Motor Mounts

[CTX-SLPR]

Howdy,

Sometime in the future I'm going to move my V6 back to where I wanted to put it to begin with but didn't have the clearance on the accessory drive. Also instead of welding the mounts to the frame, I'm going to make the frame stands bolt on.
Here's the parameters I'm going to have to design around:
I have access to only one side of the engine cross member
The motor mounts are going to end up hanging halfway off the back of the crossmember
I'm after durability and reduced weight.

Right now I'm thinking 'L' channel brackets with the long leg laid down on the top of the crossmember with 4 bolts hold it down on the long leg with atleast 2 more on the short leg. The tube for the lower poly bushings would be pretty much welded directly to the top of the plate to keep the engine as low as possible. The back end that hangs off the crossmember would have a support brace under the entire remaining length of the tube.
Questions:
Would 6 3/8in bolts threaded into the roughly 1/8in thick crossmember be enought or should I put something like a Nutsert in?
Would 3/16in Al plate and 1/4in Al wall thickness tube (probably 6061) be strong enough?

Thanks,

[Caveman Tony]

How much torque is it making?

I'm leery about anything 1/8".... 3/16 & 1/4 are more likely to put me in my happy zone.

[Loren]

Is this for the Skylark?

Three 3/8" bolts per side worked stock, of course. Adding support where the mount hangs off the back of the x-member is a good idea, another bolt back there would help too. Try to put forces into the center of some structure and within a bolt hole area, not hanging off the side.

Access to the insides of those crossmembers, for putting nuts onto motor mount bolts, sucks...still, I wouldn't use nutserts there. They pull on the edge of the hole too much and if one of those things ever loosens and spins you'll never get the bolt back out. (I use masking tape to hold nuts to wrenches or set an angle to a u-joint on a socket, and reach up there with a nut/washer taped to that through the a-arm holes.) Also I wouldn't use aluminum for a motor mount, unless it were a solid machined piece. Welded aluminum just doesn't take the vibration/stress cycles as well as good 'ol iron.

[DanStokes]

Instead of a nutsert you can thread the ID of a piece of heavy wall tubing and drill a hole the diameter of the OD of said tubing into the crossmember. Then weld the threaded tubing in place. It's like a nutsert only strong.

Dan

[squirrel]

THe drawings didn't show up on my browser.

[anotheridiot]

I used 1/4" 6061 to make motor plates on a dragster, but no way would I trust an angle that thin to hold up a motor if its hanging off the cross member. I understand wanting to save weight, but not in the case of a motor mount. If you can come up with some kind of motor plate between the engine and trans to a point on the rear of the frame I would trust 1/4 inch no problem.

[OldMachinist]

The rule of thumb for depth of threads is 1 1/2 times the diameter so 3/8 should have 9/16 deep threads minimum.
I should qualify this by saying this rule is for maximum clamping pressure. If you look at engine fasteners like head or main bearing bolts you will see how the factories address bolt length. Rod bolts or head or main studs have nuts shorter than 1 1/2 times dia. but are fine threads for more contact area.

[CTX-SLPR]

Loren:

It's for my '64 Riviera. While they might not be exactly where I want them, a Skylark would have the frame side holes already there since it was optioned with the 300/340/350/231. The only way into the inside of the crossmember is to take the lower control arm off and force the bushing out and go through the hole that leaves. The original mounts are welded to the front of the frame since the Nailhead mounts are just behinds it's front cover.

Squirrel:

No drawings, I was in a hurry to post between rocking my 6mth old to sleep and going to bed myself

All:

I see the point about aluminum that thin being a bad idea. Per Metalsupermarket.com, I can get 0.250in 'L' channel as well but not 0.375 I don't think. Oh well, I can weld Steel myself, Aluminum would have to be outsourced. I could cut a billet mount if I had a rotary table but that's more material loss that I'm really wanting to pay for.

I'll have to give more thought to how to reinforce the frame where I want to put the bolt holes. Backside access like I pointed out to Loren is a royal pain!

[squirrel]

weld nuts onto the back of a plate, and weld the plate onto the crossmember.

[anotheridiot]

looking at how they have the motor in the 36, basically on a piece of moly tubing with an angle welded to it, if you want light weight and can cut into that cross member, what about using a hole saw to cut about an 1 1/2 hole thru the cross member, push moly tube thru the crossmember and weld it to both sides leaving enough length out the back to locate your motor. That way you can fab a flat piece or a mount from the tubing for your lower mount. With the moly welded to both sides of the cross member an extra 6 inches is gonna stay solid.