Hepolite Powermax Pistons

[TeeJay]

Can anyone improve on my mathematics or explain the variation of diameters?

Hi,
I am in the process of rebuilding my 1968 +2. Having completed the chassis and its sub-assemblies, I have moved onto the engine.

The workshop manual states that the Piston clearance (Type A) in cylinder bore is 0.0027/0.0033"

My maths calculates that 3.2500? + 0.0400? = 3.2900?

3.2900 - 0.0033 = 3.2867?

Reading the forum:-
rgh0 (Rohan) advises that ---- ?measured at gudgeon pin height at right angles to the gudgeon pin which is the normal point for specifying piston diameter for bore clearance.?

This measures 3.2865? which would give the correct clearance.
But measured at and just below the gudgeon pin, it is 3.2700? i.e. very oval by 0.0165?

With reference to the other 3 piston diameters the LARGEST is 3.2723? which is 0.0144? less than 3.2867??? and is not oval.

Can anyone shed any light on these measurements?
Are the measurements correct for Hepolite powermax pistons?
Are they OK to fit? (Subject to checks on piston rings etc)

Any advise very welcome.

[CBUEB1771]

Your pistons appear to be just fine. Pistons are round in the top and oval in the skirt area. The diameter at the top is smaller than the maximum dimension in the skirt which is found at 90 degrees from the gudgeon pin axis. The rings keep the piston centralized in the bore and the skirt keeps the rings square in the bore. I am well aware that my description is very elementary and that modern pistons have much more elaborate skirt profiles due to the possibilities offered by CNC turning. However the basic principles still apply.

[Rob_LaMoreaux]

Your forgetting to take into account the expansion of the top of the pistons when the engine is running and it gets hot. The skirt doesn't expand much since it is cooled by the crankcase. The top of the piston is made smaller diameter than the skirt so that they are the same once the piston is up to temp.

The skirt is slightly oval being narrower than the skirt that needs to ride against the cylinder. I'd have to ask my father-in-law for why, since he is the Internal combustion engine expert not me. I suspect it is because all it does is hold the piston pin in.

Rob

[TeeJay]

Hi Russ & Rob,

Thank you both for your comments.
I can now with confidence carry on with the engine measurements / checks and move forward with the project.

Although I understood that the coefficient of expansion is greater for the aluminium pistons than the steel block, I had some concern with the ovality and dimensions.

Thanks for your assistance.