Conversion - 1/2 Hapi

Rev - Nov 11, 2017

 By Jan Zumwalt (EAA #66327)


See Also

Taylor - 10 articles appeared in EAA's "Sport Aviation", 12/1979 thru 9/1980, 44 pages (15mb): Taylor - What about Volkswagen conversions.pdf
Hoover - 119 page manual (1mb):  Hoover - VW aircraft engine building.pdf


H API engines are Volkswagen auto engines converted to full 4cyl or 1/2 2cyl VW aircraft engines by Homebuilt Aircraft Products, Inc. (H.A.P.I.), initially of Calexico, California and later of Eloy, Arizona. HAPI was founded in 1977 by Rex Taylor and his son Patrick while they were designing a light aircraft kit, the HAPIcraft. The aircraft never materialized, but they became deeply involved in VW conversions in both kit and completely assembled form.


Rex Taylor wrote an extensive series of articles in the EAA Sport Aviation magazine about VW conversions in general, and HAPI conversions in particular. The articles can be found in a ten-part series from 12/79 through 9/80.


H API engines had a reputation for being problematic. However, if someone considers the original aircraft conversion occurred prior to 1980 when there was very little knowledge about VW conversions and much experimentation was taking place (proven designs still lay decades ahead), it would seem they were no more prone to problems than other engines of similar vintage. Production run for the HAPI kits and fully assembled engines ended in 1989 (assets were sold to Mosler) and so improvements and evolution that would have normally occurred also never took place after that.

HAPI engines are still flown and well represented on homebuilt aircraft. As of 2010 the high failure rate and poor reputation of HAPI engines appears to be a thing of the past. It is unknown the number of engines being flown are what modifications may be attributed to the improved safety record. There is clearly no-longer a high accident or incident rate.

Hub Design

The hub is a predecessor of the ForceOne produced by Great Plains and was called the "HAPI UltraHub" or "SuperHub". It was an improvement over the stock VW but once HP was pushed past the original Volkswagen manufacturer's designed horsepower of 65hp (i.e the Magnum) the crank earned a reputation for early failure. Of course, this issue can be eliminated by attaching the propeller to the flywheel, or relieving crank stress by using a reduction drive.


As HAPI engines began gaining high time other problems and weaknesses where also noticed. Two dowel pins were used to attach the aluminum flywheel (steel outer ring gear) to the crank. The original VW used four dowel pins and eight pins has since become standard. There are now several after market kits for about $15 that allow a builder to convert to any dowel pin arrangement.

HAPI used an aluminum flywheel with steel starting ring, and it cracks sooner or later. The now deceased Steve Bennett had provided a rebuilding service for the HAPI accessory case including a special steel flywheel and case rework to fit both the flywheel and the standard Great Plains alternator.

Head & Valves

HAPI used high performance SCAT heads designed for short life times in drag racing. Some experienced engine re-builders feel SCAT heads are prone to cracking because very heavy valve springs were used to prevent valve float at high RPM . There are also a significant number of engine builders that feel the valve guide clearance is too tight and leads to heat and galling problems. The valve spring problem is easily remedied by after market products. It is a simple procedure to ream the existing SCAT valve guides so they have the ideal .001" - .002" intake and .002"-.003" exhaust clearance.


Valve guide re-pair is common on all VW's using the original rocker foot because it places a side load on the valve stem and these surfaces receive inadequate oil. Installing new valve guides is common practice with all VW heads. Some builders expect to do this every 500-1000hrs of operation. The practice of knurling valve guides, then re-machining is also common - though the guides are sure to wear faster than normal if this is done.

Over Heating & Oil Volume

The Achilles heel of all VW's engines is overheating. This is partly due to design issues but most certainly caused by limited oil flow of the original VW block design. HAPI engines have a oil flow restriction plug (next to oil filter mount) that reduces oil flow to the 4th (pulley now prop hub bearing). With time, it has been learned this should be removed. Some consider it a possible cause of the original crankshaft cracks.



The cranks where known to have a higher than normal failure rate and as larger HP engines were developed the problem of cracking worsened. This problem however is not just limited to HAPI cranks or engines. Several other VW crank manufactures have suffered similar problems as street hot rods and aircraft conversions have pushed for higher horsepower.

