Engine Mount Washers

[Tom]

Pressing on with my Clipper rebuild and there are large area fiber washers between the engine mount and firewall on both the front and back sides of the firewall. Have checked all the parts manuals and blueprint CD but can't seem to locate. Any suggestions?

Tom

[Steve Pierce]

I just looked in several engine installation drawings and they are not depicted nor does the part number show up in the numerical index in the back of the parts manual on the PA22-150. It does on the Super Cub. I know they installed those washers on PA22s and PA16s at the factory because I have found them multiple times there.

[N2709P]

I also did not find any washers required on the engine mount to firewall drawing for our 55 TriPacer. I have seen them installed on tube fuselage Piper’s to bring the engine thrust line into spec’s. Not sure if that was a factory practice or a field modification after an incident.
N2709P

[Steve Pierce]

I also did not find any washers required on the engine mount to firewall drawing for our 55 TriPacer. I have seen them installed on tube fuselage Piper’s to bring the engine thrust line into spec’s. Not sure if that was a factory practice or a field modification after an incident.
N2709P

I don't see how they would change the engine thrustline. All I have seen are on all 4 corners except on the Lycoming powered Super Cubs with swing out mounts where they committed the one on the lug that has the part that stops the mount from going to far that is the same thickness as the washer.

[N2709P]

If the washers are mounted between the motor mount and firewall they can change the engine thrust line. Obviously if the same thickness and number of washers are used on all four courses there would be no angle change. I have found different thickness of washers and numbers of washers installed. If you put a washer on each right side leg of the mount, you will angle the thrust line to the left as the firewall is the base for measurements. Many different combinations of washers can be used to bring the thrust line into tolerance. You will have to replace the motor mount bolts with ones that are the correct grip length. The drawing shows the factory desired thrust angles. A perfectly straight fuselage and motor mount should not need any adjusting washers. That being said, I would imagine there are very few aircraft that have not had an incident or two or have repaired motor mounts. It does not take much of an incident to tweak the parts and without a factory fuselage or motor mount jig it would be difficult to make it perfectly straight again. The engine thrust line can have a major affect regarding aircraft rigging issues. It is easy to check engine thrust line angles with a digital level and protractor.
N2709P

[Steve Pierce]

I have rebuilt 5 Super Cubs that had never been apart and installed numerous Thrustline kits as well as disassemble several PA16, 20 and 22s and never found the washer stacked to change any angles. I always figured they were an insulator of sorts between the galvanized firewall and the steel engine mount. I have also jigged more Piper fuselages than I care to remember and they are all over the place, wrecked or factory original. Have to use a plate at the tailpost because they are all a bit left or right of center. Also discovered that wing incidences are all over the place. That is why the Thrustline mod doesn't work on all Super Cubs and some airplanes are faster than others. I have always been able to rig any rag and tube Piper to fly hands off using Piper's rigging data and no trim tabs. Worked on a Cardinal many years ago that would not rig and finally figured out they had installed the Lord mounts at the wrong position and once installed correctly it rigged out with the ball in the center.

[jasongould]

Thanks all. On the lower two engine mount to firewall mount points my '55 22-150 had two stacked thin steel washers of what I'd estimate to be maybe 3/32" total each stack. On the upper two mount points had one fiber at what I think was around the same thickness.. (rough estimate, would have to check again at the hangar). I will say that my mount had a noticeable sag to the upper two arms of the mount. I've since had it rebuilt by AWI/AMI and is nice and straight now, though my wallet is a bit lighter since they had to replace nearly every tube.

I will go back with new fiber and also check the thrustline. Any tricks to ball park it before we get the engine mounted?? Maybe mount a flat plate to the engine mount once the mount is on? Or an engine length bar?


Also I'd imagine it'd take quite a bit of washer stacking to appreciably modify the thrustline, no? Though I guess the distance arm from the firewall to the crank pulley is maybe 2.5-3ft?


Pretty weird that Piper excluded them from the drawings!

[Steve Pierce]

4 degrees down, 2 degrees right.

[N2709P]

Jason, I would install the mounts and engine without any washers. Once the engine is mounted, put one leg of an 18 inch carpenters square against the firewall and the other at the aft section of the upper case split line. You can use a protractor to measure the angle between the square and the case split line. Should be 2 degrees right angle. Use an 8 inch digital level along the upper case split line. This should be 4 degrees down angle. These measurements will give you the thrust line of your engine to airframe. If it is out of tolerance, you can place washers as need to improve the thrust angle. Make sure that you have properly torqued the engine mount rubbers prior to checking the angles. It takes about 5 minutes to check with the upper cowling off. If you need more than a washer or two to bring it into line, you probably have something bent pretty good. Rebuilt motor mounts are a common issue. Some jigs are not so accurate, if they jig them at all.
N2709P

[Steve Pierce]

I check firewall to zero and then put the digital level across the two rocker covers if the engine is mounted. If not put the firewall side of the mount on a flat surface and being it is a conical mount the digital level should give you the angles when layed on the engine conical mount washers.