Lift That Engine… Have you ever wondered?

– A point pondered and drawn by Lanny

Lifting an Engine

Some of us occasionally have the need to lift an engine, possibly with the transmission attached. How often have do we use a lifting plate that attaches to the intake manifold by four 5/16th bolts? How about if that manifold is made of aluminum? Have you ever thought about just how much weight is riding on those four little bolts screwed into aluminum. I know I think about it every time I do it.

Well, I guess someone else thought about it too. In the July issue of Car Craft, 2002 there is a letter to the editor asking this very question. The answer is pretty involved. For those of you that are mathematically inclined the answer can be found using the following equation:

As=0.7854(Dbsc-0.9743/n) 2

Got that? Now how about that in English?

First lets assume that you run the bolt in at least twice the depth of its diameter. In the case of a 5/16″ bolt that would be 5/8″. Lets also assume that you are using a grade 5 bolt, the most common type. When you torque the bolt you add a certain amount of stress to it, lets say that is about 20%. The tensile strength of a grade 5 bolt is 120,000 lbs psi. That means that if the bolt was 1 inch thick it should hold 120,000 pounds. But, of course it is not. A 5/16″ bolt would hold “only” 6300 lbs. and if we subtract the 20% torque load we place on it, only about 5000 lbs. That’s one bolt, we have four. So, assuming that all is well, we should be able to hold about 20,000 pounds before the bolts would fail. According to the article that applies even in the case of an aluminum manifold.

Most of the engines I work with weigh a lot less than that, so I guess I will worry about something else.

© 2007