Why You Want to Use Standard Size Bores - Modular Fords

  • Why You Want to Use Standard Size Bores

    One of the more frequent and emotionally charged PM questions I get on a regular basis is why can’t I bore my block 0.020” or some even ask 0.040” or 0.060” oversize. The simple answer is there is insufficient cylinder wall available to support those dimensions. That is usually followed by a proof point being offered that my good friend has bored his block that much and he makes XXX RWHP and is very happy.

    The statement begs two questions;

    1.) If you are so comfortable, why are you PMing me asking for my blessing to do the same thing?
    2.) How frequently and how long has his engine run at max power with this configuration?

    The simple truth is that when you have an engine with 100mm bore centers and a 90.22mm bore there is precious little material left to support any bore increases let alone bore increases on supercharged engines that are asked to perform at power levels several times Ford’s original advertised power level.

    Let’s look a little deeper into the how and why side of the issue using some Ford data. The image below, from a Ford print of a 4.6 block, illustrates the cylinder bore dimensions and geometry of a typical aluminum OEM 4.6L engine block.

    As you can see from the drawing the cylinder bore centers are set at 100mm. The combination of the liner and the supporting aluminum register the liner is located in come to 3.15mm or 0.124”. The print calls out a 3.5mm (0.138”) gap between the outsides of the two supporting aluminum registers.

    When we start to do the math beginning with the 100mm bore centers and then subtract 6.3mm (for liner and register) we come sown to 93.7mm. From the 93.7mm we need to subtract an additional 3.5mm (1.75mm on each side) for the coolant gap between the two aluminum cylinder registers which brings us to our standard bore size of 90.2mm or 3.551” in imperial units.

    Now lets go back to the 3.15mm liner and liner support between cylinders. Remember 3.15mm is 0.124 inches. Let’s say we leave a 0.062” thick aluminum register to support the cylinder liner. That means the iron cylinder liner is 0.124” – 0.062” or only 0.062” thick on a side! The aluminum register is actually thicker than 0.062. Let’s be conservative and hold to only 0.070” which means our cast in place sleeve has 0.124” — 0.070” or 0.054” thick walls.

    Imagine boring that block 0.020 over size let alone larger. At 0.020” over you only have 0.044” of liner wall left! For a daily driver you might be OK. For a mild 650 WHP car you are in the deep end of the swimming pool and you don’t even know you have to swim! A 650 WHP car is essentially a 750+ HP engine. That works out to a little over 3HP per inch and it only has a 0.044” thick cylinder containing all that power!

    On high powered, supercharged engines, thin walled liners like that distort under power allowing combustion gases past the rings and into the crankcase. Repeatedly doing this to the liners will ultimately cause them to work harden and crack. For any supercharged engine you want to maintain the best ring contact possible and the most rigid liner possible. That means the least overbore.

    It is also the reason I always encourage a new engine builder to find the worst cylinder in the engine and use that as the guide for piston and finished bore sizing. My suggestion will always be to go just large enough to clean up the bore top to bottom and then no more. The challenge with this historically has been that piston manufacturers only make standard, 0.010 and 0.020 oversize pistons.

    The reason I originally contacted Gibtec to ask them if they would consider offering a ModMotor line of pistons was that CNC manufactured pistons cut from billet stock had none of the limitations that forgings brought to the design table. It was possible to start with a clean sheet of paper and without compromise build the exact piston the engine needed!

    The fact that the CNC process would allow a custom pistons to be built in 0.001” increments to any size was a huge plus! Equally as attractive was the ability to order replacements that would match size and weight of the originals to a gram and a thousandth of an inch.

    The reason that having pistons in 0.001” increments is so valuable for OEM blocks is the preservation of the already oh-so-thin cylinder walls in the OEM block. The real fix for the problem is the use of an aftermarket liners like LA Sleeve offers.

    The LA Sleeve Modmotor liner has a outside diameter of 3.793 inches. Using a 3.551 inch bore will provide a cylinder wall thickness of 0.121 inches or just shy of an eighth of an inch!

    The near eighth inch liner wall thickness and improved nodular iron liner material provides a substantially stronger liner than the thin less than a sixteenth inch OEM liner wall alternatives. The more robust liner stays round under high boost and high power and if necessary can be easily replaced on an individual basis. Additionally the full flanged approach to registration provides the necessary real estate for a receiver groove allowing copper gaskets and a stainless o-ring to be used for a dramatically improve head gasket seal.

    Could you go 0.020” over on a replaceable liner — sure. You would still have 0.111” of cylinder wall left but it begs the question why? If you do, you need eight new pistons, a new set of rings, a complete boring and honing and a rebalance because the rotating assembly has just changed. This is a look from the crankcase side where you can see just how heavy the LA sleeve cylinder walls are.

    The replaceable liner lets you replace just a liner if it gets wounded and reassemble. Worst case you order a single replacement piston from Gibtec and it will arrive identical to your original including weight. Better yet order ten pistons from Gibtec on the initial order — it is less expensive. Slide the new piston in the new liner and you are good to go. There is no need to rebalance the crank or any of the other usual rebuild monkey motion.

    This is what a block looks like with aftermarket liners installed from the top.

    And here is a picture of the block with liners and a few pistons installed

    When you add head gaskets this is what they look like on the deck,

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