There's No Replacement for Displacement

Stroking the Slant-6 Engine

Written by Doug Dutra (1996)

Edited by Dan Stern (1997)

Revised by Doug Dutra (2001)

There's an old axiom that says that the fastest way to get more power from an engine is to increase its piston displacement. The displacement of an engine is measured in litres or cubic inches. We'll use cubic inches in this article as determined by:

  1. Crankshaft KitThe bore, which is the size (diameter) of the piston.
  2. The stroke, which is the distance the piston travels from Top Dead Center to Bottom Dead Center.
  3. The number of cylinders.

The engine's cubic inch displacement is abbreviated "CID".

Since the number of cylinders is fixed, to gain more cubic inches from an engine it is necessary either to increase the bore or to increase the stroke.


Boring (actually, "over-boring") is the easiest and least expensive way to increase engine size, but, like anything that's easy, boring won't yield large gains. All three Slant 6 engines, 170, 198, and 225 cubic inches, use exactly the same 3.40" diameter bore size and piston. The Slant 6 is considered a thick-wall casting and can be bored considerably more than most modern engines. Pistons are readily available in 0.020", 0.040", and 0.060" factory oversize. However, boring a 170 engine 0.060" will only increase its displacement to 176 cubic inches, and boring a 225 0.060" yields a total of 232 cubic inches, not enough to make a noticeable difference.

It is possible to bore a Slant 6 as much as 0.100" over, making the piston size 3.50", which increases a 225 to 238 cubic inches. Another size range to look at is the 87.5 to 89mm metric offerings (3.445 to 3.504). Silv-O-Lite PistonsThese metric offerings now provide an even wider selection of pistons and rings to choose from. Note that a .104 overbore is not a problem with a well-cast SL6 block that has no core shift, but when building a race engine, it's advisable to have the block sonic checked to ensure that the metal thickness is sufficient around all the cylinder bores, especially on the major thrust side of the cylinder (camshaft side). As noted, there is a much larger selection of 3.50" and newer metric sized (87.5 to 89 mm) pistons and rings.

There are several known instances of 0.130" overbores, including Mark Goodman, who has turned a 10.87 second quarter-mile time with the resulting 242 Slant 6. This is only for all-out racing engines and definitely requires all the expensive block tests.


Not much has ever been written on stroking, i.e., increasing the stroke of the Slant 6. I could only find a short statement in the Mopar Performance Engine Book:

  • "The only production part available to stroke the [Slant] Six is the installation of the 225 crankshaft into the 198 engine. This requires the 225 crankshaft and rods. Likewise, any specially made stroker crankshaft will also require either special rods or special pistons or both. The assembly should be rebalanced."

This does not give any new information. What this does is turn a 198 into a 225, undoing what the factory did when they turned the "raised block" 225 into a smaller 198, which replaced the "low block" 170 engine for 1970.

Let's review what the factory has given us to work with: The Slant 6's letter designation is "G". 


Low block, "LG" engine: 170 cubic inches.
Nominal 8.4:1 compression ratio
Raised block, "RG" engine: 198 and 225 cubic inches.
Nominal 8.4:1 compression ratio.


170: 3.125"
198: 3.640"
225: 4.125"
(divide above figures by 2 to obtain actual crankshaft throw)


170: 5.705" 708g
198: 7.005" 756g
225: 6.700" 750g


An engine's "rod ratio" is the length of the connecting rod divided by the crankshaft stroke. The higher this ratio is, the better. Higher rod ratios mean less side loading on the piston and less mechanical drag from swinging the piston and rod assembly back and forth. In addition, a higher rod ratio gives more "dwell" time at top and bottom dead center.

At top dead center, more dwell time allows the pressure in the combustion chamberConnecting Rods to build higher, giving more power. The tradeoff is that longer rods weigh more. Also, the length of the rod is limited by the overall block height. As the connecting rod gets longer and longer, the pin bore in the piston must be higher and higher to fit the whole assembly into a given engine. When you've moved the pin bore as high as can be, making the pistons as short as possible, a longer rod will not fit. An engineering rule of thumb is to stay above a 1.5 rod ratio, because below this point piston side loading and tensile stresses from rapid directional change at TDC and BDC become excessive.