It was eventually learned that vibration, gyroscopic forces, and torsional stresses from metal propellers was a major contributing factor to all VW crank failures. It is now universally understood that wood propellers (dampening qualities) should always be used with VW engines. At the time of HAPI production this factor was not fully understood and may have been a significant factor with the crank issues; this may also explain the improved safety record since 1970 - 1990.

Whether the metallurgy, supplier or forging process of HAPI crankshafts was a contributor to failure will never be known. What is known is the following factors...

  • Intolerable stress was imposed by metal props
  • The #4 (pulley) crank bearing is inadequate for 65hp or larger prop loads
  • An oil restriction plug should not have been used
  • Improved oil volume and cooling were not used
  • Improved cowling and baffles were not used
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W ith the passage of time and increase in experience VW engine conversion concerns have improved, been made manageable, or completely eliminated. For example, the limited oil flow and over heating may be partially or completely resolved in any or all of the following ways...

  • Adding a large external oil cooler (usually below engine).
  • Adding full flow oil filter
  • Drilling larger tappet oil galley
  • Adding oil grooves to rocker threads, cam follower and cam web
  • Installing higher volume oil pump
  • Improved cooling baffle design
  • Improved rocker foot design (elephant foot, ball foot or roller)

Note: The solutions mentioned may or may not be applicable to some or all HAPI models. For example, it is unknown if the oil galley can be drilled on these engines.


External oil cooler and improved oil pan cooling


Drilling larger tappet oil galley


Oil grooves in tappet and cam web


Small details can mean large improvements in cooling

Model History


HAPI Hornet 2cyl; 25-35hp@3250rpm; 1983-1989; Wt = 74#
Bore 94mm, Stroke 78mm, Vol 1083cc
Bore 3.701in, Stroke 3.071in, Vol 66.1ci
Same bore and stroke as O-132
Same bore as O-117
Carburetor engine with single ignition
Applications: None found.


HAPI Model 50 4cyl; 50hp@3200rpm; 1979-1989; Wt = 152#
Bore 85.5mm, Stroke 69mm, Vol 1585cc
Bore 3.366in, Stroke 2.717in, Vol 96.7ci
Same stroke as O-102, O-112, O-117
Carburetor engine with optional single or dual ignition;
Starter and alternator were standard equipment except on Model 50-E
Applications: None found

Reference: Sport Aviation 2/79


HAPI Model 55 4cyl; 55hp@3200rpm; 1983-1989; Wt = N/A
Bore 88mm, Stroke 69, Vol 1679cc
Bore 3.465in, Stroke 2.717in, Vol 102.4ci
Same stroke as O-97, O-112, O-117
Carburetor engine with optional single or dual ignition;
Starter and alternator were standard equipment except on Model 55-E
Applications: None found


HAPI Model 60 4cyl; 60hp@3200rpm; 1979-1989; Wt = 152#
Bore 92mm, Stroke 69mm, Vol 1835cc
Bore 3.622in, Stroke 2.717in, Vol 112.0ci
Same stroke as O-97, O-102, O-117
Carburetor engine with optional single or dual ignition
Starter and alternator were standard equipment except on Model 60-E
A turbocharged version also was available
Applications: (US) Bounsall Super Prospector

Reference: Sport Aviation 2/79


HAPI Model 75 Magnum; 4cyl; 75hp@3200rpm; 1983-1989; Wt = N/A
Bore 94mm, Stroke 69mm, Vol 1915cc
Bore 3.701in, Stroke 2.717in, Vol 116.9ci
Same bore as O-66, O-132, same stroke as O-97, O-102, O-112
Carburetor engine with optional single or dual ignition;
Starter and alternator were standard equipment except on Model 75-E
Applications: None found


HAPI Model 82 Magnum Plus 4cyl; 82hp@3400rpm; 1983-1989; Wt = N/A
Bore 94mm, Stroke 78mm, Vol 2165cc
Bore 3.701in, Stroke 3.071in, Vol 132.1ci
Same bore and stroke as O-66, same bore as O-117
Carburetor engine with optional single or dual ignition
Starter and alternator were standard equipment except on Model 82-E
Applications: None found