Datsun 240Z 3.1 Stroker
Datsun 3.1 Stroker
This is a page documenting how I built my Datsun 3.1 liter stroker. I'm not an expert on these by any means, however, I have been working on cars for about 30 years. In that time I have built many motors including several Datsun L series. This is not intended to be a "How To Guide", it simply contains some of the knowledge I have acquired while researching, along with hands on experience with this particular project. Consider this just another piece of the information puzzle.
This motor is definitely not for everyone. It's expensive to build and, when finished, you won't end up with the horsepower that can be achieved with a turbo or a V8 conversion for the same amount of money. So why build a 3.1 stroker? One word....TORQUE!!!
The stroker is an extremely torquey motor. Unlike a turbo, which produces most of its torque at higher rpm's, or a stock V8 that has a relatively steep torque curve, the stroker when properly setup, has close to maximum torque from 2000 rpm all the way through the rpm range. This makes for a car that is just flat "fun to drive". If you enjoy "seat of your pants", "head snapping" driving, you might consider the 3.1 stroker.
With that said, lets get our hands dirty.
The block of choice for the 3.1 stroker is the F54. Although some people have used the N42, the F54 block has webbing built in between the cylinders for extra rigidity. The block will need to be bored 3mm over stock. It is strongly recommended that the boring process is done with the main caps torqued to specs and with the aid of a torque plate. This will ensure that the cylinders will remain true after the engine is assembled. Don't shop around for the best price on your machine work. Go with the best you can find.
Nissan used press-in plugs to seal the front and rear main oil galleries. I'm using a higher pressure turbo oil pump, therefore, I opted to remove them by using a slide hammer, and drilling and tapping the hole to accept a standard threaded plug. When installed with Lock-tight, it will eliminate the possibility of blowing out a plug and loosing oil pressure under extreme conditions. This needs to be done prior to the machine work so all the metal shavings are removed during the cleaning process.
The front plug will need to be shortened so that it doesn't close off the # 1 main bearing oil passage and, at the same time, it will fit flush with the front face of the block.
To check the ring gap, insert the top ring into the cylinder and use a piston to square the ring by pushing it down approximately 2 inches. Using a feeler gauge, measure the gap to ensure that it is within spec. Use a good quality flat file clamped in a vise to remove excess material. Be sure to deburr the corners of the filed end. Repeat the procedure with the second ring. Because this is a "non typical" set-up, you won't find a factory specification on this in any Z service manual. If the manufacturer of your rings didn't supply the ring gap specs, here is a good general rule to follow for compression rings.
Top Compression Ring.
0.005 end gap per one inch
Second Compression Ring.
0.004 end gap per one inch
Oil ring-rail gap is not as critical.
They can have anywhere from
0.0010 - 0.040 in. end gap.
This formula works well with any L series engine, however, if you plan on racing your motor on a consistent basis, it's a good idea to add .002 in. to the gap to cope with the increased expansion due to the prolonged heat.
The heart of the 3.1 stroker is the LD28 crank. The crank can be identified by the "V07" embossed in one of the counterweights. It has an 83mm stroke as opposed to the 79mm stroke of the stock L28 crank. These cranks were used in the 81-84 diesel Maxima with the LD28 block. The LD28 crank, often referred to as the "stroker crank", shares the same 49.97mm rod and 54.95mm main journal sizes as all the L28 engines. It also has the same type snout, pulley-retaining bolt, keys, front seal, rear register-flywheel flange-rear seal, and transmission input shaft pilot bushing. This means that the dampers, pulleys, crank sprockets, flywheels and drive gears are interchangeable among all of the L28 engines.
If your LD28 crank has a flywheel dowel installed, it will need to be removed.
When installing the rear main cap, apply a sealant such as Permatex in the area shown here.
DO NOT use silicone!!!
After the main caps have been torqued to specs, (33 - 40 ft-lb), check for proper crankshaft play by using the handle of a hammer to force the crank towards the front of the block. Use a feeler gauge to measure the amount of play.
Acceptable play is .0020 - .0071.
When purchasing 89mm pistons for a 3.1 you have a few choices. One is the LZ24 from the Nissan 720 truck, (Nissan part #12010-30W01). This is a 15cc dished piston that will produce a 9.4:1 compression using a 2mm gasket and N42 head. Using this piston will require the removal of some material from the top of the rod to achieve clearance between the rod and piston pin boss.
Second, and probably the most popular, is the KA24E piston from the 240SX. There are actually three versions of this piston. Part #12010-30R10 has a 2.8cc dish and uses a floating pin. This piston with a 2mm gasket and N42 head will yield a 10.14:1 compression. Part # 12101-40F10 is the flat top version of the same piston and will bump the compression up to 10.6:1. The third version, and the one I went with, is the KA24E from the mid to late 90's model Hardbody truck. It also has the 2.8cc dish but uses a pressed pin instead of the floating pin and clips. All three of these pistons have the same 34.0mm pin height and will result with a 0.6mm positive deck height.
