Chassis: Motor and Transmission Mounting
From The Scratch-Built Hot Rod
Chapter 4: Chassis - Motor and Transmission Mounting
Mounting the engine
With the front suspension in place and the rear of the frame on solid jack stands, the engine and transmission can be mocked up in position between the frame rails, and motor mounts can be designed and fabricated. For this particular project, the engine and transmission will be positioned so that the lowest point on the oil pan is even with the bottom of the frame. It is important to make sure the position of your engine will not extend below the scrub line once the car is at final ride height.
To understand the concept of scrub line, visualize your car without any tires, just rims, sitting on a sheet of glass. Anything on your car that would extend down below the sheet of glass is below your scrub line. In the event of a blowout or other tire loss, that element of the car would come in contact with the pavement and result in partial or total loss of control, as well as other potentially catastrophic results. The actual scrub line can be measured by stretching a line from the lowest point on the front left rim to the lowest point on the rear right rim. Then, do the same for the other two corners of the car. There should be nothing on the car extending below the plane created by those two lines.
To temporarily set the motor and transmission into position, use supports or blocks to hold the engine at the correct height. For this project, two sections of 2x4 rectangular tubing were clamped to the bottom of the frame rails on each side of the chassis as supports for the engine. One support was placed approximately under the lowest point on the oil pan and the other was placed under the main section of the transmission (but not under the transmission mounting holes.) The engine and transmission were slowly lowered onto the supports until their full weight was on the frame and supports. The engine may be a bit unstable, so keep it chained to your hoist so that it can't possibly roll over or slip and fall.
|Next, the tie rods are attached to the front spindles in order to determine crank pulley clearance (Photo 4-1). By turning each wheel from stop to stop, you can determine if the crank pulley will clear the tie rod in all wheel positions. If the tie rod contacts the pulley, the engine will need to be slid rearward in the frame. If the pulley-to-tie rod distance is greater than anticipated, the engine can be slid forward in the frame. To locate the engine side-to-side, a steel rule, such as the one shown in Photo 4-2, is set across the frame rails, squared up and then clamped in place. Use one at the front of the engine and one at the tip of the transmission tailshaft. The steel rules are set so that an inch marker is at the exact center between the two frame rails.|
|Then, place a floor jack or transmission jack under the tailshaft section of the transmission. Place a level on the carburetor or the intake manifold as shown in Photo 4-3, and then jack the tailshaft up or down until the engine is level. On this stock Ford carburetor, the vent tube can be used to determine level, or the carburetor can be removed and the machined mounting surface of the manifold used for finding level. Note that when the intake manifold or carburetor are level, the crankshaft and transmission tailshaft will be pointing a couple of degrees down and to the rear. So, do not be alarmed if you put your gauge on the tailshaft and it is not level. It's not supposed to be; it will normally point 2-3 degrees down.
With the engine sitting level, the motor and transmission can now be wiggled left and right until the crank pulley and the tailshaft are centered on your steel ruler. This will likely require a lot of jiggling, and is a job done best with two people: one to hold the front of the engine in place while the trailshaft is moved into location and then vice versa as the crank pulley is moved into position. During the process, make sure the engine is still level, and that the front-to-rear positioning has not changed.It should be pointed out that in some applications, the engine may be offset to one side of the frame or the other, rather than centered exactly between the frame rails. In such a case, make sure that the front of the engine is offset the exact same distance as the rear. Just remember that the centerline of the crank and the centerline of the tailshaft must always run parallel with the frame rails.
|With the engine and transmission centered between the rails, level, and set back the desired distance from the front of the car, you are ready to make the motor mounts and transmission mounts. The Ford 302 stock engine mount includes a hard rubber cushion unit which has a flat steel face molded right into the rubber. This cushion bolts directly to the engine. Then, a stamped steel mount bolts to the cushion and spans the distance to the frame. For most applications, you'll be using the cushioned portion of the mount, but the stamped steel portion will need to be replaced with a new "bridge" section to span the distance from the cushion to the frame.
