Conversions - DIY Design Considerations

Rev - Nov 05, 2017

By Jan Zumwalt (EAA #66327)
excerpts from


See Also


We are going to have to cover allot of information and we need a reference. Our discussion will generally pertain to a displacement of approximately 1800cc unless otherwise stated. This provides a good reliable 10-20% performance increase over the stock 1600cc engine. 1800cc does not require extensive machining or modifications, neither does it require costly custom parts. The primary parts supplier for this article is I don't recommend them for any particular reason other than they are a good one stop shop and I need to use some specific parts for examples. I shop around and recommend you do the same; SAMBA ( is another good parts source. If you re-use your exhaust, carb(s), and engine ignition, you can build the engine we discuss here for ~ $1k, which is pretty amazing!

Performance engine building is, by no means, a simple endeavor. Many of you may have noticed this information has been difficult, if not impossible to find, online or otherwise. Well, I’m committed to sharing all the information I can find and categorizing it so it is easy to find,

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



Prior to 1966 VW installed 36-40 HP engines on Beetles and Karmann Ghias. They are too small and have physical differences from larger engines. You should not attempt to build a performance engine from this starting points. Starting in 1967 VW began the 1300/1500/1600 series engines and these should be the basis for all performance modifications.

The cylinder hole in the case of 1967+ is the same for all 3-engine sizes! Both the 1500 and 1600 engine use the same bore size in cylinder head, but the 1300 cylinder head is smaller. This means that a 1500cc engine can be upgraded to a 1600cc engine by simply installing the 1600cc piston and cylinder set! The 1300cc engine can also be upgraded if you bore the 1300 cylinder heads out to the 1600cc size, or replace them with the 1500 or 1600 cylinder head.

Line boring is known to cause overheating and may in overheating and low oil pressure problems later. If you can offord it, start with a new case and you will iliminate many worries later. Used ones are worn out, and prone to cracking at the the rear web. So just factor a new case and machining into your budget from the beginning. You will also need an engine case installation kit, which includes all the miscellaneous hardware for assembly.

Use 8mm head studs. New cases have case savers (Helicoils) built in, and these are far stronger than head studs threaded directly into the case. Use factory VW 8mm head studs (used are fine), since they expand and contract at the correct rate, and keep head torque constant. 10mm studs don’t do this, and that’s why 10mm stud engines have a problem with pulled studs. The VW factory changed to case savers and 8mm head studs in 1972 to solve this problem.


A quality machine shop is capable of doing the required machine work to make an 82/84 X 94mm engine (2275/2332cc) no more difficult to assemble than a stock engine for a novice. You heard that right! Many folks won’t consider something larger than a 74mm stroke because they are afraid of “clearancing”. When the parts arrive from the machine shop, the hard work should already be done for them. All you need to do is assemble the engine, make sure everything clears, and that’s it. The mystery and concern regarding parts clearancing for large engines is overrated and a highly misunderstood part of performance engine assembly. 86 mm and longer strokes, however, do require more attention to assembly detail.


74mm Crankshaft, VW Rod Journals. This is the biggest stroke you can go with, with minimal case machining or other hassles. If you are on a super tight budget, you can re-use the stock connecting rods, but these will require more clearancing of the case because stock rods tend to be “fatter” on the big end than some other rod options, The next best connecting rod option is the 5.4″ I-beam Connecting Rods. You simply don’t NEED more than this for 1800cc engines. A good solution is to use rods made just for THIS application! You can get an easy 5mm increase in crankshaft stroke buy going from 69 to 74mm. This is split with 2.5mm up top, and 2.5mm at the bottom. 2.5mm = .100″.

If you want to run a 76mm crankshaft, we recommend using 5.5″ rods and B pistons (Pistons designed for 76-84mm strokes). It is true you can build a 76mm crank with 5.4 or 5.325″ rods and A pistons, but you are going to find out (the hard way), why we recommend 5.5″ rods and B pistons!

Crank & Rod

Crank and rods need to be treated and selected as a set, not as individual components. The stock crankshaft stroke on the 1300/1500/1600 engine is 69mm. The first consideration is that you MUST have a forged, counterweighted crankshaft. Some companies are selling cast crankshafts. A performance engine will put out more power than stock, and cast cranks will break or flex, it’s just a matter of when. Even if the crank doesn’t break, the flexing stresses other engine components and can cause their failure.

Some people get mislead by this situation since the crankshaft didn’t actually fail, and they assume that the crank is fine and attribute the failure to the broken part when in fact, the flexing crankshaft is the cause. Two examples are: pounded out cases and popped out wrist pin clips. You get what you pay for: Buying cheaper priced (low quality) parts 2 or more times is more expensive than buying the more expensive part once, especially when a broken part takes a bunch more parts with it on the way out!

