[Dimis]

I work at a Uni & today I bumped into a mate of mine at lunch who is a mechanical engineer academic (Masters degree) who happens to also work on the drive-lines for Caterpillar (he is just as hands on as theoretical & one of few academic engineers who actually knows how to twirl a spanner), so I asked him the question (Duh... I really should have thought about this earlier).

Below is a summary of his theories:

The I beam is stronger in compression and can therefore transfer a greater load to the crankshaft during the power stroke & the H beam has slightly less strength on the power stroke but has more strength in tension, provided that all else is equal.

He also concluded that it's all about piston speeds.
If you're building a low piston speeds ie: low RPM or short stroke motor that makes lots of power or torque use I beam.
If the motor is going to experience high piston velocities - ie: high rpm or long stroke, then use H beam.

Practical summary -
A NASCAR for example would most likely experience faster piston velocities than an F1 engine with high revs but very short stoke.
Thus NASCAR most likely got for H-Beam & F1 most likely for I-beam theoretically (if they didn't custom design).

The H beam will fail first if the rod is overpowered on the power stroke. The I beam will fail first if the weight of the piston & pin assembly is too great for the rpm. This assumes that all else is equal.

He was also adamant that the quality of the material & composition is more important than weather the design is an I or H beam & that either would work equally well in most cases. Thus in 100% agreement with Barry R.


I asked this question as I'm trying to research the parts I'll use for my engine build.
My conclusion - I or H beam is not relevant as brand quality in this instance and for my application.

Many thanks to all for your responses thus far.