Aligning Your Engine Using Shims

Many Pre-J model Mooneys have the engine out of alignment with the fuselage. This is evident by a prop spinner that doesn’t align properly with the exterior of the cowling. Learn what Don Maxwell has to say about getting your engine back in proper alignment. Good looks and improved cruise speeds are the result.

Figure 2

When your “J” or pre “J” Mooney left the factory, the engine was shimmed so that the spinner was centered in the cowling. Shimming at the factory is done by trial and error and is considered satisfactory when the spinner aligns correctly with the cowling. The shims used by Mooney are really nothing more than “large area” washers. They come in two thickness. Part number 600364-001 is one-sixteenth of an inch thick and part number 600364-003 is one-eighth of an inch thick. Figure 1 shows these two shims. The way they work is really pretty simple. These washers are placed between the engine case and the rubber isolators on the engine mount to align the engine with the cowling as indicated by the prop spinner alignment. Shimming is normal on just about every Mooney and is very important. It keeps engine/cowl contact to a minimum. (See figure 2 for an example of contact between the starter and lower cowling.) Shimming should be considered mandatory before installing cowl mods such as Lasar’s cowl closure or a new Southwest Texas Aviation cowling. Engine/cowling contact is the number 1 cause of excessive vibration in Mooneys.

I receive lots of calls from owners or mechanics that have removed an engine for one reason or another and were unaware of the shims. They fell on the shop floor when the engine came off the airplane. Now they don’t know what shims to use where or when reinstalling the engine. At this point it’s anyone’s guess. Trial and error may be the only way to get the engine properly aligned. Its hard to tell by looking at shims that were removed exactly where they were located. There may be a pattern of the engine case or markings that will give you a clue as to it’s original position, but I doubt it.

So, you’re going to have to start all over with the shimming process. To keep from having to do this, make sure to carefully notice where the shims are in the first place before they fall on the hangar floor during an engine removal.

Most installations have a one-eighth shim at each mount position. In addition, there may be a one-sixteenth shim on one or more of the other mounts. I usually start with a one-eighth shim at each position and check the alignment of spinner and cowling.

Figure 3

On pre “J” models proper alignment will have the spinner centered in the cowling and approximately one inch of clearance between the bottom of the starter and the lower cowling (see figure 3 for this proper clearance).

Figure 4

Figure 5

On “J” models the spinner will line up with the bulkhead on the front of the cowling. (see figure 4) Also on the “J”, the ram air inlet and ram air door should be in alignment. Figure 5 shows an example of the ram air not in alignment with the ram air door. Not Good.

In most Mooneys I see in my shop, the engine is sagging due to the age of the mounts or improper alignment when the engine was installed. To raise the engine using shims, first determine how much it needs to be raised at the spinner. Hear is an important rule of thumb for raising the engine with shims: a one sixteenth inch shim will raise the spinner one-quarter (1/4) of an inch. A one-eighth inch shim will raise the spinner one-half (1/2) inch. Remembering this rule of thumb will certainly make the trial and error method of aligning your engine much easier.

An example would be that we have a one-eighth shim under each rubber mount and we have a spinner that is one-half inch low. Support the prop with a hoist using straps or generator belts and place a support under the tail skid. Remove the top engine mount bolts and remove the one-eighth washer from each top position. Replace and torque the bolts. You may have to add washers under the nut of the mount bolts for proper torque. Install the cowl and check for alignment. This time alignment is better, but the spinner seems a little low on one side.

Now that the shims have been removed from the top mounts, we must add a shim to one of the lower mounts. This procedure is a little more difficult because once the engine mount is installed on the firewall, there is not enough room to remove the lower engine mount bolts.

To add a shim to a lower mount, we must first modify our shim by cutting a slot from the side to the middle making our shim look like a “U”. (See figure 1) You can cut the slot using a hack saw or Dremel tool. Once the slot is cut, bevel the washer on both sides the slot is cut on.

Again, we support the prop with our sling and a support under the tail. Loosen the lower mount bolts, raise the front of the engine using the prop sling. Place the modified shim between the engine rubber mount and the one-eighth washer that is already installed. Using a hammer and extension bar, tap the shim in until it bottoms out. Re-torque the mount bolt and make sure that threads extend past the locknut.

Figure 6

Figure 7

One final caution, make sure that it’s not a lose cowling that is making it look like the engine is out of alignment. I’ve seen this sometimes and it’s deceiving. Especially on J models. I’ve seen cowlings riding high because they are loose, giving the impression of a sagging engine. But it’s not the engine riding low, it’s the cowling riding high. If you can move the cowl upward by hand, the floating cowl fasteners can be replaced with fixed fasteners to keep this from happening. If you’ve got an engine that you think is out of alignment, five me a call at 903-643-9902 or e-mail me at

You can have it fixed by someone who knows Mooneys, with a little help. Or you can bring it by my shop and we will definitely get it fixed. Your airplane will look better and should be a little faster as well.

MAPA Log, Volume 24, Number 6 (June 2001)