Engine Rebuild: 5.0 GL Marine Gas - Boring, Honing and Ring Gaps

Now on to the block...

This thing was a mess! This is how the block looked when it showed up at our shop. The customer said it was sitting in his yard for over a year. So we knocked off the mud and grass and then it was time for a good cleaning in the hot tank.



We placed it in our Hotsy hot tank which sprays the block with heated degreaser solvent which removes the grit, dirt and rust.

Then we placed it in our boring bar and bored it .030 over stock.

Then it was installed in our power honing machine which uses stones to sand the cylinder walls straight.

This removed any tapering and/or cut marks caused during the boring process. It also creates cross hatch marks that file the rings, helps seat them into the cylinder walls and to seal the combustion. Ring end gap size is important because too much creates crank case blow by. Too little ring end gap will cause the ring ends to butt together and cause cylinder scaring or the piston to break at the top ring groove.

Cylinder Bore Refinishing info via Hastings Rings

----------------------------------------------------------------------------------------------------

On this particular engine, we used hypereutectic pistons which require a larger ring end gap of 0.008" per inch of cylinder bore. In this case the cylinder bore was 4.030", therefore the ring end gap would be 0.032".

Hypereutectic performance pistons will expand less than typical cast or forged pistons. Because of this and the wear characteristics of the hypereutectic alloy, you can run tighter piston to wall clearances which helps the engine run more efficiently and last longer.

Most engine rebuild shops try to get away with using the cheaper components and inaccurate, obsolete machinery to preform their work.

WE DON'T!!!

Got any questions?

Leave a comment

Engine Rebuild: 5.0 GL Marine Gas - Hypereutectic Pistons

First the pistons were pressed off of the rods. Then they were sand blasted and ummmm yea that's all

Then we resized the rods to factory specifications.

Here they are lookin' all nice and pretty. 

I mean, look at it, it's shiny. That can only be a sign of good things to come.

These are the new pistons we selected for the engine. They're Speed Pro Hypereutectic pistons.

What the heck is a Hypereutectic piston???

Here's a little info for ya!

A hypereutectic piston is an internal combustion engine piston cast using a hypereutectic alloy–that is, a metallic alloy which has a composition beyond the eutectic point. Hypereutectic pistons are made of an aluminum alloy which has much more silicon present than is soluble in aluminum at the operating temperature. Hypereutectic aluminum has a lower coefficient of thermal expansion, which allows engine designers to specify much tighter tolerances.

The most common material used for automotive pistons is aluminum due to its light weight, low cost, and acceptable strength. Although other elements may be present in smaller amounts, the alloying element of concern in aluminum for pistons is silicon. The point at which silicon is fully and exactly soluble in aluminum at operating temperatures is around 12%. Either more or less silicon than this will result in two separate phases in the solidified crystal structure of the metal. This is very common. When significantly more silicon is added to the aluminum than 12%, the properties of the aluminum change in a way that is useful for the purposes of pistons for combustion engines. However, at a blend of 25% silicon there is a significant reduction of strength in the metal, so hypereutectic pistons commonly use a level of silicon between 16% and 19%. Special moulds, casting, and cooling techniques are required to obtain uniformly dispersed silicon particles throughout the piston material.

Hypereutectic pistons are stronger than more common cast aluminum pistons and used in many high performance applications. They are not as strong as forged pistons, but are much lower cost due to being cast.

Advantages

Most automotive engines use aluminium pistons that move in an iron cylinder. The average temperature of a piston crown in a gasoline engine during normal operation is typically about 300 °C (570 °F), and the coolant that runs through the engine block is usually regulated at approximately 90 °C (190 °F). Aluminium expands more than iron at this temperature range, so for the piston to fit the cylinder properly when at a normal operating temperature, the piston must have a loose fit when cold.

In the 1970s, increasing concern over exhaust pollution caused the U.S. government to form the Environmental Protection Agency (EPA), which began passing legislation that forced automobile manufacturers to introduce changes that made their engines run cleaner. By the late 1980s, automobile exhaust pollution had been noticeably improved, but more stringent regulations forced car manufacturers to adopt the use of electronically controlled fuel injection and hypereutectic pistons. Regarding pistons, it was discovered that when an engine was cold during start-up, a small amount of fuel became trapped between the piston rings. As the engine warmed up, the piston expanded and expelled this small amount of fuel which added to the amount of unburnt hydrocarbons in the exhaust.

By adding silicon to the piston's alloy, the piston expansion was dramatically reduced. This allowed engineers to specify a much tighter cold-fit between the piston and the cylinder liner. Silicon itself expands less than aluminium, but it also acts as an insulator to prevent the aluminium from absorbing as much of the operational heat as it otherwise would. Another benefit of adding silicon is that the piston becomes harder and is less susceptible to scuffing which can occur when a soft aluminium piston is cold-revved in a relatively dry cylinder on start-up or during abnormally high operating temperatures.

Hypereutectic piston definition via Wikipedia