170: 1.83
198: 1.92
225: 1.62

As previously mentioned, the bore size of all three stock Slant 6 engines is the same, 3.40". The main bearing journals and rod bearing sizes are also the same on all three stock Slant 6 engine sizes. There is one major exception: the mid-1976 and later Slant 6 engines, all 225s, have cast iron crankshafts instead of the earlier engines' forged steel crank. None of the following information will apply to them. They are not as strong as the earlier engines and have different bearing sizes. These later engines should be avoided when building a special Slant 6 engine. (Note: There are now many examples of cast crank SL6 engines being used in SL6 racing & other "high output" applications, with good success. My preference is to use the forged steel crank engines when possible but note that the cast crank engine is also a very strong unit.)

As discussed previously, boring alone won't yield a big increase in displacement. But if you combine an overbore with an increased stroke, appreciable gains in engine displacement are possible. I will present a few stroking options, all of which I have built and successfully run.


One overlooked way to pick up a few more cubic inches and get some additional compression (without milling the head) is to offset grind the crankshaft rod journals (pins) outward by 0.025". That will make the stroke 4.150", and the rod journal will now be 0.030" undersize. The 0.005" difference is needed to clean up the journal after offset grinding. On really nice cranks, you may be able to move outward even a bit more. You should also be aware that this is the largest undersize bearing available, so you are at the end of the line as far as more crank grinding later. Doing this offset grinding and a 0.100" overbore will give you a 9:1 compression, 240 cubic inch Slant 6. This isn't a bad way to pick up some cubes and compression, especially if you spend extra money and have the reground crank nitrite hardened ("Nitrided") so that future wear or damage will be minimized.


You don't think a 4.15" long-stroke engine is where it's at? You want a fast-revving, lightweight engine? How about a 210 CID low-block 170? I built one of these for my '68 Hyper-pak equipped Barracuda drag car. This was done using a 198 crankshaft in the 170 block with a 0.100" overbore (3.50" bore). I used Chevrolet Vega 144 CID pistons on 170 connecting rods, which gave 9.2:1 compression. Sounds easy? It's not, because the longer-stroke 198 crank does not quite clear the 170 block's crank pocket. To make it fit, I ground metal off the crankshaft counterweights and a small amount of metal off the bottom of the block's cylinder bore area. Most of the crank clearance grinding work was done on the first and last small crankshaft counterweights. Approximately 0.300" had to be trimmed off these counterweights so they would clear the bottoms of #1 and #6 pistons, solving the interference between the crank counterweights and the bottom of the piston's pin boss area. In order to rebalance the crankshaft after the modification, some weight-reduction holes had to be drilled into the large center weight in the middle of the crank, 180 degrees opposite the work done on the end counterweights. After this, everything cleared and was rebalanced with no added heavy (and expensive) Mallory Metal. This engine runs well and is 50 lbs lighter than a 225! It's proved a strong engine with no problems in five seasons of 7,000-RPM-redline drag racing. If I were to guess at a weak spot in this engine, it would be those Vega pistons, but so far they've held up.

Update: I did end up pulling this engine out of the Drag car after its fifth year of action. The oil pressure was starting to drop and there was a "metal-to-metal noise" coming out of it. Upon tear-down inspection, the noise ended up being the windage tray. It had developed some cracks, shifted and was hitting a rod. The oil pressure drop was due to a "trashed" oil pump, which had sucked up "rock-hard" pieces of nitride hardened roller camshaft iron. (That's another tech. article.) Bottom line is that this LG 210 engine is still "alive and well" with a new flat tappet cam, oil pump and a repaired windage tray.


