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Cylinder Head machining to get 9.5:1


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Has anyone machined a 2.5 TC head (529879 / 219016 / 313248) from 3.475" thick down to 3.300" thick and if so has there/was there any issues? Having a look at the casting I think I can see where it was just increased in thickness on the bottom (block face) from 0.197" (5mm) to 0.355" (9mm) thick. This junction is in the region of where a 3.300" thickness would be. So it looks like Triumph did not change the internal casting former occupied by the water in the head. Any confirmation or not would be helpful. If there was an issue I might be able to use dome top pistons.

ta Kevin

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I was told they suffered from lots of core shift so you may well end up with a scrap head.

Might be better to look for a flat top head to start with.

There was a very long no a detailed thread on the old forum about someone's issues with a skimmed head that broke into the waterways. Can't remember who it was sadly.

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  • 1 year later...


Did you pick a skim of 175 thou out of the air, or did you calculate it?     It sounds to me like a lot more than is needed to get 9.5:1.

I would humbly refer you to my article, "How To Raise The Compression Ratio, Safely And Effectively" at https://sideways-technologies.co.uk/forums/index.php?/topic/7551-how-to-raise-the-compression-ratio-safely-and-effectively/.

If you have measured the chamber volumes, and done the sums, then fine!  Go ahead!


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Presumably this is to use on a 2L engine with flat-top pistons?  Otherwise you don't need to take anything like that much off.........

I can tell you for sure that it can be done.  I have a 219016 head on my Vitesse engine which is 2L with flat-top pistons.  That has a CR of 10.25:1, though that is partly achieved with a 0.005" piston pop-up.  I'm not sure what the final thickness is, but it's less than 3.30" for sure.

However - it was HUGE skim (two in fact).  It's razor thin under the plugs and I was lucky not to strike water in the squish areas of the combustion chambers, which seems to be the usual problem area.  Blind luck really as I didn't really know what I was doing at the time. It's lasted well - done in 2008-ish and probably 45k miles since.  It's been off and refitted once (bottom end swap) - I was concerned about bits breaking off under the plugs, but it survived.

As mentioned above, it was Bruce who had problems with the squish/fire-ring area collapsing.  Not on one head, but two.  The story of the second is here.


Eventually resolved by starting with a factory 3.30" head.  Which is to be much preferred, if you can find a decent one.  Not so easy these days.


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