Graham Posted June 8, 2017 Report Share Posted June 8, 2017 Just looking at this http://www.chriswitor.com/cw_technical/head_applications_chart.pdf which of these heads would be suitable to go straight on a 5/6, which ones would need machining and which ones are a no no? Quote Link to post Share on other sites
YankeeTR5 Posted June 8, 2017 Report Share Posted June 8, 2017 (edited) The simple answer is to focus on head height. The factory PI cars had a head height of 3.4 inches resulting in a 9.5 to 1 compression ratio. The heads with the lower height (3.3) are going to give you too high of a compression ratio (something like 11.5 to 1 I think someone once told me??). The thicker heads can be simply milled down to the factory PI height of 3.4. Any of the heads will "bolt on" to a 2.5L motor, so its not a question of fit. The other "no go" for you is the early North American cylinder heads (the one listed on the bottom of the list for the 250). These early (thru 1970?) NA heads have a narrower spacing between the intake ports than all the other heads. So not compatible with a PI setup and the intake manifold used on these heads is also the least efficient. Still, they will bolt on to any of the 2.5L motors so can be used with the proper intake setup. Edited June 8, 2017 by YankeeTR5 Quote Link to post Share on other sites
john.r.davies Posted June 8, 2017 Report Share Posted June 8, 2017 Any head that was fitted with domed pistons, to a 2L, will fit straight onto a 2.5, that will have flat ones Triumph stopped machining the extra height away - to save money? - and the domes occupy the extra volume so CR stays the same. John Quote Link to post Share on other sites
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