To make our bridge, we first cut a piece of 1/4" flat stock to match the size of the steel face of the cushion mount as shown in Photo 4-4. Then, drill a hole near the center of the 1/4" plate to match the attachment bolt, which is welded onto the cushion mount. You can then bolt the 1/4" plate to the steel face of the cushion mount. This provides a surface for you to weld on the bridge section.The bridge is made using 2x3 rectangular tubing. To determine the length and angles of the end cuts, use cardboard or heavy paper stock to create a pattern. With the engine stationary, you can cut and trim the pattern until it fits snug against both the inside of the frame and the face of the steel plate, which is bolted to the mounting cushion. Then, use this pattern to cut the 2x3 tubing and tack weld the tubing into place.
|Before final welding, move to the opposite side of the engine and construct that mount using the same steps. But don't assume you can use the same pattern for your bridge piece. In some cases the left and right sides of the engine block do not have identical mounting positions, so a separate pattern may have to be made. Tack weld the second bridge in place and then re-check all of your measurements to make sure nothing has shifted, and that the center of the crank and tailshaft are still centered between the frame rails and that the engine is sitting level. Then complete your final welds. Photos 4-5 and 4-6 show the completed motor mounts.|
Transmission mount and crossmember
Next we move on to the transmission mounting bracket. In choosing the size and location of the tubing for your rear mount, you need to pay special attention to the space requirements under your car. Most cars need space for mufflers, tail pipes, emergency brake cables and other mechanicals. This is doubly important if your car is going to sit really low to the ground. Most exhaust pipes alone will be 2 1/2" to 3" in diameter, and they have to fit under or over your crossmember and still allow adequate ground clearance. So, every inch counts.
To conserve space, 1/8" wall 1x2 rectangular tubing, laid flat, was used to construct the crossmember/transmission mount. The mount was also attached to the top of the frame rather than to the inside wall of the frame, again to raise the crossmember out of the way as much as possible.
No matter where you locate your crossmember, it is quite likely that your transmission mount will require a "dip" in the center so that it can fit under the transmission for support, and provide an attachment point for the transmission mounting bolts.
The transmission mount and "dip" for this chassis is made from the following five pieces of tubing: a "mounting block", two "upper members" and two "angle pieces".
|Begin by measuring the width of the mounting pad or mounting base on your transmission, and then add two inches to your measurement. Cut a piece of tubing to that length but make each cut at a 22.5-degree angle, as illustrated in Photo 4-7. The distance between "A" and "B", which is the top of the block, should be the width you calculated above. Next, you need to drill holes in this block to match the mounting holes on your transmission's base. You can do this with a pattern, or by marking the center of this block and the center of your transmission's base and then holding or clamping the block in place while you mark the hole locations. Install whatever rubber cushioning or vibration dampening you might be using and bolt this block to the transmission, making sure it is square to the frame rails. It should now look something like the illustration in Photo 4-8 and the image in Photo 4-9. (Note: Photos 4-9 and 4-11 have been altered in Photoshop to better illustrate how this mount is made.)|
|Now, measure from the tip of the mounting block to the outside edge of the frame, and add a couple of extra inches to your measurement. Cut a piece of tubing to this length but again make the end cut at a 22.5-degree angle. We will call this piece the "upper member". Place the upper member on the frame as illustrated in Photo 4-10 and as shown in Photo 4-11.|
|Using an angle gauge like the one shown in Photo 4-12, slide the upper member inward or outward until the pieces line up, tip-to-tip, as illustrated in Photo 4-13 with the gauge set at 45 degrees. Then clamp the upper member to the frame rail. At this juncture you need to use a steel rule, square and/or chalk line to ensure the upper member is exactly perpendicular to the frame rail, and that it is in exact alignment with the mounting block you bolted to the bottom of the transmission. You may need to loosen the clamp a bit to jiggle things into position. Go back and forth between your angle gauge and your square until everything is lined up properly and you have firmly tightened your clamp again. Then measure the tip-to-tip distance between points "A" and "B" as illustrated in Photo 4-13. This distance should be the same for the two top tips as it is for the two bottom tips.|
|Cut your "angle piece" to this length, making the 22.5-degree angle cuts parallel at each end of the tubing as illustrated by the red "angle piece" in Photo 4-14. When cut, the piece should fit correctly between your mounting block and your upper member. If not, loosen the clamp on the upper member and slide it slightly inward or outward until the pieces fit.
Once fit, mark the upper member where it extends over the outside of the frame, and then cut off this excess. To give a more finished look, this cut should be made at an angle, and the open end of the angled tubing should be filled with a plug. I used a 45-degree angle for this mount, as shown in Photo 4-14.Reassemble your pieces, squaring everything up with the frame and your mounting block, and tack weld the parts together. Then move on to the other side of frame and repeat the steps shown above. You should end up with something resembling the illustration in Photo 4-15.
The completed transmission mount and crossmember are shown in Photos 4-16 and 4-17. Many builders will also fabricate whatever "X" member or "K" member supports they are going to use at this stage of the build. For this project, we waited until a little later, when we knew exactly how all the real estate under the car was going to be utilized. Although it was actually done later, a photo of the completed "X" member is included here so that you can see how it was constructed (Photo 4-18).
|< Chassis: Front Suspension || Chassis: Rear Suspension >|