Conversion - 1120cc 1/2

Rev - Nov 11, 2017

By Jan Zumwalt (EAA #66327)


See Also

DIY 1120cc 1/2 VW

HP Rating: 45 @ 3700 5min
Continuous HP: 40 @ 3500
Cruise rpm: 3200 +/- 200
Idle rpm: 900 +/- 50
Displacement: 1120 cc
Bore: 92mm (THICK WALL 88mm cylinder )
Stroke: 84mm
Compression ratio: 8:1
Firing order: 1 – 4 – 3 – 2
Timing: BTDC 25 degrees
Ignition: Single or dual ignition
Alternator: 20 amp
Dry weight: 65 lbs.
Flying weight: 85lbs.
Fuel min octane: 91 octane auto or 100LL
Oil: multi-grade 20/50
Spark plugs: 12 X 3/4 reach DCPR7E
Carburetion: Zenith, RevFlow, Makuni,
      Bendix / Stromberg, Model-T, etc.
Battery: 12V min. 20 amp
Crank flange: SAE #1 4-3/8″ bolt circle
Cam: > 240-250º duration @ .50
Oil temp min: 130 degrees F
Oil temp max: 220 degrees F (measured at sump)
Oil pressure min: 10 lbs/1000 rpm
Oil pressure max: 80 psi
CHT cruise: 350 – 375 degrees F
CHT climb: 450 degrees F
EGT full rich: 1250 degrees F
EGT max; 1400 degrees F

PLEASE NOTE: The information contained here is a compilation of three different builds, 1000cc, 1100cc, and 1200cc. It became difficult to tell what information pertained to a certan configuration. So, there may be a few areas that I have not completly smoothed out the differences or contridictions. I also have additional information to add as I sort through my saved notes After finalizing this article, I hope to follow it with a 1200cc engine build which will clearly seperate different issues from this engine. - Jan Zumwalt (2017.10.08)


This link (Engine Design Sheet.pdf) is also at the top of this article. It is a form that helps design your custom engine.


This article will provide all the information needed to build a two cylinder 1200cc 40hp (continuous) VW engine. That’s right, we cut out the back two cylinders and weld the case back together so only two cylinders exist. For those without the funds or resources to have the magnesium case welded, there is also an option to leave the case intact and remove the back two cylinders and block off the holes in the case (weighs about 5lbs more).
The purpose of such an engine is for ultralight or very VERY light aircraft (under 700lb empty weight). “But, why?” Because this engine can be completed for less than $1500 dollars and many pilots wanting to fly simply don't have a comfortable budget. Comparable engines from manufactures will cost between $4000 and $6000 dollars. This engine will match the performance of the factory engines but weigh about 10-20lbs more. That's a reasonable tradeoff for the pilot with a tight budget.

Once the job is finished, the whole engine ready to fly weighs about 85 pounds. For those efforts, you have about 45hp@3,500 rpm with reliability and low operating and maintenance costs, So what good is a 1/2 VW engine? Well, there are quite a few ultralights and small single-place planes that have been specifically designed to use this type of engine. There are at least 200 aircraft types that could use this engine. A couple examples of very popular aircraft are the Legal Eagle which weighs 244 pounds, Hummel Bird (310lbs), or Hummel H5 (460lbs), Teeny Two, and the Fisher Avenger.

Typical 1/2 VW aircraft



Illustrated Parts List

Some parts or kits that are idealy suited for this engine will only come in sets for four (4) cyclinders. With a little effort, some parts houses might sell a set of two (2) but the cost is likly to be 3/4 of the full set. In this cirmustance, I recomend that a full set be purchased and set aside for a later overhaul. A well maintained 4 cylinder VW can be expected to last 1500hrs. The two cylinder variety is likly to run a bit harder and there is certainly more viberation. An expectation of 1000hrs TBO is probably realistic. If you have an extra set of studs, cylinders, pistons, lifters, and other assorted parts left over, a complete overhaul can be done for pennies and you should be good for another 1000hrs!

Case: AS41 dual pressure relief oil valve (1970 or later). Case savers.

