Engine Briggs & Stratton engine design

[dlsevier]

Why doe Briggs use aluminum intake and steel exhaust push rods? Would it be better to replace the bent aluminum push rods with steel push rods?

 

[bertsmobile1]

In theory the aluminium should expand at the same rate as the engine so the valve lash does not change as the engine gets hot.
In reality I swap them all with steel ones when they start to wear then set the vales slightly tight.

If the engine was doing 10,000 rpm then the weight and inertia of the push rods need to be considered but at < 4000 rpm not worth worrying about.

 

[dlsevier]

In theory the aluminium should expand at the same rate as the engine so the valve lash does not change as the engine gets hot.
In reality I swap them all with steel ones when they start to wear then set the vales slightly tight.

If the engine was doing 10,000 rpm then the weight and inertia of the push rods need to be considered but at < 4000 rpm not worth worrying about.

Thanks, for sure the temperature expansion coefficient is different for steel and aluminum. The problem I see with the aluminum is the same as experienced with aluminum electrical wire, if get hot it deforms and there is no returning it to it's original shape. It is a real design dilemma. The best to hope for is replacing the aluminum push rods with steel and hoping the aluminum cylinders do not expand to where the push rods are overly loose, or annually replace the aluminum rods and adjust the valve clearance. Maybe Briggs will make a steel block diesel engine.

 

[bertsmobile1]

They really do not give that much trouble.
I have 40 motorcycles and all the OHV ones have aluminium pushrods.
When converting a Briggs engine into a high performance go cart engine or racing mower engine you use two aluminium pushrods to reduce the reciprocating weight.
If your pushrods ever get above 400oC then you got big problems.
The thing that needs looking at is the support sleeve.

 

[dlsevier]

Interesting, one of the aluminum push rods in my lawn mower was bent. I checked the valves and they are not stuck. The rod seat on the rocker arm had come loose. I could see where the push rod had rub against the cylinder wall. The other cylinder is still good, but valve clearance was about .007 inches. Perhaps allowing the engine longer warmup and cool down periods before mowing will help. Thanks for you input!

 

[upupandaway]

Maybe cuz exhaust is hotter so steel expands more to be equal to the aluminum while cooler expands the same???
just a guess.