Pistons tappered

4sd911 · 07-29-2009, 05:02 PM

I see from my measurements that the width of the piston at the wrist pin hole and then the top of the piston there is a difference of about .012", does this sound normal? At the wrist pin hole I measure 3.542", this is for a 2.7 piston, the size stamped on top is 89.97mm, that converts to 3.5421, the diameter of the cylinder measures 3.5435 which is spec. These have a little more than 100,000 miles on them.

cgarr · cgarr's Garage

Always smaller where the rings are, so that part is normal

fredmeister · fredmeister's Garage

MAHLE PISTONS ARE ACTUALLY ovalized by design. They are not round intentionally. Reason being is they expand due to thermal and pressure loads so they should go round under operating conditions. the piston profile is a proprietary design feature. Thats what makes Mahle so good.
Good catch though. Just make sure they meet specs when measured at the points outlined in the spec manual, that should be enuff to determine if they are useable.

4sd911 · 07-29-2009, 06:55 PM

Thanks, that's what I thought, good to know.

Henry Schmidt · Henry Schmidt's Garage

All high quality pistons are both cam ground (wider at the front and back, narrow at the pin) and barrel ground (smaller below the ring land, larger at the middle of the skirt and narrow again at the base of the skirt). All of this is designed to reduce piston slap (keep the piston from rocking) and friction (reduced contact area).
Back in the early GP days, Honda developed an oval piston and cylinder combination to exploit these very principles.

The first JE pistons were not barrel grounds and the result was a huge clearance issue. It wasn't until they purchased the proper tooling that their product became usable in air cooled engines like Porsches. A lot of the stories you hear about JE are left over from those days some 15 years ago.

fredmeister · fredmeister's Garage

Hondas oval piston was mainly designed to take advantage of the increase in available valve area with the oval shape of the cylinder head. the biggest problem was getting the rings to seal properly and survive with the oval shape to them. Gotta give them credit for such an innovative approach.