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Saturday, 8 December 2012

The Hillman Imp low compression piston

I have many early pistons from various Imp engines I have stripped but one is different from the rest.


The one on the left has a shorter skirt and the distance from the top deck to the gudgeon pin is shorter. Instead of 18.80mm it is 18.26mm,so 0.54mm shorter, so a lower deck height in the bore and lower compression.



Both pistons are standard bore and both carry the same number inside, 7101025 this tells me they just skimmed the required thickness of the standard piston to get a low compression piston?


This is further prooven when the top lip of each piston is measured, the standard piston is thicker there, 0.153mm to 0.126mm on the lower piston.



This is the lower compression piston, look closely and see that there are half moon marks in the crown of the piston, it was fitted and run the wrong way around!

Roy

1 comment:

  1. that's very interesting Roy. I'm guessing if it had been the other piston things would have went pear-shaped pretty soon

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