Conversions - 800cc 1/2

Rev - Nov 11, 2017

By W. J. Spring
edited - Jan Zumwalt (EAA #66327)

See Also

PDF version of this article - BUILDING THE 1/2 VW ENGINE


I have been unable to find three drawings or images that should have been part of this document. If someone has a copy available, please contact me through the comment / message center located here.



    HP: 28 HP @ 3500 RPM
    Bore: 92mm
    Stroke: 69mm
    RPM: 3500 max
    Idle: 1000rpm
    Oil pressure: 35-45 PSI (hot, high RPM)
    Oil temp: 200F max.


The user of this manual and the associated three drawings assumes all liability in the construction and operation of this engine.


 "Outrageous" you say? "Cutting an engine in half"?  In these days of nearly out-of-reach flying costs, such claims for a reliable engine, even an aircraft engine are enough to invoke the raised eyebrows of aviation buffs everywhere.  As bizarre as it may seem, it is possible to cut an engine in half and end up with something that will give aircraft reliability.  Even the outlay of $500 - $600 is enough to make most ardent flyers regard you as some kind of nut and the coup de force will come when you casually mention that the gas consumption is just 1.8 US gallons per hour.


Generally, look for a 1600 cc engine from a 1970 VW or later.  The engine should have dual-port heads and case savers for the cylinder hold-down studs.  VW engines with serial numbers beginning with AE or AK are excellent choices but don't discard other engines because they fail to meet the above criteria. Case-saver studs can be added to a case and single-port heads which convert with slightly more difficulty than dual-port heads, are quite satisfactory.  The only parts that you will use from the original engine are the crankcase, heads, crankshaft, connecting rods, and oil pump.  Everything else is discarded.


-     6" outside dial caliper
-     dial gauge
-     access to a metal lathe
-     access to a welding equipment
-     a copy of HAPI'S book "How to build a reliable VW aero engine" will be quite helpful but is not mandatory.

If this list is scaring you off, don't panic.  If you have to buy the caliper and dial gauges, the outlay can be as low as $100.  Before you resort to this, check around and maybe you can borrow or rent them.  Access to a lathe may not be as difficult as you might think and again, ask around.  You will probably have to pay for the welding required but in so doing, you'll be able to seek out a competent welder and the piece of mind from this expenditure is well worthwhile.


-     1/2 set (2), 92mm pistons and cylinders:
-     1 - C-20 camshaft: SCAT or Great Plains' equivalent
-     2 - exhaust valves: be sure to specify face and stem diameter
-     2 - intake valves:      "      "      "      "          "        "         "          "
-     4 - valve pushrods:
-     4 - pushrod tubes:
-     1/2 set (4), cam followers (lifters):
-     1 - main bearing set (specify O/D & brg journal size):
-     1 - rod bearing set (specify O/D & journal sizes):
-     1 - cam bearing set
-     1 - tapered prop hub and faceplate: Great Plains Aircraft
-     1 - prop hub seal: Garlock model 63x1114 21128, or CR Indust. 19852
-     1 - POSA 26mm super-carburetor (mixture adjustable):
-     1 - Fairbanks Morse magneto model FMP1-2B10: Morry Hummel
-     2 - NGK B6HS sparkplugs, set gap to .016" Suggested Suppliers - Write for Catalogue

Great Plains Aircraft P.O. Box 545, Boystown, NE.,
USA  68010
Phone: (402) 493 6507
Fax: (402) 333 7750
SCAT Enterprises, Inc.   (ask to speak with Pat)
1400 Kingsdale Ave., Redondo Beach, CA., USA  90278
Phone (213) 370 5501  FAX (213) 241 2285


