Another piston option for the 273.

Than

273 said

Have you checked out onewildcrazyguy@ how to port 318 (273) heads he got around 205 cfm with 1.75 valves. Which should get you at least 1.7 hp per cfm with street cam and cr (220/9:1) and around 1.15 lbs-ft per cid. The smaller bore might lower the cfm somewhat but 1.7 is conservative.

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thanks for that address.
I just took a photo and width of the two valves. I know the bores to head won't all perfectly line up but assuming the valves and bores are all central, the best will be an average clearance to bore.
The photo where the readout is white should have been the standard bore of 3.625.
I will work out the existing clearance then using the over bore and the larger valves of 1.88 and 1.6 this will give me an idea I'm open to overbore 60th if that gives me clearance. Then I will truely have a 5litre or 305ci.