The next step in our Datsun Engine build series is to balance each of the piston assemblies. Previously, we gapped the piston rings to each of the engine cylinder bores. The goal of balancing the piston assemblies is get each of them within 0.1 grams of each other. Matching the piston assemblies can be achieved by mixing and matching certain components and when necessary slightly modifying the components to take out a little bit of weight on the heavier assemblies to match the lightest.
Tools and Equipment:
Scales 2000+/-0.1 gram
Piston Gudgeon or Wrist Pins
Dremel with Optional Flex Shaft
120 Grit Flap Wheel
Powder Free Nitrile Gloves
Compressed Air and Spray Nozzle
Marker & Note Pad
Weighing the Piston Components
Before weighing your components make sure they are clean and free of dust. You can wipe them down or wash them. Make sure to blow out with clean compressed air to remove any lint or dust.
Starting out we had previously measured each of cylinder bore diameters and then the diameters of the piston skirts. We then determined the combination of piston and cylinder bore that would give the best clearance values across the engine as much as possible. The Datsun L-Series motors have different exhaust/intake valve orientation depending on the cylinder bore so you have to take that into account when doing the assignment. With the pistons assigned we removed the letters A-F and numbered them to match the assigned cylinder bore.
With the pistons now assigned we can weigh each piston and record the value. Makes sure to weigh each piston several times until you get consistent readings, write the value down and then move on to the next piston. Continue until all piston weights are recorded.
At this point we have labelled each of the piston pins a letter A through F to make it easier to keep track of each part. Then repeat the process weighing each of the gudgeon or wrist pins, the name depends on where you live.
With the piston and wrist pin weights now known, you can allocate the wrist pins to each cylinder bore. You will likely have component weights that are close but there will be a few a little lighter or heavier. Match the lighter pistons with heavier wrist pins and vice versa. Here you can see we ended up with a variance of just a few tenths of a gram between components.
Remove the letters from the gudgeon pins with isopropyl alcohol. To keep track of the wrist pins, write the new assigned cylinder number on the wrist pins using a marker or sharpie.
Weighing The Piston Assemblies
Now that you have the pistons, piston rings and wrist pins assigned to each cylinder bore you just need to add the wrist pin retaining locks to each set of parts to complete the assignment of components for each piston assembly.
Weigh each assembly multiple times, slightly repositioning each time until you get consistent readings. Record the weight for each assembly. We found it easiest to turn the piston upside down and place the wrist pin and retaining locks inside the cup and then lay the various piston rings on top of the piston skirt.
You will likely find that each position assembly has a slightly different overall weight. We had a minimum value of 468.9 grams and maximum value of 469.1 grams with several assemblies weighing 469.0 grams. Not bad only an absolute variance 0.2 grams. Often if you have the piston assemblies within 0.5 grams you call it good and move on. But since this engine build is a learning experience we decided to match all the assemblies.
Balancing the Pistons
The easiest way to match the assemblies especially when you are only talking a few tenths of a gram is to take a little weight out of the pistons.
The best place to remove weight from the piston is by radiusing or rounding the edges around the inside of the pin boss, which holds the piston in place. This is the least structural area of the piston and will have the smallest impact on strength and heat dissipation.
You do not want to drill out material out of the bottom of the piston creating stress risers or uneven heat distribution. You also want to stay away from the ring lands and not hog out large amounts of material from the skirt or any other structural areas.
These pistons are not being used in a race motor so we weren’t looking for the absolute lightest piston possible just to match the assembly weights. If we were building a race motor there are lighter weight piston designs that are more suited to that application.
To remove material start by weighing just the piston itself and then working out how much weight needs to be removed. In our case we had three pistons that needed 0.1 grams removed and one piston that needed 0.2 grams. The pistons had an individual weight of 347.1 grams so we needed three 347.0 pistons and one 346.9 gram piston.
We used a 120 grit flap wheel with a flex shaft Dremel. Start by taking small even amounts of material out of the piston. A nice smooth radius on each of the inner pin boss sides.
We worked one edge of the piston boss a little bit then rotated and worked the opposite edge and then the next edge and then the next opposite edge. This way we could even out how much material we took off each corner and edge.
Blow the dust and debris out with compressed air and then weigh the piston again.
Repeat this process several times and sneak up on the amount of material removed. You do not want to remove too much material and then have to go back and remove even more material from all the other pistons.
In the close up you can see we rounded the squarer outer edges of the pin boss and then blended all the edges with a nice smooth radius. You want to make sure you do not generate any sharp edges or deep scratches that can be stress risers and potentially grow into cracks.
Keep slowly and evenly removing material until you reach the desired piston weight. Double check the desired assembly weight by adding the additional components. Here you can see we have reached target 468.9 grams with a modified piston.
Here is the comparison of a modified versus unmodified piston assembly. You can see the small variances in radius and curvature around the non structural areas of the pin boss. You will note that very little material was removed to reach the desired weight.
Repeat this process for each piston until you have all the piston assembly weights matching. Double check the measurements for each individual piston assembly, including the ones you did not modify to ensure the scales have not drifted.
Take a moment to admire your work! Make sure the components are clean and then put them away together as a matched set. We mark the individual parts with a sharpie or their covers, bags or boxes for the smaller parts.
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