After the engine has been completely disassembled and studs removed, clean the case with a solvent and have it bead blasted (like sand blasting but with glass beads).  Inspect the case carefully for cracks and for bearing saddle wear.  The #2 bearing saddle (middle main bearing) seems to wear more than the others as evidenced by a ridge around the surface of the saddle near the oil gallery.  If this ridge is more than .001" high, have your case line-bored.  Otherwise, the new bearings will not be held in place with the proper "squeeze" which will allow too much clearance between the bearing and the journal even though the bearing is new.
Measure and record the bearing saddle diameter (O/D of the bearings) so that the proper size bearings can be ordered. It would be best to discard the case if it is beyond the figures shown below.
    Standard size saddle: 2.565" (brgs 1,2,3), 1.965" (brg 4)
    1st line-bored:        2.585" (brgs 1,2,3), 1.985" (brg 4)

Bolt the two halves of the case together and mark the cut-line as shown in the prints. Now drill a 1/4" hole as shown which will provide access for a new dowel pin.  Make one from 1/4" steel rod (eg. drill rod).  Remember, the original dowel pin on the top of the engine will be discarded when the case is cut.  Separate the case halves and cut behind the marked line using a hacksaw or a bandsaw.  The cut can be fairly rough but don't cut too close to the line.  The case is made from magnesium alloy and cuts easily even with a cross-cut saw.  Honest, it doesn't hurt the saw at all so don't be afraid to use it.
There are two ways to fill the unwanted holes left by the two cylinders cut off and to beef up the lower engine mounts so, each one will be described separately.  In both situa- tions, you will need to machine plugs from aluminum to fit in the front and rear main bearing holes (#2 & #3). Be sure to machine centers in each plug.  These will allow the case to be mounted in a lathe so that the rear surface can be machined.

METHOD ONE - welding

Bolt the case together, complete with new dowel pin and using the previously-made plugs, mount the case in a lathe.  Take a very light cut to even the rear surface. Now cut a piece of material from the discarded portion of the case to fit the hole left by the rear cylinder and bevel the edges for welding.
Blocks of magnesium for the lower engine mounts can be cut out of the discarded bearing saddle and should be filed for a snug fit.  Again, bevel all edges to increase the weld surface. Clamp these blocks in place and drill and tap for the two 1/4"-20 bolts used to anchor the blocks.
Welders who can TIG-weld magnesium are scarce and when you do find one, instruct him to weld all pieces and to fill the holes/depressions left by the other rear cylinder with weld. Again, bolt the case together and put it in a lathe to make the finishing cut to the rear surface.  As light a cut as possible should be made to preserve the integrity of the welds.

METHOD TWO - drill and tap

Cut a notch out of the rear of the case on either side of the rear cylinder hole so that a piece of 1/2"x 3/4" aluminum can be set into it.  Drill and tap for #10-24 countersunk screws to secure it in place.  This piece will actually form part of the rear surface flange that mates with rear cover plate.  Make a cover plate for the hole left by the rear cylinder from 1/8" aluminum then drill and tap in place using  6-32 screws.  Seal with silicon sealer/cement (RTV clear silicon rubber).  On the other half of the case, cut and file a depression about 1/4" deep and 3 1/2" long to accommodate a piece of 1/4" thick aluminum.  This will fill the hole left by the other rear cylinder and it's hold-down studs. Drill and tap for one #10-24 countersunk screw.  Before any of these pieces are secured, coat all surfaces with silicon sealer.
Make blocks for the lower engine mounts from aluminum or magnesium and file for a close fit.  Drill and tap for two 1/4"-20 bolts on the side of the case and for another, through the web inside the case and into the block.  Seal the blocks in place with silicon and coat the threads of the bolts as well. These bolts should be safety wired. Now bolt the case together, complete with the new 1/4" dowel pin and place in a lathe to true up the rear surface.
Of the two methods, I prefer method #2 since it requires less lathe work and eliminates the cost of welding (it cost me $50.00). As well, the case should end up being much stronger.
Plug the exposed oil galleries by drilling and tapping for short grub screws.  Only tap as deep as required to cause the screws to bottom out so that the top of the screw is just below the rear surface of the case.  Also make sure that the screws don't restrict the inner oil passages.
If you are going to use the 92mm cylinders which are larger than the stock 1600cc ones, then now would be a good time to have the cylinder holes in the case opened up. To preserve as much wall thickness as possible for the cylinder hold-down bolts, the cylinder bores should be opened up via a two-operation cut.  Make the first cut at 3.700" to a depth of 1.125" followed by a second cut at 3.787" to a depth of .75".
Earlier cases (pre 1970 or so) were single-bypass, that is, there was only one spring and piston (located just to the left of the oil pump) to control engine oil pressure.  Later cases contained an additional spring and piston located at the flywheel end of the case. Such cases were termed dual-bypass.  If yours is a dual-pass case, then you must drill a 3/16" hole about 1 7/8" directly above the existing hole in the bypass adjacent to the oil pump.  Examine the discarded portion of the case removed earlier and note the two holes in the bypass area. Basically, this extra hole drilled restores it's function to the portion of the case that we are using.  Buy an adjustable oil relief valve assembly and set it for 60 PSI.