Crank: 84mm stroke cut in half, optional 3deg end grind for Force One Hub.

Case bearings: Stock to match case & crank.

Case hardware kit: Dual pressure relief kit.

Rod: Stock I-beam

Camshaft: 240-250º duration @ .50


Oil system: Full flow

Cam Bearing kit: Stock, sized for case and camshaft


Cylinder:88mm thick wall, no machining of stock case but heads must be machined for 92mm

Ignition: Slick model 4316, 4001.

Seal & gasket kit:

Exhaust: 1-1/2in pipe (i.e.

Carburetor: Zenith, RevFlow, Makuni, Bendix / Stromberg, etc.(Model-T shown)



The good news is just about any VW case originaly designed as a 1300/1500/1600cc CAN be used. BUT, if you want the engine to last 1500hrs instead of 200-400, you won't use anything other than a case marked AS41. There are many reasons but here are a few.

  • The early VW cases (AS21) had inferior metallurgy problems[1] that caused deforming at normal operating temperatures. There are some unmarked cases that are "good" - but how would you know?
  • There are a variety of cases VW allowed to be out sourced such as Brazil and Mexico. Many are poor quality, but some are good - how would you know?
  • It has been reported that some of these out sourced cases have slight differences in design. For example attempts to enlarge the main oil galley (a recommended mod) has resulted in drill bits exiting the case!

Use stud savers! VW got smart and installed them on all their later engines. Remember to drag your fingernail across the case bearing pad and if you feel your fingernail catch on any scratches the case will need re-boring. I do not suggest re-boring without a good reason. The VW case does not have enough dowel pins and bolts to prevent the case from shifting over time. The bearings are designed to crush fit around the crank during the bolt torquing and I believe the bearing will adjust a thousand here or there as needed.


The best crankshafts are forged, Nitride surface, and made from 4340 Steel. Experience has shown that VW engines below 60-65hp really don't require a custom prop hub and will fly the full lifetime of the engine from an unmodified crank off the pully end. A Force One Hub is the preferred prop hub solution but requires machining. In all likelihood you are going to have the crank cut in half, it is very little extra work to have the 3deg taper machined at the same time to fit the Force One Hub. Then all your prop mount headaches are taken care of for you. Once again, this decision is a tradeoff between parts design, time, and money.

Main Bearings

Stock bearings are fine and a good way to trim the budget. Remember to drag your fingernail across the case bearing pad and if you feel your fingernail catch on any scratches the case will need re-boring. That in turn will require oversized bearings. A test for the proper sized bearing is to place the bearings solidly in a case half and place the crank on the bearings. There should be absolutely no side play to the crank, but you should be able to give the crank a twist and it should want to spin freely - no bearing pressure. If the crank feels tight or loose, it is the wrong bearing.

Case Hardware Kit

For about $30 you can get a case hardware kit. All springs loose tension over time and this is a good way to start out with good oil pressure relief springs and new case studs. You will also need a good seal & gasket kit. Don't get the one with cork valve cover gaskets - go for silicone seals whenever possible.


Unless you are re-using a $500 set of rods, it does not make since to overhaul them - it is cheaper to buy new ones. Auto parts suppliers would love to sell you new "H" beams but they are only needed for engines turning more than 5000 rpm. Many "H" beams are designed to solve Stroker clearance problems and so an engine builder with little restraint on their budget can pay to solve a problem the rest of use will need to use a little hand work to fix. At the engine RPM and loads for aircraft, inexpensive stock rods are fine.

Camshaft & Lifters

Stock cam is good here to. A little extra duration is a good thing. Some folks have never been told the advertised degrees of lift is meaningless until the cam is open 1/2 inch (.50). Cam makers provide two numbers, the first is the total duration and the second is duration after .50 inch. This last number is the number to watch! The ideal cam for this engine has a duration after the cam is lifted .50 of 240-250 degrees.

