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Old 02-18-2018, 09:17 PM   #198
burnitg
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Join Date: Nov 2009
Location: Black Hawk, CO
Posts: 215
Re: Project Screamer - '72 GMC K2500 + DD 4-53T

Transmission Part 3/Cross Member Part 1:

I re-installed the transmission. Installing a transmission from above into an open frame is easy. I'll have to remember that if I ever have to pull it from below. I'm sure that it will be difficult.

The last thing to do before calling the transmission complete was to check the operation of the air shift for the auxiliary section. I dialed the shop air pressure down to 50 psi, hooked the supply up to the slave valve and attempted to shift - nothing, not even a hiss. The slave valve was frozen up. And to make matters even worse, the manufacturer, Parker, had used steel screws into the aluminum body to hold the cover on. After years in a corrosive environment, the screws and the body had chemically combined into one. So, the valve was headed for the recycle bin. When I received the new valve, I decided to tube it up instead of using hoses. Here it is installed:

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It functions flawlessly - the auxiliary makes a nice clunk when I move the switch on the gear knob and shift through neutral. The transmission is now complete. One more thing to check off of the to-do list.

Time to start on the transmission rear support and cross member. It turns out that this is a multifaceted problem. It involves both the rear transmission support and the front drive shaft.

My first thought was to support the rear of the transmission from above. There is just enough clearance between the bottom of the cab and the top of the frame to sneak in a cross member. The support for the rear of the transmission would be tricky but doable.

Then I had a look at the front drive shaft. The truck is currently sitting about 1" above the bump stops (stock springs with a much heavier engine & transmission). This gives me a good idea of what will fit and what will not. Here's a look from the transfer case toward the front axle:

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Here's a look in the opposite direction:

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After some measuring, I concluded that the largest drive shaft that I could comfortably fit is 2" in diameter. The problem is that the distance from the center of the transfer case yoke to the center of the front axle yoke is 53". A 2" shaft that long is essentially a piece of spaghetti. A quick visit to Spicer's maximum RPM calculator confirmed this. Their calculated max RPM for a shaft with these dimensions is 2558. That will never do for a truck whose engine has a maximum governed RPM of 2800 and a transmission with a 0.80 to 1 overdrive. Conclusion - I need a center bearing for this shaft.

As an aside, I love Spicer. While some manufacturers act like data on their products are trade secrets, Spicer shares everything and provides helpful tools like the RPM calculator and torsional vibration analysis calculator.

That changed my transmission support design. Now I'm looking at supporting it from below. It's actually going to be a lot easier to connect to the transmission. It looks like Fuller intended that the rear support be from below. Here's my rear support mocked up in cardboard:

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I purchased the parts to make the front drive shaft. Spicer has a data sheet that lists all the parts necessary for a 2 piece drive shaft. It's as simple as they could make it. Here are the parts:

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Now, it's time to start cutting some steel...
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