Remove the oil pickup tube and cut a portion from the rear of the bell.  Make a piece to weld up the open end of the bell.  For the oil screen, clean up a used one and cut the flange flush with the rear cover.  Bend over the screen to fit inside the bell.


Make the rear cover plate from 1/4" aluminum as per the full scale drawing in the prints and trim to fit the outline of your particular case. Leave sufficient material on the bottom edge so that it can be filed flush with the sealing surface for the oil sump cover. Make a cardboard template of the rear cover marking all the holes to be drilled. Place this template on the rear surface of the case and transfer the hole positions to the case with a centerpunch (very lightly only).

Now examine carefully each punch mark to ensure that it is located accurately and will not allow the bolt to fall too close to an edge or to restrict an oil gallery. This step is important since there are slight manufacturing differences in VW engine cases and a correct hole in one situation may be inaccurate for another. If you determine that all is okay, transfer the hole locations from the cardboard template to the rear cover and drill 1/8" pilot holes at all locations. Lay the cover on the rear surface of the case, position it properly and secure it with tape or whatever. Use it as a drill guide to drill the 1/8" pilot holes into the case. One way to do this, is to first drill two holes at opposite corners then drill out to size, and tap. Two mounting screws can be used to hold the cover in position while the rest of the holes are being drilled.
The two 5/16" holes on either side at the bottom are motor mount points. For these two holes only, drill and tap through the back cover as well as the crankcase to provide a little extra strength here. The top motor mount is made from a piece of 1 1/2"x 1 1/2"x .090" steel tubing, 6" long. One side is cut out to form a "U" channel that fits over the flange along the top of the crankcase. Make up as per the drawing.
Locate the center of the magneto on the rear cover and use a 2 1/2" or so, hole saw to make the initial hole. Now screw the cover to the case, mount everything in a lathe, and open up the hole to 3 1/4" for the magneto mounting flange. This step is necessary to ensure that the magneto will be on center with the crankshaft.


(see note in last paragraph)
Completely disassemble the crankshaft by removing the snap ring at the pulley end and removing the cam-drive and distributor-drive gears. You'll need to use a gear puller for this. It would be advisable to have your crank magnafluxed to ensure that it is crack free, however, there is another simple test that will yield the same results. Insert the bolt in the pulley end of the shaft, the one used to secure the pulley, and use it to suspend the crank by wrapping a piece of wire around it. Holding the crank vertically in this fashion, strike it with a piece of wood and listen for a definite ring. If you hear a dull clunk or thud, the crank is defective and is not suitable.
Inspect the bearing journals for grooves and use a dial caliper or micrometer to measure them for size and out-of-round.
    Standard size: 2.165" (brgs 1,2,3, & rod journals)
    1.575" (brg 4) First regrind: 2.155"
    1.565" Second regrind: 2.145"

