Restoration of The Aermotor 4/8 Cycle


I'm restoring this engine for a friend, Frank Luecke.  When we got the bones of this project which consisted of about one and one-half engines, we discovered that one of them had the "normal" 8-cycle cam and the other one had a cam with two lobes on it which made the engine run as a 4-cycle.  We'd never heard of this option before (and it is not mentioned in the Aermotor booklet) so we decided to restore one engine but make it so the cams could be swapped easily, allowing it to run in either mode.



Here's a "before" picture of the engine we're restoring.


The other engine was missing most of its parts.  About all that was there was the block (with the rocker support broken off and missing) and the gears.  The important parts we needed were one of the oil cup covers for the mains and the 4-cycle cam.


Frank saved some timbers sawed out of a windfall wild cherry tree so we stole a couple of pieces and had his neighbor build a set of skids for the engine.  Methinks that this skid is a little fancier and nicer looking than the ones that were shipped from the Aermotor factory.


It's coming along nicely and ready for the first trial fit of the Webster Tri-Polar magneto setup.  The magneto was an option for the Aermotor engines and cost, I think, $15.00 extra (We sure wish we could get one for that price now!).  Not many were sold and fewer are left.  Frank located a magneto and bracket but we haven't found the magneto trip rod.  The trip rod  shown is for the regular battery ignitor.  It won't work with the Webster.


The muffler was gone so I scaled off of the photos in the Aermotor manual and came-up with something close.  When I was doing the first trial fit of the Webster, I made the revolting discovery that the muffler hits the trip roller bracket.  I'll had to use a 45-degree elbow to get it out of the way.  I haven't seen a photo of the Aermotor with the magneto setup and the original muffler so I can only guess that this was how the Aermotor folks moved it out of the way.


This is how it is as of October 9, 2003.  As you can see, the body of the muffler will hit on the trip roller bracket.

(This picture is with the battery ignitor trip rod)



E.C. Mims, another Aermotor owner, kindly sent this photo along with others of his engine with the original Webster magneto.  The photo above is of his engine.  I made a CAD drawing of the pushrod assembly from E.C.'s photos.  E.C. put some dimensions on it so we now have a dimensioned drawing that I used to make a replica.  As you can see, the rocker arm is different on the Webster-fired Aermotors so we need one of these, too.  The rocker for the Webster is much uglier and the rocker casting goes over the bearing in the block and is clamped to the shaft with a "U" bolt on the magneto side to stiffen it.


This picture shows the "new" trip rod for the Webster.  It works but needs the uglier rocker arm because the non-mag rocker shaft (the black part) is just too limber.


Here is the Aermotor as it looked just before it was started.  Slick, eh?


Along with the magneto parts, we need an original gas cap.  It's one of those cheap pressed tin jobs that were common on kerosene cans back then.  The diameter of the cap is 5/8" at the outside of the thread.  Can anyone help us on these two items?


A friend and I barked her off on the 10th of December 2003 and found out that, without the ugly rocker, the rocker shaft is too springy to cock the magneto properly.  Because of this, the timing is very advanced (due to running out of ignition rise in the cam).  This makes the engine hard to start and it runs poorly.


The next day, I did some "artistic filing" on the trip block to make it rise faster.  Now, the timing is just slightly fast but the engine is easier to start and runs better.


Here it is getting greased-up after the "artistic filing" of the trip block.

The home-made muffler really works well.  No bark - just a nice soothing "putt-putt" sound.


22 December 2003:  After running the engine for a couple of hours, it started running strangely (if it can run more strangely than normal for an Aermotor).  I found that the taper pin holding the rocker shaft to the rocker arm had worked loose.  I took it apart and discovered that the hole in the shaft was a little egg-shaped, requiring the use of a taper reamer to true it up.  It runs all right for now, but I think that, without the 'ugly rocker arm', the rocker shaft is going to continue to wear because of the increased torque of the magneto 'wind-up'.  It will eventually be ruined.


I'm still looking for an 'ugly rocker'.  If you have one, please get in touch with me.  If necessary, and if I can borrow someone's 'ugly rocker', I'll make a pattern and have some cast.  Let me know if you're game for this.


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Some of us technically inclined (Nerdy) guys are interested in how the Webster Tri-Polar magnetos actually work so I did  some tests on the magneto setup.  I used a Tektronix 468 storage oscilloscope for these tests.  You might want to re-visit this page in a while because I have my doubts about some of the scope pictures.  The probe may have been bad.  I can't believe that the later pics don't show the high voltage spike.  It's gotta be there and I'm going to hook it back up and re-do the tests to confirm the results.


Click on the links below if you wanna get technical:

Techno-Geek Page


Drawing of Muffler


Drawing of Webster Magneto Trip Rod


The "Ugly Rocker" Cloning Project

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If anyone needs drawings of the Webster trip rod assembly or the muffler for an Aermotor, I can supply them.  Since the 8-cycle cam is rust-pitted pretty badly, Frank's going to use his CNC mill to make a new one of both of them.  If there's interest out there in Aermotor-Land, Frank may make a limited number of both the 8-cycle and the rare 4-cycle cams for collectors.


Also, if you have detailed information about the Aermotor company, serial number lists, etc. I can publish them here.


Thanks for looking - Elden & Frank

[email protected]

Visits Since 8 February 2010