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73-87 Chevy _ GMC Trucks => Engine/Drivetrain => Rear Drivelines, Drive Axles and Gearing => Topic started by: unbludictable on April 03, 2008, 11:06:21 am
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I had a 350 long tail transmission put in my '76 GMC, which had a short tail in it, and
I was wondering would it be better to have the original driveshaft cut to fit, or find a
used shorter one?
My drive shaft is 2 part, not the single shaft. Which is better, 2 part or single?
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I did a similar switch in a 79 with a 3 speed stick to a long tail shaft 350. I used a single piece drive shaft which is standard with that transmission, at least it came in the truck I got the trans from.
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This transmisson came out of a '71 C10 Stepside short wheel base, which is the reason why I did not buy the driveshaft with the transmission.
My truck is a long wheel base, mostly original! Not sure if I want to cut my 2part driveshaft to fit or buy a shorter single driveshaft.
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either way is acceptable. Weigh out the cost of shortening it versus if you can find one locally in a salvage yard.
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Whenever possible it is better to use a double drive shaft. The bearing setup in between the two is to minimize vibration which in return puts less wear and tear on the transmissions output shaft. Vibration on the output shaft has a domino like effect on the rest of the transmission. No matter how "true" a drive shaft there will always be vibration, the longer the drive shaft the greater the vibration. Other factors such as age of the transmission components and factory machined quality of the parts are also vibration inducing factors. For example, even if you get a high quality part the tooling it is made with has replacment specs. Let's say for example that the blade used to machine a part (though well oiled/watered) is only good for 1000 cuts. Cut number 1 will be more true than cut number 1000 even if by hundreths of an inch. This could be a big factor in vibration depending on the situation.
Similar drive shaft setups are used in older commercial HVAC setups where motors operate continuously. Hour totalizer numbers can really benefit from dialed in balancing. For example, before joining the Air Force I was a building superintendant. One of my HVAC motors needed to be replaced every three years as opposed to the others averaging eight or nine. I couldn't figure it out, the unit seemed to fuction smoothly and quietly. Through mentors I got cantact information for a "Vibration Tech", a career filed that I didn't even knew existed at the time. He had a special stephascope and a special meter, similar to a Fluke meter with a thermal probe only the probe picked up vibration levels. He ended up finding the problem: a previous employee had replaced the bearing between the two shafts with a generic unit boasting the same specs as the OEM unit to save money. As it ended up, the bearings were the same ABEC grade but the race was of a lesser grade. It was not off by much but enough that over hundreds of thousands of RPM's it made a difference, enough to shorton the motors life cycle. Seven years later that motor is still in service. I am not insisting that a truck needs that exact type of precision but it does make you think.
The point is, GM put that there for a reason and it is probably better to leave it. Vehicle manufacturers are in the business of keeping production cost down, especially GM in comparison to a company like Mercedes. We are not talking about relocating a stationary shock mount, this is a moving part under a tremendous load. Custom applications are "good stuff" but you always have to overthink the long term effects and not get to cocky and jump right into it. Hope this post benefited someone- Mr. Blocks
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Uh yeah, I had the original driveshaft cut to fit and put it back in the truck! It's running like a deer!
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Hopefully not like a John Deere :P