The first crank upgrade above the counterweighted 69mm crank is the Stroker crank. “Stroker” refers to any crankshaft with a stroke longer than stock.  A stroked crankshaft allows the engine to make power without higher RPMs. High RPMs stands for “Ruins Plane's Motor”. Any stroker engine at low RPM will have more usable power out than a small engine running at high RPMS.

Here comes your first decision point, and as I warned, it’s dependent on your budget and goals. All sizes of crankshafts are comparably priced (around $400) – the cost difference is in what additional parts they require for reliable operation. I never recommend reusing or rebuilding old rods. Other companies may suggest the use of reworked stock connecting rods (which will cost around $120 for a good set) – but when you can get new 4340 I-beam rods for $150 a set, it doesn’t make any sense. If your rods fail, you get to START OVER FROM SCRATCH since all your expensive parts are junk – new everything – so why risk it?

78mm: When you stroke an engine, there is no point in using a crankshaft smaller than 78mm. Stroker crankshafts are all the same price from 74mm to 84mm.  78mm is the largest stroke you can reliably use with reworked stock connecting rods (but remember I don’t recommend that!) With the 78mm crank you can get away with using a $140 set of reworked connecting rods (or 4340 I-beam connecting rod set).

82mm: crankshafts require the use of I-beam or H-beam connecting rods. H-beams are lighter and stronger but cost about $150 more than I-beams! If you are on a tight budget, and want to maximize your displacement you should go with the 82mm with I-beams.

84mm: strokers require a different connecting rod, which costs more. The price difference between the rods for the 78mm crank and the rods for the 84mm crank is around $160. The decision between I-beam or H-beam connecting rods depends on the maximum RPM; 4340 Chromoly I-beam rods are good to 6500 RPMs, and if you plan on going higher than that, (or want peace-of-mind) you need the 4340 Chromoly H-beam rod (about $300-320 a set of 4, depending on length), which is good to 9000 RPM or 500 HP.

86/88/90mm is the next increment in crankshaft. For this size engine, hot-rodders prefer a type 4 center main bearing on cranks of this size, or even type 4 mains all the way along (special engine case machining is required for this installation). The larger main bearing makes the crankshaft stronger, and you need it if you have a stroke of this size! Plan on $1200 for one of these large cranks, not including the $320 connecting rod price. If you get one of these cranks, you also need a SPECIAL set of long cylinders which run about $300 a set of 4, and do NOT include pistons or rings. One more thing: things start getting pretty tight in that engine with a crank of this size, so you had better know what you are doing if you get one!

Connecting Rod Length

“Rod Ratio” is the length of the connecting rod divided by the crankshaft stroke. The small end of a connecting rod is affixed to the center of the bore of the piston, and the big end is attached to the crankshaft. If you increase the crankshaft stroke, and do NOT also increase your rod length, the rod angle is increased. 78mm is the maximum that the stock rod can withstand; beyond that the bolts will fail. Note that I said stock rod, not stock rod LENGTH. The stock rod length is fine as long as you use a stronger than stock rod. Longer crankshaft strokes require a longer connecting rod, and/or a stronger or a better-designed rod connection system. This is exactly what 4340 Chromoly rods accomplish.

69-82mm crankshafts can use the VW (5.394″) or Porsche (5.354″) length rod, IF it is made of Chromoly. Once you start using an 84mm crank, you are required to use a 5.5″ or longer connecting rod so you don’t overstress the rod and rod bolts! I recommend a 5.7″ or longer rod for strokes longer than 88mm.

Another thing to consider when you increase connecting rod length is that the longer the connecting rod, the further out from the crankshaft the piston is, and could potentially stick out the end of the cylinder. Since the cylinder is attached to the crankcase, you need to use cylinder spacers to adjust the compression ratio to where you want it to be.
The longer the connecting rods are, the wider your engine will be.

1800cc Application

One trick is to put the piston close to the same place at TDC is to use a 5.325″ long rod, which is .075″ shorter than the stock 5.4″ rod. The 74mm crank and 5.325″ rod combo puts you within .025″ of the original TDC location! In the past you had to use more expensive “H-beam” rods, since only those were available in the 5.325″ length. has a 5.325″ rod made in the affordable I-beam style, 5.325″ I-beam Connecting Rods.  The net result is an engine that assembles very close to stock! This means no special work on push rods, or exhaust system. If you have a little more $, you can use a 76mm Crankshaft, VW Rod Journals along with 5.325″ I-beam Connecting Rods. The 76mm stroke will get you to 1848ccs.

If you try to run the 76mm stroke with 5.4″ rods the cylinder base shim will get too thick, and you’ll start running into hassles you weren't expecting due to the wider engine. You'll need new longer push rods. And you will need thicker (more expensive) cylinder base shims. If you want to run a 74mm crank, do this with A pistons (Pistons designed for 69-74mm strokes), and EITHER 5.4, 5.394, or 5.325″ rods. If you use the 5.4 or 5.394″ rods, you’ll wind up needing around .090″ of cylinder shim for everything to work out. If you use 5.325″ rods, you’ll need NO cylinder shims, but you may have minor piston skirt clearancing, to make sure the skirts clear the counter weights on the crankshaft.

