> For 9:1 compression on a 1500 motor, you want to replce the US spec
> dished pistons with the UK spec flat-topped ones. You may still need to
> mill the head a bit, but this will get you close to the stock UK 9:1
> spec.
Roger,
I used +0.020 flat pitons and milled the head on my 79 Spitfire to get 9.5:1
CR. Originally, I had asked the machine shop to mill the head for 9.5:1 CR
but when I got it back, all they had done was take off 0.012" to flatten the
head. When I called them on it, they asked if I knew the answer so I went
off reading everything I could on the subject. The most useful was an
article by John Davies called "Skimming Your Head For A Higher Compression
Ratio".
http://www.totallytriumph.net/spitfire/skimming_your_head.shtml
Toward the end days of Triumph, there were a lot of different head and
piston combinations and you never know if a PO may have swapped heads so it
is best to measure the amount empirically. I determined that I needed an
additional 0.050-0.060" skimmed. When I got the head back, I believe I
measured about 9.3:1 but based on how the engine runs (or doesn't run) on 87
octane and how well it runs on 93 octane, I'm pretty sure it is closer to
9.5:1. If you want to see my struggle with this, check out my website
(starting with 23 January):
http://webpages.charter.net/eighmy/spitfire/
Gene Eighmy
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