Dee, I recently went through an engine rebuild with a '69 GT6+, and after
careful consideration decided against the domed pistons due to both the CR
problem, and the wealth of unknowns concerning the domed pistons (got flat
tops for $275.00 + s&h). The recommendations I received amounted to this;
the engine already has a 9.25:1 CR, and if that's not sufficient, there's
enough information available in the competition preparation manual to get a
flat top piston engine up over 12:1 if that's what you want, and none of it
involves domed pistons. You can already anticipate a jump in CR after you
have the block decked following cleaning and crack-checking, and the head
should be cleaned up too (machined to ensure flat mating surfaces - improved
sealing). Potential problems would include having to ensure there's no
chance of an interference between pistons and valves, and the need for
higher octane gas, expensive and not readily available to the general
motoring public. If your intention is to drive it on the street regularly,
or primarily, stay away from the dome.
BTW, the '69 GT6+ had flat pistons, I believe the domed ones were for the
non-exported variety, Nigel at SPIT Bits (800-201-0494) probably could tell
you, we had this discussion when I was planning the rebuild of mine.
Dave Korzun
'69 GT6+
-----Original Message-----
From: Dee Jackson <dee_jackson@gilbarco.com>
To: triumphs@autox.team.net <triumphs@autox.team.net>
Date: Friday, September 25, 1998 1:42 PM
Subject: Compression Ratio/Octane
>
>Fellow Triumphanatics,
>
>In rebuilding the lump on my GT6, I've just found out that I need new
>pistons. This motor is from a '69 + (engine KD55XXX). In it were +.020
>domed pistons. The head had been off the motor for 15 years prior to me
>getting it, so I've no idea how it ran. I understand that this motor
>likely originally had flat pistons (thanks, Barry). If I go back with
>+.030 domed pistons, am I going to be able to get by running on 93
>octane or is there going to be too much compression? To my knowledge,
>only .010 has been taken from the head and the block has not been
>decked. Thanks for any insight.
>
>D.
>Spit FM90204UO
>GT6KF20632
>
>
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