Cylinders & Pistons

The aircooled VW engine uses cylinder sets that are removable; they are not cast into the block like most engines. This makes them fairly easy to replace. I will refer to “machine in” and “slip in” piston/cylinder sets. “Machine in” sets require engine case and cylinder head machining before they can be used, since they are significantly larger than the original sets. “Slip in” sets do not require any machining, since they use the stock VW case and head hole sizes and increase piston size by “thinning” the walls of the cylinder.

Almost all piston and cylinder sets are available in two versions: short stroke (these are considered “A” pistons), and long stroke (these are considered “B” pistons). A pistons are used on up to 76mm stroke engines and B are used on 78 and longer stroke engines. The difference between the two versions is the location of the wrist pin hole in the piston. Be sure to get the matching set for your crank and rod combo.

77mm, 83mm, 85.5 mm: Stock VW engines (aircooled type 1 1300/1500/1600 cc) came stock with these piston sizes, respectively. Slip in piston and cylinder sets are available in ‘size upgrade’ of 87mm and 88mm, but we do NOT recommend them EVER, PERIOD. There are NO special cases or exceptions to this recommendation. Aircooled.Net doesn’t sell or support applications that use the ‘size upgrade’ slip in sets on 1300/1500/1600 cc engines. When slip ins are made, the cylinder walls are thinned so that the larger piston will fit. The cylinder walls become too thin to maintain their integrity as the engine gets hot, and the piston, cylinder, and piston ring seal breaks down. Overheating and loss of power are the result.

88mm: The next bore size up from stock is the machine in 88mm set. These work VERY well, but you are looking at a lot of machining (this costs approx. $100) for a very small displacement increase! If you’re going to go to that work or expense, you should opt for the larger piston set! However, 88s are very good for busses and type 3 engines, since these engines run hotter than beetles, ghias, or buggies/rails.

90.5mm: This is a very common bore size. You are finally beginning to get a substantial displacement increase for your $. These sets have the same cylinder wall thickness that a stock 1600 cylinder has, so they are VERY reliable. These are an excellent choice, and routinely last 100K miles or more.

92mm: For 30 years, this piston was essentially a “slip in” set for the 90.5 bore size. It works ‘okay’ for low mileage race applications, but it’s prone to the same problems as the 87/88mm slip in combination. However, now the 92mm P&Cs are available in the old version (which we recommend against), but also in “Thick Wall” versions, with a 94mm register at the top, and EITHER a 94mm register at the bottom, or a 90.5/92mm register at the bottom. These 92s are ultra reliable, and are an excellent choice for convertibles, busses, and type 3s.

94mm: NOW we’re talkin’! 94mm cylinders have the same cylinder wall thickness as the stock 85.5mm set (1600cc). A slight drawback, however is that since the fin area is the same, and the engine is now larger, these do run slightly hotter than stock or 90.5 piston sets. When these were first introduced, I did not trust them! 92s had problems, so how were 94s going to be better? Well, the jury is in and 94s work. Reliability is very good; you can expect about 50K miles before a tear down and replacement is needed. Some sets have gone over 100K miles when low compression and sane driving are exercised. The machining for 94s costs slightly more than 90.5s — in addition to “boring”, you must have the case “decked”, so they do cost a little more to build (even though the piston set is about the same price as the 90.5mm). The additional displacement is definitely worth it!

1800cc application

88mm Slip-in THICK WALL P&Cs. These beauties are designed to fit into the STOCK ENGINE CASE at the bottom (they are REALLY thin), but up top they are super Thick, using a 92/90.5mm Head Bore at the top, with a 88mm piston! Can you say THICK CYLINDERS? The cylinder is over 5mm thick on the top (that’s more than .200″)! We put this option on the table because you are putting new heads on this engine, and when you buy them you’ll simply order them already bored to the 92mm Bore size!

You’ll need 85.5mm Cylinder Base Shims, AFTER you measure your deck height, with a Deck Height Measuring Tool. You can use a dial indicator or calipers if you are careful ,because this isn’t a race engine! You want a deck height of .050-.070″. The actual deck height will depend on what rods and crank you decided to use, it’s easiest with the 5.325″ rods. We recommend you mock up the engine and measure, don’t try to buy the cylinder shims/spacers beforehand. If you need them, you’ll do some quick math and figure out which set of 85.5mm Cylinder Base Shims you need to put between the case and cylinders to get there.

You need around .100″ of shim. Since this is not available in the 85.5mm size, what we suggest is that you use a .040″ Cylinder Base Shim AND a .060″ Copper Head Gasket in the 90.5mm Size, to achieve your goal. Combined they get you the .100″ thickness you need! IF you want a little more than 1800cc, you can use everything in this Engine Guide the same but use as large as 94mm Pistons, resulting in a 2054cc engine!

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