You say revving the hell out of the engine is not for you? You want low-RPM torque? How does a 260 CID, 4.50" stroke Slant 6 grab ya? The key to this is a specially welded and reground crankshaft - to the tune of $500. Block NotchesThe block needs to have clearance notches at the bottom of the bores to allow for the extra swing of the connecting rods. In addition, the left (opposite the camshaft) side of the block's crank pocket needs to have clearance "dips" ground approximately 1/8" deep from the oil pan rail down approximately one inch to clear the sides of the connecting rods and rod nuts. This is because the rods come so close to the side of the block. I ground a little additional material off the sides of the connecting rods and used smaller diameter ARP 12-point nuts to help gain additional clearance. The 225 connecting rods were used along with super-short, dished-top, 1mm oversized pistons from a 2.2 litre MoPar 4-cylinder turbo engine. This ends up with a 3.485" bore and 8:1 compression ratio with an unmilled head. This is a strong, lightweight (565g) piston, made of hypereutectic cast aluminum intended for a turbo 2.2 application. It has thinner metric rings for less drag and is a much higher quality piston than the cast junk Vega piston. On the downside, these pistons are expensive compared to the stock Slant 6 units - approximately $32 each. The ring sets are "turbo quality" and therefore, also pricey. The point here, however, is that a 4.50" stroke Slant 6 IS possible.

I have installed this engine in my 1964 Dart. The engine is set up with the stock 1bbl Carter BBS carburetor, single exhaust system and point-type distributor. The car has the original 904 automatic transmission and 2.93 ratio in the 7 1/4" rear axle. Basically everything is factory stock except for the 4.50" stroke shortblock assembly. The camshaft is the stock 1971-'80 Slant 6 grind (244 degrees duration, 0.406" lift intake, 0.414" lift exhaust, 26 degrees overlap).

On-the-road performance of this combination is impressive, especially the off-idle to 3,000 RPM range. My best description of the feel of this engine is that it reminds me of an average-running 273 2bbl V8 in the 600 to 3,000 RPM range. After 3,000 RPM, I get the feeling of an undercarbureted engine. It just stops pulling hard as it continues to rev up. At about 4,800 RPM the power really starts to fall off. I think this is where the cam runs out, but I need to increase the carburetion before I really know.

The engine has over 3,000 miles on it now with no problems or failures. The car idles and runs smoothly and is very responsive to throttle openings in the lower RPMs. The only funny thing I've noticed is a minor amount of piston noise or "slap". I can hear it during the first cold startup drive-away. After the first minute of running, it goes away. In fact, the intensity of this noise seems to be decreasing as I get more miles on the engine. My guess is that the long stroke combined with the short and lightweight hypereutectic pistons are the cause of the noise. Add some heat and oil and it goes away, or maybe the pistons' wrist pins are a bit snug and are "working in". Either way, it's minor and I'm not really worried about it. Time will tell! (Ed. note: My freshly-rebuilt stock Aluminum 225 did this, and got quieter as miles piled up. Hemi Anderson reports that this sometimes can be the result of the camshaft walking back and forth in the block when cold, and that it is not a problem. -Dan)

As of this update, this engine is still running flawlessly but has been swapped into my "every option you can think of" 66 Dart wagon. The 3500 lb. wagon has taken some of the "seat of the pants spunk" out of this engine but let me tell ya, it still has gobs of low speed grunt. Future plans for this engine include intake and exhaust improvements. I feel that this will unleash some additional power. Another idea/plan is to use this engine as a test bed for a Slant 6 Turbo kit. Future plans for this engine include intake and exhaust improvements. I feel that this will really unleash some additional power. Another idea/plan is to use this engine as a test bed for a Slant 6 Turbo kit.


So you say these special engines sound neat, but you're just not up to all that crank welding and grinding work? Well, here is another combo to think about and it is based on the increasing availability of metric sized pistons. Here you use a 225 block and crankshaft, along with the 7 inch center to center 198 connecting rods and 2.2 Turbo pistons. This is not really a "stroker" engine but does have some advantages. The main one is a light, strong piston and a very long connecting rod. This combination produces an advantageous 1.7 rod ratio and will push the piston close to the top of the bore. (.026 to .010 negative deck). A stock 225 piston stops 0.140" down in the cylinder (negative deck height). Bringing the piston closer to the top can be used to provide some quench in the Slant 6's combustion chamber. Quench or "squish" helps create turbulence and breaks up the mixture for better efficiency and power.

