Motor/Trans mounts

[Monkeynono]

Question for the group.

What is the general consensus on how to mount the engine and trans?

On my jeep I have 2 poly (leaf spring) bushings for the motor, the trans has 3. One on each side out by the frame and a 3rd at the trans. I have been wondering if this 3rd mount is unnecessary and also allowing for excessive twisting of the trans.

 

[Bebop]

I've always liked to mount the engine/trans with 3 points of contact.

2 at the engine, 1 under the trans.

This is what makes the most engineering/physics sense and why OEMs do it that way.

I know plenty of people have had success doing it differently, but I don't like the various other solutions I've seen.

I've done it like that on my own buggy, and have been been happy with the outcome.



Imagine the engine/trans combo is a table, you want 3 legs, not 4 as it's really hard to keep all 4 in the same plane. With 3 you are guaranteed to have always no added stress from the mounts being in different planes :

In your particular case, I'd say the 2 bushings at the frame side of your trans crossmember are unnecessary, but they aren't hurting. They help your frame flex. Depending on how your cage/framework is, that may actually be a good thing.

I would definitely NOT delete the one under the trans, this is the one that does most of the work.

 

[shawn]

They help your frame flex. Depending on how your cage/framework is, that may actually be a good thing.

Could also be a bad thing. Thinking about the typical TJ frame, that would mean no crossmembers between the one at the steering box and the one above the rear axle.

 

[rodney eppes]

Not sure how your set up, but I'm using the factory CJ set up. One on each side of the motor, wide GM mount, under the transmission, & the single mount supporting the transfer case. The transfer case mount is bushed so there is some movement allowed. And I've always used rubber bushings, even in the springs. Tried poly once in the springs, & you can really feel the shock load. They probably last longer, but I'm going for the ride! I've heard the poly motor mounts, will rattle you teeth! "To each, his own"!

 

[rodney eppes]

I'll have to eat Crow, on the rubber spring bushings! I'm having my springs changed & was going to reuse my present ones, that look perfect. [and low mileage] Shop guy called me, saying mine wont fit the new springs. Crap! And in the discussion, he guaranteed me, the bushings he took out of mine, Are Poly! All I can say, I guess they got changed, last time they were rebuilt. They are black, I would of swore, Rubber. Anyhou, he tells me it's best to go with the poly, as they will slip with the spring eyes, & not stick & get ripped up like rubber. He's installed 100s, & I've installed 2. So I ordered the Correct Polys.

 

[Caver Dave]

I'm using the factory CJ set up. One on each side of the motor, wide GM mount, under the transmission, & the single mount supporting the transfer case.

Confused as you've list 4 mounts (2 motor mounts and 1 each for the tranny/TC)...
ALL the factory setups I've seen had a single mount, usually off the tranny/TC adapter, for both of those components...

Not pushing you for another crow incident, just curious... currently working on RE-doing the single tranny/TC mount after the drivetrain swap

 

[rodney eppes]

Like this >

I consider this 2 mounts, being they have 2 different bolted areas. With my custom built crossmember, I separated the torque mount, & bolted it on the side of the transfer case. If it was with me right now, I'd take a picture.

 

[WARRIORWELDING]

Hey, I have this question about the mounts made of poly for the engine and some cross members. Is most fabricated from the same bushing as springs and shackles?
Could a man stock Dom and tabs to cover them all?

 

[Mac5005]

The big idea to wrap your mind around when mounting the drivetrain, is to avoid creating stresses that the components were not designed.

The engine block is designed to handle the torsional and axial loads thru the mounting points. This is twisting, and front/rear, side to side and up down.

The transmission case and bell housing is not designed to support or control any of those movements and forces other than basic up down of the tail housing.

Depending on transfer case, some can help handle some of the stress and some cannot.

For example.

An atlas, d300, or 205 etc can be supported by the tail housing (rear output), the side of the case, bottom of the case or even the front output, while a np208,231,241 etc is not designed for these loads and forces.

You don’t want to use the tcase mounts to try to stop the engine twisting in the chassis. Think rotating around the crank centerline.

You also don’t want the motor mounts to try to control the tcase front output trying to rotate and climb a gear when bound up and in 4wd low range.

Both of those scenarios create a huge torsional load on the transmission case, and typically break it where the trans case is smallest diameter near the output shaft.

You also don’t want a heavy unsupported tcase to be unsupported hanging off the back of the trans, with the mount under the back of the trans tail housing.

The same can break the back of the trans off as the tcase tries to bounce all around but the back of the trans is the only thing supported.

When I do mounts, I just try to look at the loads and forces and the components and what can handle what stress and what can’t.

I do not want the trans case itself doing any work other than physically holding the components together.

I also try to keep the mounts that control twisting as wide as practical and possible from the centerline. I also try to match the widths in places to the amount of deflection, and load on each is similar.


The other consideration mentioned above, is you do not want the drivetrain restricting the chassis movement.

So a lot depends on each chassis, each drivetrain, etc, but the principles at play are all the same.

This coming from a guy who has a decent scrap pile of broken parts from myself, my dad, and my friends of figuring out what not to do, and then what to do.

 

[Mac5005]

Hey, I have this question about the mounts made of poly for the engine and some cross members. Is most fabricated from the same bushing as springs and shackles?
Could a man stock Dom and tabs to cover them all?

Yes most are just repurposed leaf spring bushings.

The big difference is the bolt size and bolt sleeve thickness. Most use 2.0, .250” wall DOM for the sleeve around the bushing but not all. Some are 1/2,9/16, and 5/8 bolt sizes. And various widths.

I like using the the ones that use 1/2” bolts with .250” wall bolt sleeves and 2.0” .250” DOM for the bushing sleeve.

 

[WARRIORWELDING]

Yes most are just repurposed leaf spring bushings.

The big difference is the bolt size and bolt sleeve thickness. Most use 2.0, .250” wall DOM for the sleeve around the bushing but not all. Some are 1/2,9/16, and 5/8 bolt sizes. And various widths.

I like using the the ones that use 1/2” bolts with .250” wall bolt sleeves and 2.0” .250” DOM for the bushing sleeve.

Thinking of having a small amount of these on hand. The DOM would make good trail links too.