It has a splasher (babitted) 235 engine that replaced the original 216 in the Spring of 1950 (engine is a 1949 manufacture, BTW). The tranny is the original SM420. The rear axle is the original HO72 Eaton axle with the 5.17 ratio. And the wheels are the 17-inch bias plys on 3-piece rims. The truck could cruise at 55 in stock form and sprint to 60. But it was at max RPM and screaming to do it.
Amtrakjoe sourced a rear axle from a '72 3/4-ton Chevy pickup with an automatic trans. We removed the center section (with the desired 4.10 ring and pinion gear)and took it to Stovebolt HQ. For peace of mind, I even took the new center section by our local transmission expert and Friend of Stovebolt, Sean at The Gearbox in Mechanicsville, MD, for a check. Sean looked at it and inspected the gear faces and the bearings and said it was good to go as is and to bolt it in and go. Of course, with a wink and a smile he offered to rebuild it for me anyway, being the good businessman he is, but he added it didn't need it.
As part of Homecoming, we removed Charlie's original center section and replaced it with the donor center section. Stay tuned for the tech tip on that, btw.
The operation, supervised by Sweet and involving myself, Mike B, Billy Marlow, Amtrak Joe, Mike Roache, Dr. Dave and a few others (I was under the truck mostly throughout the operation and only saw feet and ankles), was a complete success! The switchout was a simple unbolt, remove, replace, rebolt affairs. four new gaskets (both axle hubs, the inspection plate and the center carrier) and we were back on the road for a test.
We were a little cautious because we didn't know how the original steering, straight axle and bias plys were going to react, so we conservatively expanded the performance envelope to 67 MPH with only a slight buffet as we transitioned through 61 MPH. Engine RPM seemed beautful (not sure what the numbers were, but it seemed to be achieveing 60 MPH effortlessly.
Acceleration proceeded smoothly until plateauing at 67 MPH. Engine still not screaming but appearing to run out of HP. Controllability seemed marginal at best towards the upper right of the power/speed curve ... so we decided to end the test. Thus ended the first test drive. We returned to the shop and inspected the axle. No drips! No excessive heat!
We decided to give the steering box a tighten. After that, we went out for a second run. The steering adjustment did wonders for controllability! With much steering authority restored, we again made a run at the upper right of the power/speed curve. The truck absolutely will not pass through 67 MPH in level flight. So we maintained 67 until a down grade. Entering a down grade, a vehicle appeared to be entering the roadway from the right, so we had to slow down in caution. Vehicle did not enter the roadway, we by then, we had reached the bottom of the downgrade and had to build back to 67 anyway. Another down grade came along, and with my test engineer, Mike Roache, reading off the data from the GPS speedometer, and our chase vehicle, Sweet, verifying ... Ole Charlie hit 70 MPH! With a 90 HP cast-iron, splash-oiled wonder!
So we can verify that the '67-72 3/4-ton center section swap not only is a very easy upgrade, but does wonders for the driveability of the truck.
Even though Ole Charlie lacks the HP to exceed 67 MPH except in a dive, cruising at 65 is comfortable on both driver and engine, wheras before, even 55 was hard on the engine.
I am very pleased with the outcome. And it was really very easy as all had said. A very doeable upgrade for even a novice wrench turner.
Caveat: I would only recomend this upgrade if your other components are in good shape to withstand increased speeds: steering, brakes, tires, transmission, u-joints, engine, etc. My next upgrade for Charlie will be seatbelts. Mike and I were both a little uncomfortable hurtling down the road at 70 MPH in an AD cab with no seatbelts!
BIG THANKS to Sweet, Denny Graham, Amtrakjoe and the rest of the crew at Homecoming. I have a trailer for sale now
Pictures are here
