this is a question for anyone here who has made their own engine cradle
for a swap into their vehicle and did not use the stock 914 engine bar

what thickness of steel did you use for the cross bar? (connecting the mount locations in the bottom of the bay)
the rest of the cradle?

question applies mostly for people who did separate mounts for trans and engine.
if you made a single more complicated cradle for both then the same metal info
probably doesnt apply.

I was thinking I would use 3 guage square tubing for the cross bar and similar thickness tubing for the section that connects the cross bar to the mounts, then 1/2" sheet steel to make the mount components.
. I am using a subaru eg33 (H6) engine.
overall load is probably around 500 lbs for engine and trans.


thanks

I used 3/4" & 5/8" .120 wall DOM tubing... Super light and strong. Cradle weighs 25lbs.

I made it semi modular so it can be used for any engine if you rebuild the motor mounts...

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My engine is in the same weight category as the EG33. I used 9ga (.15) thick square tube (1.25") for the engine bar. Also my entire engine dolly is made from 13g square tube. Plenty thick and durable.

I made the style your not interested in but it varies with the type of steel. DOM vs. Chromoly are apples to oranges. + Chromoly is a bitch to bend but less weight.

Sorry, I have to comment since this is one of my pet peeves...

In this application there is really no strutural reason to use chromoly over mild steel. For an engine cradle you need a stiff, non-yielding structure. The modulus of elasticity (stiffness) for mild and chromoly steels are very similar (205 GPa for 4130 chromoly and 200 GPa for 1020 mild) which means that for similar geometry, the structures will be equally stiff and weigh the same.

The difference comes in when looking at ultimate strengths; 670GPa for 4130 chromoly and 394 GPa for 1020 mild. This means that once the structure has yielded the chromoly will take more stress to break. You'll never reach this in an engine cradle. Note that if not properly normalized, welded chromoly is more brittle than welded carbon steel and will fatigue crack more readily when cyclically fatigued.

That's the end of my soapbox rant.

In regards to your question, stiffness/strength increases as a cubed quartic function of size. This means that it's always better to go with a larger OD tube and smaller wall thickness when you are looking for stiffness with minimal weight. 3 ga steel is way thicker than necessary, 3d914's tube sizes should be just fine in your application. That said, I would be tempted to go to a 1.5" or larger tube with smaller wall thickness, say 1.5x0.095", which would theoretically be 3.6 times stiffer while only 80% of the weight.

I used 1.25" square tube with my rotary conversion. I found the bar bent in the middle due to the weight of the engine bouncing on the bar from normal driving. It did not bend much but it was noticeable. That's why I use small OD round bar and triangulated everything. Nothing is bending now and the Audi engine weighs more than the rotary.

This is why the factory bar is a cast I beam design. Reduced flex and dynamic oscillation.

I think 1 1/2 round X .090 ~ .120 wall should handle your needs.
I went looking for 1.5 X .090 for my X-bar. I settled for what they had, .120 wall.
Maybe it was best. I would have to see and feel the .090.
I just remember .090 from the old days, as the wall to use for off road race cars.
A lot will have to do with triangulation in your design.
For areas that do not carry as much load I would go .090