Engine and transmission mounts
 Motor mounts
 Motor mount construction
Some OEM mounts are made of a steel frame which one side will bolt into the cast iron engine block and the other side will be clamped to the frame by way of a thru-bolt. The upper and lower mount halves will be sandwiched with a layer of rubber and cotton fiber reinforcement that is vulcanized and molded to the metal frames.
The motor mount may be bolted to the crossmember and attached to the engine by a through bolt to a metal bracket that is bolted to the block, or the motor mount may be attached directly to the block and be mounted to the chassis by through bolt to a stand or bracket that is bolted to the crossmember.
In most cases aftermarket motor mounts will have some sort of overlapping of the upper and lower frames to hold the units together in case of accidental separation.
 Early Chevy motor mounts
Early type Chevys offered two types of motor mounts: one is short and wide, the other is tall and narrow.
Oldsmobile also offered short and tall models that could be mixed to offer a lower than normal mount or a taller than normal mount. Here is an idea of the rough measurements that you can get:
- Small block engine mounts with small block frame mounts - OK
- Big block engine mounts with big block frame mounts - OK
- Small block engine mounts with big block frame mounts - the engine will be 1-2" low
- Big block engine mounts with small block frame mounts - the engine will be 1-2" high
 Broken motor mounts
Often a broken/separated motor mount will go unnoticed until the vehicle is brake torqued or accelerated hard in reverse. This causes the engine torque to lift the engine up off the broken mount. This can result in the fan blade contacting the radiator shroud with a resulting grinding noise and possibly a bent fan blade, or possibly even damage to the radiator could result.
In the '60s, GM changed from a rod operated throttle linkage to a cable, in part because in the case of a broken motor mount the engine lifting up off the broken mount could jam the throttle open. This could cause a loss of control and result in damage to the vehicle or worse.
Along about this same time, a different type of interlocking motor mount was introduced. It would physically hold the two halves of the mount together in the event the rubber insulator became debonded. This type is used to this day.
 Causes of broken motor mounts
Motor mounts live in an environment of heat, oil and fuel leakage, and constant vibration and stress. They reside in close to 1000+ degrees of heat from the exhaust manifolds or headers and are subjected to deterioration by oil and fuel leakage from the valve cover gaskets and fuel lines. Most OEM motor mounts are made from compound rubber and steel frames. Many motor mounts will spend a couple hundred thousand miles and decades of use before ending their life. The final result of their life will be 'shear', whereby the upper and lower units of the mount will be torn in two, usually caused by engine torque finally overcoming the mount's ability to remain bonded together.
 Replacing Motor Mounts
Usually motor mounts do not require the engine to be removed from the vehicle, but in some cases the lack of clearance requires it. Besides lifting the engine up, the vehicle may also need to be raised to allow parts to be disconnected and reconnected. In some cases, radiator shrouds, exhaust manifolds, fuel and water lines, linkages, etc. may have to be loosened or removed. There are times that engines have to be removed to replace mounts, but this is rare.
Always replace mounts in pairs. Even though the other looks good, you have the equipment available and spending the time now is better. Also look at replacing the rear trans mount, especially if they are OEM equipment. In some cases, mounts may come as a complete kit for your application.
- Gather all equipment together to do the job. This includes engine lift, roller jacks, jack stands, drain pans, speciality tools, wood blocking and planks.
- Find a level surface, block rear wheels and jack up the front of vehicle to a level which allows you a safe access under the vehicle. Block up the vehicle using jack stands under the frame.
- Place engine lift at front. Remove air cleaners, fan shrouds, and battery cables. Loosen or remove exhaust pipes, water and fuel lines if necessary and drain them. Install lifting chain or lifting plate to engine block, do an initial lift to engine and then inspect.
- Check for electrical wires, lines and hoses that are pulled. Check for clearance between engine and firewall, and transmission and floor pans. Check for rad to fan clearance. Check steering shaft to manifold clearance.
- Continue raising engine to a suitable working and mount removal height. Remove thru-bolts from both mounts. Make engine stable using jack stands, wood blocking and planks. Wiggle engine around to simulate working on it. REMEMBER: YOUR FINGERS WILL BE IN CLOSE PROXIMITY TO THE FRAME AND OTHER APPARATUS.
- Remove mounts from engine and frame. Compare old parts to new replacements. Inspect for any damage to engine and frame. Chase out the threads in the block with a bottoming tap and use new grade 8 bolts and washers of the appropriate size.
- Install new mounts. Apply lubrication to all bolt threads and tighten to specifications. Lower engine and align lower mounts, insert thru-bolts and tighten up hardware. Thrugh bolts are usually a knurled grade 8 type for maximum shear resistance.
- Continue lowering engine and reassemble parts and equipment as required. Lower vehicle to ground after final inspection.
Note: If replacing transmission mounts, raise transmission after loosening bolts and slide in new mounts, then finger tighten bolts before lowering vehicle. Tighten and torque down bolts. For added security use lock nuts or loctite to ensure that these bolts don't back out.