One of those tiny differences is the distance between the centerline of the crankshaft and the centerline of the camshaft. Because of that difference Volkswagen used nine sizes of cam gear, from +4, through 0, to -4. (The size is stamped on the back of the gear. It reflects a change of .01mm on the diametrical pitch.) About 95% of factory-built engines use cam gears near the zero size, with a nominal range of about +2 to -2. Align boring, which Volkswagen used to do on all their rebuilt engines, dictates the need for the other sizes. How well the gear fits determines how rapidly it wears. Spec for cam gear mesh is .000" to .002." The zero clearance reflects the fact that thermal expansion causes the two shafts to move farther apart at operating temperatures.

Robert S. Hoover reported checking more than thirty after-market gears obtained from a number of retailers in southern California. Most of the gears were from Taiwan, some from Germany. None had the size marked. Of the Taiwanese gears, all were about a +3 and woul be too big for 98% of all crankcases.

Good fit, slow wear. Good fit also means good performance since the fit effects your cam timing and valve train geometry. Immediately after checking the fit of the main bearings to the crankshaft and case, the driver gear is installed on the crank and the crankcase is gauged to discover what size cam gear is needed. One of the most practical ways to do this to is to obtain three stock cams for use as gauges. With a +2, a 0 and a -2, it takes only a few minutes to figure out the right size cam gear for any crankcase. All you have to do is install your gauge-cams in your crankcase and check their lash against your crank.

[ INFORMATION TO BE ADDED: There is a difference in Cam Gear design that effects distributor gear clearance. Some designs have the center mounting screw protruding 1/8inch and it may hit the distrubor gear - I will add pictures later...]

Cam Bearings

Nothing special here, go with good quality stock parts.


Dual port heads are the only way to go. OK, before you get upset, consider this. Single port heads have not been made for decades, most have been run hard and long. While a single port engine provided 40% more flow for a 4 cyclinder than was required by stock valves, why on earth would you shoot yourself in the foot with this old part? If your useing used heads, be sure to check for cracks near the valve stem. Quick and dirty shops force the valve guide in at room temprature (do I hear a crack?), it should always be installed after sitting in an ice box for two hrs. Stock rocker arms can be used but if the budget allows some extravagance, 1.2 up to 1.5 to 1 rockers will really help the engine get maximum performance.

Oil System

For 2-3lbs extra weight, why would you not want to put a Full Flow oil system on your engine and extend the engine life significantly. For $15 bucks your engine might last 500hrs longer. Also, do not be misslead by thinking a large capacity oil pump is neccesarly better. A stock 21mm oil pump only has to deliver oil to 2 cylinders instead of 4 and that is more than adequate. A higher volume pump might easly blow the oil filter on cold starts or damage other engine parts.


Your most likly going to receive the pistons as a set when you purchase the cylinders so you won't have much choice. For this engine build, the pistons are not critcal and any good parts supplier will be adequate.


For this engine build we are recomending the 88mm thick wall. Thes provide a 92mm bore but the neck is 88mm so no machining of a stock case is needed. The heads must be machined for 92mm.


There are quite a few options here. The stock distrubtor converted to electronic ignition would be the least expensive. I personaly think electronic ignition is as safe as magnetos but I don't have hard facts to support that claim. If you feel more comfortable with mags, ther are lots of options. About the cheapest mag that has been used on a VW is the Slick model 4316. They can be regularly found on ebay for $100. Slick 4001 have also been used.


I can't really remeber ever seeing a 1/2 VW with a muffler. I can tell you a 1/2 VW without a muffler is not much louder than a regular engine with a muffler. The sound is a bit more of a pop-pop but it has a pleasent deep rumble simular to a high compression motorcycle. 1-1/2 in pipe is recomended and at the time of this writing is the only inexpensive supplier. I checked with local muffler shops for pipe costs and bending; they where all about 2-3 times more than GreatPlains.


The carburator is as important as the whole rest of the engine. Unfortantly the carburator must be matched to the type, speed, prop, altitude, and other operational factors. Proven reliable chices include Zenith, RevFlow, Makuni, Bendix / Stromberg,  and even the Model-T. I can only recommend research and asking folks on some of the aviation forums such as Yahoos "1/2 VW Aircraft Engines". These forums will have experineced pilots of the same type aircraft you intend to install your engine.

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