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Am I reading this right?

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Planning for a future head swap, so with a 1975 car, #'s matching block and an E12 head with a 74 casting date, I should have a 9.0:1 compression ratio..right?


Am I reading that right?

The motor pulls real strong through to 6k, and the valve train is clean, so I can assume there has been at least a head rebuild and maybe a cam upgrade? I haven't popped the valve cover off yet to look for a cam marking..

No smoke ever from the tailpipe..so maybe a total rebuild at some point, although there is a disheartening fluctuating light knock-ish sound at idle that comes and goes.. Don't know if its bad/normal/impending doom..?

But I have a 121 head coming with a port/polish, 3 angle, double springs, and a 304 cam...for my bike carb setup. Carbs are on the way... I just want to clarify my piston set up before I make any major moves .

This will be an interesting FAQ article.

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have an 8:1 compression ratio, not 9:1 unless of course the pistons hae been changed. CR is altered on an M10 by changing pistons, not by milling the head 'cause there ain't much to mill.

Be advised that if you use that 121ti head, you'll need matching pistons. Don't think the piano top E12/21 pistons will play nicely with a 121 head. And a 304 cam is pretty hot for the street--gonna affect your idle and low end torque.

See you at Salt Fork and/or V@V?


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