I have built two more "long rod" engines since this article first appeared and since then, "better" 2.2 pistons have been released from United Engine & Machine Co., (KB-Silvolite) who makes this engine combo even better. The "older" long rod 225s I built used the Silvolite H1290 or H1291 hypereutectic pistons. Here is some info from KB describing the hypereutectic piston alloy they use:

The silicon in the base metal is dissolved with aluminum to form a 12% silicon/aluminum solution. This base metal is eutectic, (u-tec'-tic), aluminum. Adding additional silicon which no longer dissolves into the base aluminum makes the alloy "hypereutectic" with small grains of unbound silicon suspended in the alloy. The bottom line is that the 390 hypereutectic alloy gives superior wear resistance, 15% less thermal expansion and increased thermal barrier properties. KB Performance Pistons are designed around our hypereutectic alloy. The characteristics of the 390 alloy, combined with modern permanent mold process and T6 heat treating gives design freedom and strength, resulting in a superior product ...

The KB239 & KB268 have a "zero offset" pin and have lock grooves so the pin can be set up as "pressed" or as a "floater". Other points of interest are that these pistons have higher ring groove placements, they are pretty light (482 grams with pin) and are available in a 1.5mm oversize (3.504 inches).

2.2 pistonAnother great 2.2 piston choice is the Federal Mogul L2502F, (old TRW p/n). This is a forged piston with a flat top, no valve notches, a pressed pin and 1.610" compression height. It does not have pin off-set and weighs 573 grams. This piston has a .375" thick top so some dishing / valve-notching work can be done. As of this rewrite, this piston has been discontinued with approximately 150 units still showing in inventory. Like the discontinued TRW SL6 forged piston, these 2.2 forgings will soon be history. On the "bright side", I keep seeing more and more metric sized pistons coming "on-line", as well as the custom pistons which seem to be coming down in price to the point where they are becoming the preferred choice for a performance SL6 build-up.

Deck Heights, Dish / Notch Volume & Compression Ratios: (w/ .045 head gasket)





8.73 C/R





10.15 C/R





11.48 C/R

I based my calculations on a 3.500 bore size using a .045 head gasket thickness. This work was done on a calculator then cross-checked on a "Dream Wheel". Note that my Power - Speed Calculator (dream wheel) came up with exactly the same results, without all the keypunching. If you don't have one of these things, get one because it really saves time. Here are some handy numbers for different head gasket thickness:

Added CCs for Head Gasket Thickness

  • .045 = 7.10cc
  • .030 = 4.73cc
  • .020 = 3.15cc
  • .010 = 1.58cc
  • .001 = 0.158cc for every .001 of head gasket thickness (3.50 opening size)

Milling the Slant Six cylinder head .0066" gives a 1cc decrease in combustion chamber size.

Example of compression ratio calculations (see worksheet):

If you need lower compression, the dish already in the KB268 piston can be increased in order to get the compression needed for a Turbo or street driving. If you are working to develop a quench / squash zone, take metal away on the piston surface closest to the spark plug. LEAVE close piston-to-head clearance, though you need at least 0.035" clearance in the outlying areas. This will squash the mixture towards the spark plug right at TDC compression when the plug fires. Note that this clearance needs be controlled closely, between .035 - .060 to get any benefit, .060 to .100 seems to promote more detonation and beyond .100 is the open combustion chamber design the SL6 currently has.

Something to remember on all these special engines is that you need to take actual measurements and calculate all the important things like compression ratio, displacement and valve clearance. Also, you must always be working with a fresh cylinder bore any time you change the stroke. So, with careful attention to detail and some creative use of welding, grinding, and parts-bin engineering, you can create "alternative" Slant 6s that have different characteristics than the 170, 198 and 225 engines we all know and love.

Copyright © Doug Dutra, 1997-2001, All Rights Reserved