Wow, a bunch to comment on here....
I don't think the frame with have any more tendency to sag with the float-a-motor mounts than the original mounts. Although there may be others who will disagree.
The frames sag at the rear motor mounts because in the past the car had hit potholes or perhaps curbs which caused the frame to bend at the rear mounts. This happened years before the float-a-motor was widely used.
One valid concern is the frame necking, or getting closer together at the mounts. The flywheel housing and original mounts acted as a frame member at the mounts to keep the frame rails in alignment. I don't think that this a too much of a worry though.
Neil, you kinda answered your own question with your opinion. When a Flywheel Housing is installed with original Rear Engine Support Brackets using reproduction rubber cushions, vs. with a housing installed with new FAMs and a Flywheel Housing, the frame has the ability to twist WAY more when you support the rear of the frame on a sawhorse just in front on the rear crossmember and lift the front of the frame by one frame horn. We showed this one year at one of the workshops we held at my shop. It was pretty eye-opening how much difference it made.
The cause of the frame bending/sagging is due to the added flex IMO. In the original way, the Supports are solidly clamped to either side of the Flywheel Housing with two fasteners and the load dissipated out over the entire end of the block. The opposite end is attached with three fasteners and the load dissipated similarly as to the opposite end of the Support block.
Another concern that has been mentioned is the engine rotating under torque but I also don't think this is too much of a worry if the float-a-motor mounts are installed correctly.
This is where I think you answered your comment above. The #1 reason I feel that most do not understand the downfall of FAM is the main purpose of the Rear Engine Supports is to transfer the energy of the engine back to the tires and returns it thru the Wheels to the rear Axle Shaft Housings, up the Torque Tube to the backside of the Transmission into the Clutch Housing and finally the Flywheel Housing which is connected to both Frame Rails. The entire braking system along with both the front & rear suspension was designed on the engine remaining stationary in relationship to the frame.
So not only is the strength of the Frame channel compromised, but with the ability of the engine to move within the frame, every time the brake pedal is depressed, some of the pressure from the pedal is diverted to moving the engine on the mounts. In a mechanical system, surely we all would agree this is not good.
One final thing to contemplate, most frames that we see sagging or bent coming thru my shop typically also have a broken Rear Engine Support Bracket(s). I think the correlation comes as the pothole did not bend the frame, but instead I feel the sudden shock of the pothole initially cracked the Support Bracket which over time then allowed the Flywheel Housing to lose the inherent rigidity that it had. That overloads that area of the frame and causes the damage.
Wow that’s interesting. All I have heard as a newbie is the float A Motor kit reduces vibration.
Sounds like it’s the opposite.
Lenny, does it reduce vibration -or does it just mask it? That harmonic imbalance is still hammering on the rear main bearing either way. Maybe if you felt it, you would either drive faster or slower to move out of that harmonic area. If it is masked, then when in that speed range you are likely unaware you are damaging the engine even more.
One other consideration - float a motor mounts also electrically insulate the engine from the frame - if you keep the FAM’s you might want to add a ground strap between the frame and engine/transmission.
I have not found this to be true. As mentioned above the method by which they were painted originally would make a good connection difficult at best. Most Model-As today that are sans the engine support pans get their engine grounding thru the connection of the subcomponents such as crossmember into springs, springs into spring hangers, and the torque tube and front radius rod connection at the clutch housing. Pretty wild when you think of the electrical paths a ground must find.
