Editing 383/388 Chevy stroker

All small block Chevy blocks are the same measurement from the centerline of the main bearing journals to the deck where the heads bolt on, ~9.025". When we mix and match components, we have to fit them into this "Block Deck Height" dimension of ~9.025". We would use half the stroke dimension, because only half of it swings toward the block decks. So, with a stock 350 Chevy, with a stroke of 3.480", we would use the radius of the stroke (3.480" times .5 = 1.74") to start putting the stack of parts together that will fit into the block. Next, we would use a standard 5.703" rod length, then a piston with a 1.560" piston compression height (measured from the centerline of the wrist pin to the top of the piston crown). Now, if we add up all the dimensions of our "stack" components, we find that we have room in the block for them, plus a little extra which will be between the crown of the piston and the block deck. 1.740" + 5.703" + 1.560" = 9.003". So, if there is 9.025" available and the stack is only 9.003", that leaves 0.022" space above the piston crown with the piston at top dead center. This space is called the piston deck height (not to be confused with block deck height). 

Now, we decide to stroke the motor, so we install a crankshaft that has a stroke of 3.750". This can be either a 400 Chevy crankshaft with the main journals turned down to a smaller diameter so that the crankshaft will fit into the main bearings of a 350 block or it can be a specialty aftermarket crankshaft that has been manufactured with the 350 main journal size and will drop right into the bearings with no machine work at all. Now, we have a choice to make. The radius of this crank is 1.875" (3.750" times .5 = 1.875"). Common sense will tell you that if you retain the 5.703" rod length and the 1.560" piston compression height, that the additional length of the radius of the crank will push the piston out of the top of the bore at top dead center. (1.875" stroke radius + 5.703" rod length + 1.560" piston compression height = 9.138"). This sticks the piston out past the block deck (remember, our block deck height is 9.025", so you cannot bolt in a stack of parts that measures 9.138") This prevents the head from being bolted on, so we must make other provisions.

Most commonly, we would use a piston with a shorter compression height, since using a shorter rod is not the desired way to make the change. Most of the time, with a shorter rod, the skirt of the piston will strike the counterweights of the crankshaft with the piston at bottom dead center, so we want to use the longest rod we can within reason. We can continue to use the 5.703" rod and use a different piston, 1.425" compression height. Now, our stack looks like this......1.875" + 5.703" + 1.425" = 9.003" with a 0.022" piston deck height. OK, that works. 

Another way to do it is with a longer rod yet, a 6.000" rod length and a shorter yet piston, one with a 1.125" compression height. So, 1.875" + 6.000" + 1.125" = 9.000". The piston deck height becomes 0.025" with this stack of parts. I personally do not like the 6.000" rod in the 383 build because it puts the wrist pin up into the oil ring groove in the piston. Then you have to use a filler ring to make a platform for the oil ring rail to ride on and it just adds more parts to go wrong in the perplexity of an engine build. Many fellows have done the 6.000" rod stroker motors and have run them without a hitch. I just personally do not like them. For me, a 5.703" rod and a 1.425" piston works just fine. I will agree however, that there is more clearance between the skirts of the pistons and the counterweights of the crankshaft with the 6.000" rod. If you are building an internally-balanced motor, you need all the counterweight you can get, plus maybe some Mallory Metal, so a 6.000" rod begins to make sense. The least expensive way would be to build an externally-balanced motor using a 400 damper/balancer and a 400 flexplate/flywheel. 

One of the most commonly used combinations used for a 383/388 stroker is the Scat cast steel crankshaft and Scat I-beam forged Pro Stock rods. The counterweights will clear the pistons using a 5.703" rod and the rods will clear the cam, so no grinding on the rods for cam clearance will be necessary. You may need to do a little grinding on the inside of the block at the pan rail to clear the big end of the rod, but it will be minimal. A 383 is a 350 block bored +0.030" with a 3.750" crank. A 388 is a 350 block bored +0.060" with a 3.750" crank. 

It is commonplace for us hot rodders to cut the block decks to zero piston deck height and use a head gasket that compresses to around 0.040". This allows a squish of 0.040". Squish is the high speed jetting of fuel air mixture from the dead zone opposite the combustion chamber. When the piston approaches top dead center, the clearance between the crown of the piston and the underside of the cylinder head diminishes to just short of a collision. This squeezes or "squishes" the mixture that is there, across the cylinder toward the spark plug. This high-speed jetting of the mixture not only eliminates any dead spots in the chamber, but also creates turbulence to accomplish complete mixing of the fuel/air mixture so that there are no lean or rich areas in the chamber. Having the tight squish (0.035" to 0.045") will allow you to build a high performance street/strip motor that will run on pump gas without detonation.

[[Category:Engine]]