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Re: 9:1 pistons

Subject: Re: 9:1 pistons
From: (David Councill)
Date: Wed, 17 Jan 1996 08:45:45 -0700
Roland Bolian wrote:

>Hello Everyone,
>        I plan on having the engine and transmission rebuilt on my '76
>Midget. I would like to go with a higher displacement (i.e. 9:1 pistons).
>What I need to know is what is involved, if anything, in going to the
>higher displacement pistons. Is the higher displacment ratio caused by the
>shape or by the size of the piston head itself. Also, are thier any other
>special considerations I should take into account? I'd like to keep
>everything as original as possible.
>I have a Zenith-Stomberg carb- will this do, or will I have to go with a
>different carb assembly? Will I have to have the lift of the valves
>increased? I'm fairly new to all of this- but I LOVE MY CAR!! HELP!!
>Roland Bolian-

I went to a 9:1 piston on my last overhaul to my 71BGT (its been ten years
now!). The piston head is flatter (the 8:1 has a depression in the crown).
My main concern was the need for a higher octane fuel as the standard
pistons can handle the lower octane fuels sold in gas stations. However,
this has not been  a problem.

If you went to higher compressions (10:1 or more), the piston crown would
become domed and then valve clearance may become a factor. The 9:1 was
standard in the sixties.

As far as Zeniths go - I know nothing. I am a SU user (dual variety).

David Councill

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