Well you could look at it this way. A single grade 2 mild steel 5/16-18 UNC stud(they are the corse threads in an intake manifold) maintains a minimum tensile strength of 3900 LBs with an average proof load of 2900 LBs.(A proof load is a predetermined test load, greater than the average service load, to which a specimen is subjected before acceptance for use.) So looking at those data figures you could technically hold your engine and transmission up by a single carburetor stud. So four of those studs should easily hold the 450-500 lbs of a 302 and the 100-150 lbs of a C4. For a good reference table on the strengths of specific diamater bolts here is a good reference chart http://www.almabolt.com/pages/catalog/bolts/proofloadtensile.htm
this makes it sweet going back in... http://www.harborfreight.com/2-ton-capacity-heavy-duty-load-leveler-5402.html
The concern with aluminum manifolds is the threads stripping out in the aluminum. Scanning this discussion http://www.practicalmachinist.com/vb/general/thread-strength-vs-percent-thread-vs-depth-173689/ removed any concerns I had if the stud or bolt is screwed in near the bottom of the threads. Looks like the bolts would break before the threads pulled out of the aluminum.
If you are worried about it run a chain to a couple of holes in the engine as backup until you get it in
I found a link to a report concerning this from a few years ago. The web site is no longer working but the wayback machine had the test results. I have seen a video of a similar test conducted on a standard 3 ring carb lift plate on one of the weekend automotive shows with similar results to those listed below. "Load Leveling Lift Plate Destructive Tensile Testing Results Iron vs Aluminum Manifold Knowing that there are some genuine concerns associated with using an engine lift plate attached to an aluminum manifold, Kaizen Motorsports wanted to evaluate the validity of this concern. To do so, we sent two of our lift plates to a certified test facility, along with an iron and aluminum intake manifold, to evaluate the difference between the two set-ups. We think the results will surprise many people! First, one of our plates was attached to the iron intake with four high strength carburetor studs. With the plate and manifold in a level position, the leveling device was positioned at each end and pulled to 2000lbs with no deflection. The leveling device was then centered and the entire set-up was pulled to failure. The figure below shows the results (failure point highlighted in red). It can be seen that the system did not break until it reached 7,890 lbs! Having established a baseline, we were anxious to see the results for the aluminum manifold. IRON MANIFOLD TEST RESULTS The second test was conducted in precisely the same manner, except the plate was attached to an aluminum manifold. The results of this test surprised even us. For this test the load was applied progressively from 1250 lbs to 2000 lbs and then to ultimate failure. At 1250 and 2000 the leveling device was actuated from one side to the other. The load was then centered and pulled to destruction. The ultimate load was 8950 lbs. The plate in the first test failed at a lower load simply because the lifting eye was not perfectly centered and at around 4500 lbs the plate began deflecting at a slight angle which in turn reduced the amount of force that the plate could withstand. The plate in the second test (aluminum manifold) began deflecting evenly and remained even until failure. The difference between the ultimate loads in the two tests has nothing to do with the manifold material. ALUMINUM MANIFOLD TEST RESULTS Upon inspection of the manifolds after the tests, we found that the steel carburetor studs could still be removed by hand on both the iron and aluminum manifolds. No visual damage to the manifolds was apparent. NOTE: Despite these test results, Kaizen Motorsports does not recommend lifting anything over 1000 lbs. Please see our Product Disclaimer for a complete set of proper lifting guidelines."
Good find! Thanks for posting. Surprises me that the plate is the weakest element. Still think its a good idea to use studs as they did in the tests. Wonder what they mean by "high strength" studs?
The only thing you have to worry about is if the Chinese guy that welded the horizontal and vertical plates together knew what he was doing. I used one for the first time last year, could never bring myself to trust it before. Worked out well. Put decent sized flat washers on the studs between the plate and nuts.
