Alloy plate

[jimson]

If you where to use alloy plate to support a 4stroke 350 engine in the frame would you use 3mm or 6mm plate    jimson

[Billet YZ]

I would use 6mm if you have the room width wise. Peter.

[GMC]

What Pete said.
Or for convenience of supply you could consider 5mm but no thinner.

6061 would be the strongest, some places will stock this and cut you small pieces. It actually comes in old school ¼”

5083 is another alloy that would be okay to use.

5356 is further down the list.

5005 would be the softest and I would avoid using it, especially if the plates are long as the longer the plate the more it is likely to flex.

[firko]

I just made a set of plates for my upcoming Tribsa out of 6mm plate. I wouldn't go much less than that, especially not 3mm.

[Tim754]

 Jimson try to suss out your local wreckers box of engine plates and brackets. If you have a sketched plan often you find a ready made set are the bee's knees. Cheers
 Failing that no smaller than 6mm good quality I suggest for the weight of the engine you list.