Sonic Check

[jayb]

I can't agree with Rob's statements on offset boring.  I had my Shelby block offset bored quite a bit when I built my big SOHC engine.  This was offset boring the aluminum block itself, in preparation for using larger sleeves, spaced farther apart.  I was going for the maximum bore size, so the inboard cylinders (2, 3, 6, and 7) were offset bored 0.035", and the outboard cylinders were offset bored 0.105".  My shop used an old style boring machine (Sunnen or Kwik-Way, I think...), and certainly not a CNC machine; there isn't a CNC machine in the place.  Those bores are perfectly placed; I have checked them multiple times.  Also, after the rod broke at Drag Week last fall, a different shop is repairing this block, and had to touch up those same bores about .003" after the block was welded.  The guy at this shop is extremely meticulous, maybe the best engine machinist I've ever worked with.  Again, no special equipment, and the bores are right on.

There must be a way to do this accurately, without CNC equipment, because I've seen it done on my own stuff, and elsewhere - Jay

[jayb]

For the uneducated like me, can someone explain the test result sheet? What I get out of is 8 readings per hole. If your bore it .030 then you loose .015 from each of those numbers. Front and back seem right for location in cylinder, but major / minor makes no sense to me.   

Major and minor refer to the thrust surfaces of the bore.  If you think about the engine cross-sectionally, and look at the position of the piston, the crankpin, and the connecting rod right after the cylinder fires at top dead center, the force on the piston is directed mostly against one wall of the cylinder; this is the major thrust side.  This is because of the angle of the crank pin and the connecting rod.  For number 1-4 cylinders, this is the outboard side of the cylinder, and for 5-8 it is the inboard side.  Visualizing the engine from the front, and drawing a picture of the bore, the piston, the rod, and the crankpin about 45 degrees after top dead center is the easiest way to see this.

The minor thrust side is opposite to the major thrust side; this is the inboard side of 1-4, and the outboard side of 5-8.  Minor thrust refers to the force on the piston as it is compressing the air/fuel mixture on the compression stroke; there is quite a bit of force required to do this, and the piston pushes hard on the minor thrust side while it is compressing the mixture.

On a sonic check, you can tolerate thinner spots on the front and back sides of the cylinder, but you'd really like to have the major thrust side and minor thrust side as thick as possible.

[ScotiaFE]

There must be a way to do this accurately, without CNC equipment, because I've seen it done on my own stuff, and elsewhere - Jay

An old hand crank hack like me can get the hole pretty close with a dial indicator.

[fairlanegt427]

uae it send it to barry at survival motorsports.  he does great machine work. and more than likely probably has those pistons you need for your 428 in stock.  im not picking on your engine guy but barry knows the fe.

[Barry_R]

CNC does some really great things, and allows a level of easy setup and repeatability that was very difficult in the past.  I can certainly appreciate the value of the latest equipment, and agree that there are things that are "impossible" without it - like CNC porting on a 5 axis machine.

Its rather silly to assume that a good machinist is unable to do something as simple as square decks though.  Its a basic fixturing task.  New and cool equipment will allow you to be working on something else while the machine does the job, it will allow a less skilled (thus less expensive) employee to do the job correctly, and it will free up time.  All of those have great value in a shop environment and justify the investment.  But I can promise that a guy at Holman-Moody in 1960-something was perfectly able to square up a block or offset a bore without a computer anywhere in sight.

[Bolted to Floor]

For the uneducated like me, can someone explain the test result sheet? What I get out of is 8 readings per hole. If your bore it .030 then you loose .015 from each of those numbers. Front and back seem right for location in cylinder, but major / minor makes no sense to me.   

Major and minor refer to the thrust surfaces of the bore.  If you think about the engine cross-sectionally, and look at the position of the piston, the crankpin, and the connecting rod right after the cylinder fires at top dead center, the force on the piston is directed mostly against one wall of the cylinder; this is the major thrust side.  This is because of the angle of the crank pin and the connecting rod.  For number 1-4 cylinders, this is the outboard side of the cylinder, and for 5-8 it is the inboard side.  Visualizing the engine from the front, and drawing a picture of the bore, the piston, the rod, and the crankpin about 45 degrees after top dead center is the easiest way to see this.

The minor thrust side is opposite to the major thrust side; this is the inboard side of 1-4, and the outboard side of 5-8.  Minor thrust refers to the force on the piston as it is compressing the air/fuel mixture on the compression stroke; there is quite a bit of force required to do this, and the piston pushes hard on the minor thrust side while it is compressing the mixture.

On a sonic check, you can tolerate thinner spots on the front and back sides of the cylinder, but you'd really like to have the major thrust side and minor thrust side as thick as possible.

Thanks for the lesson Jay, that makes a lot more sense to me.