Often a used crank will show very little wear and if it is within one or possibly two thousandth's of spec, you can save yourself a regrind assuming of course, that there are no grooves and the journals are not egg-shaped. If necessary, there are many automotive machine shops that can regrind your crank to the next size down. If you already have a 2nd regrind or a first regrind that needs work, I would discourage going any further.
Next, cut the crank as shown and have the cut end machined as necessary. The 1/4" projection for the magneto drive can be milled if you have access to such machinery or you can use a hacksaw and carefully make the necessary cuts then file the projection to size. If you choose the latter method, mount the crank in a lathe and scribe a line 1/4" in from the cut-end of the crank. This will be used as a guide as you hacksaw. Once the projection has been formed, weld the exposed oil galleries shut and file the surface smooth.
Next, make the counterweights as shown and grind the crank as necessary for a good fit. Position them as accurately as possible using clamps and pieces of steel, then have them TIG welded in place. Before welding, preheat the crank to 450F and after welding, place back into the oven at 450F and gradually reduce the temperature over a period of eight hours. This last step is required to relieve the internal heat stresses built up from welding and to minimize the possibility of cracks forming.
The next step is to cut the 3 degree taper to accommodate the prop hub. Do not proceed until you have your prop hub because it is virtually impossible to guarantee a good fit unless you can first accurately measure the taper on the hub. It is best to seek out a good machinist to do this work since accuracy is paramount. When the taper has been cut, lap in the prop hub to the crankshaft using fine, valve-grinding compound. Be sure to clean off all traces of the compound. Install the prop hub but do not tighten the end bolt to any degree. Take the crankshaft with the prop hub to an automotive machine shop and have it balanced. If so required, it may be necessary to add a small weight to the rear of the hub (secured by the propeller mounting bolts) to achieve a good balance. Your machinist doing the balancing will be able to give you the required weight and position.

Advanced Balancing Method

Even though the above method will produce an acceptably smooth running engine, it can still be improved upon but with some extra effort. Weld up a jig to hold the crankshaft firmly then drill 7/8" holes through the rod journals. the center for these holes is 11/16" from the surface of the main bearing journal. Make the counterweight as before but use 1/2" thick steel instead of 3/8". Taper the back to reduce the thickness down to 3/8" where it is welded to the crank. If you have already made your counterweights from 3/8" steel, then make pieces from 1/8" steel and TIG-weld them to the inside face (connecting rod side) of the counterweight.
Make two "bob-weights" from strips of lead or solder, each weight equal to 55% of the combined weight of the piston, wrist pin, and connecting rod. After you've figured out what the bob-weight should weigh (it should be around 700 grams), wrap it around each rod journal making sure that the weight is distributed evenly. Use screwclamps to secure them and be sure to weigh each screwclamp ahead of time and include it in your calculations. Now your crankshaft is ready to be balanced and these "bob-weights" represent part of the connecting rod and piston mass which is a necessary consideration for good balancing of an "opposed-two" engine. Remember that accuracy in your calculations and fitting the bob-weights is very important to achieving a well balanced crankshaft so take your time and double check often.
The dual flanged bearing is used at the #3 bearing position (next to the small bearing at the prop hub end). Both flanges are used as thrust bearings to control the endplay of the crank (about .008"). The first step is to machine the thrust surface of the crankshaft so that it is true and smooth. One of the original thrust washers (removed when the engine was disassembled) will be used here but first, the inside diameter of the washer must be increased with a file to allow it to fit over the #3 bearing journal and any radius between the journal and the thrust surface.

Make sure it fits flat against the trued-up thrust surface. Measure the distance from the thrust surface to the forward edge of #3 journal. Subtract the flange to flange distance of the bearing from it. This is the amount (plus allowance for three thrust washers and the required endplay of .008") that must be removed from the face of the gear that drives the camshaft so make your measure- ments carefully and double check.

For Example:

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