When installing the pistons, here's a couple tips. Turn the block on it's side. By doing this, you lessen the chance of damage to the rod journals caused by the piston falling through the bore once the pistons clear the ring compressor. For extra protection, use fuel line to cover the rod bolts during installation.
Big End Play
When securing the rod caps, there are a couple items that need to be addressed. First is the big end play. Like the crank shaft end play, there needs to be proper clearance between the rod big end and the crank shaft. Factory specs are 0.008 - 0.012 in.
Second is clearance between the block and rod bolts. The combination of the longer stroke of the crank, coupled with the larger 9mm rod bolts, can sometimes cause the rod bolt nuts to actually scrape the side of the block. This picture shows where to look for clearance. I personally didn't have a problem with them clearing. I have heard that some people have, so check all of the rods for clearance. If needed, remove some of the material on the block using a die grinder.
When designing a stroker motor, one very important factor is the final "deck height", or in other words, how far the piston is relative to the top of the block with the piston at TDC. This number can be either positive or negative. The block height of an un-shaven L28 block is 207.9. Next you need to know the "installed piston height". This is calculated using the following formula.
Installed Piston Height
Piston Pin Height + Rod Length + Stroke/2
Here is an example of how this formula works with a stock L28 motor.
38.1mm + 133.3mm + 79.0mm/2 = 207.9mm
The "Deck Height" is the "Installed Piston Height", minus the "Block Height" or in this case...
207.9mm - 207.9mm = "0" Deck Height.
Here are the numbers for a 3.1 stroker set up. As mentioned earlier, both of the KA24E and LZ24 pistons have a pin height of 34.0mm. The 9mm 240Z rods have a length of 133.0mm and the stroke of the LD28 crank is 83mm. Using the formula above...
34.0mm + 133.0mm + 83mm/2 = 208.5mm
208.5mm - 207.9mm = 0.6mm
You'll notice that you end up with a positive deck height of 0.6mm. This can be dealt with by, either, shaving the tops of the pistons or by using a thicker head gasket to achieve the proper piston to head clearance and compression ratio.
Due to the 89mm bore required for the 3.1, you must use the "Big Bore" head gasket in order for the pistons to clear the gasket. This gasket comes in either a 1mm or 2mm thickness to allow for altering compression ratios. Both have a 91mm bore and are made of multiple layers of steel and coated with fluorine rubber. These gaskets are not cheap, however, they can be reused several times. Courtesy Nissan has the best price on these. (Sorry MSA, but $50.00 is $50.00)
Cam-Lobe Wipe Pattern
Having the correct cam-lobe wipe pattern is extremely important to ensure the longevity of your cam. This is accomplished by using the procedure below.
Coat the contact surface area of the rocker arms with lay-out fluid or black magic marker. Install one rocker arm and set the clearance to the cold specifications. (Intake .008 Exhaust .010) Using the cam sprocket bolt, rotate the cam one complete revolution. Compress the valve spring and remove the rocker, check the wipe pattern to see if it is centered and does not extend off the rocker arm pad. Adjustments are made by using different-thickness lash pads. These are available in 0.010 in. increments and range from 0.150 in. to 0.330 in. Stock lash pads are 0.118 in. (3mm). If the pattern is towards the valve end of the rocker, a thinner lash pad is needed. If the pattern is closer to the pivot end, a thicker lash pad is required. Continue this procedure until all the rockers have been checked. It is not unusual to have as many as five or more different size lash pads to achieve a correct setup.
Timing Chain Installation
With the cam and the crank at their TDC positions, install the timing chain, sprockets, guides and tension block. The chain should be positioned so that the bright links are facing the front. Align one bright link to the crankshaft-sprocket dot, which should be at about the 4 o'clock position. Align the second bright link to the selected cam sprocket, start with dot 1. The link position should be at approximately 2 o'clock with the corresponding dowel hole in the sprocket hub aligned with the cam dowel. Install the cam sprocket to the cam nose. Once everything is in place, there should be 42 link pins inbetween the two timing marks, and the dash mark should be slightly to the left of the cam sprocket notch.
Distributor/Oil Pump Shaft
Assemble the oil pump and drive spindle, aligning shaft mark...(small dot)...with the oil pump hole, and then rotate the shaft to the right one notch. At this point, fill the pump with clean oil. With the motor at TDC, install the pump and shaft as a unit. If done correctly the shaft tang should be at the 11:25 position and directed tangent to the backside of the top distributor-adapter mounting bolt.