Series III Cylinder Head Weight

Tall Ratbag said:

Thank you for providing an independent check of my calculation/numbers - that's very helpful!

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I just checked the figure for the unswept volume - I'm not brave enough to think about the shaving off of metal calculation - bit complicated as I don't know what the combustion chamber looks like

rob1miles said:

Yes, but I seem to recall the combustion chambers are a lot less cross section than the bore so the volume and CR will be correct but the amount to come off will be a lot less.

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I've roughly taken that into account - that's the 83% I was on about.

The way I did this was to calculate the amount which needed to be removed from the head assuming the combustion chamber took up the full bore area, and then factoring up to meet the 100 thou quoted in the expedition web page - following the same routine gives 75 thou for changing from 8:1 to 9:1.

>>the amount to come off will be a lot less.

It's actually the other way round. The best way to see this is to imagine a cylinder head with a flat surface - skimming it wouldn't change the CR.

As soon as I read this I knew you were right; I was erring on the side of caution, being uncertain I opted to advice a smaller cut. I'm nervous of posting on-line comment that leads someone else into a costly mistake. Of course the smaller cross section requires a larger reduction to get the same volume change. Can't see how I missed the 83%; senior moment. I think I'd opt for posting a cheque to Turners!

Before you send your head off for skimming, make sure it hasn't already been done in the past.

Col

Hi, I've managed to find a local machine shop that will skim the 7:1 series 3 head for £45+ vat plus extra cost if they need to take a LOT off they said! So that seems ok, but they again don't know the stats for this engine.

So does anyone know what I need to tell them to get this to 8:1 compression? They said they need to know how thick the metal left (which makes contact with the rocker and bottom engine) so they can skim down to that exact measurement...

Thanks if anyone can provide any measurements I can give them - is this covered in the land rover green bible book, I'll dig a copy out if it helps

Sounds overly complicated to me. I was once offered a job as a performance head polisher for minisport in Padiham and I asked them if they used a pipette to measure and balance combustion chamber sizes as each was ground out and polished. Oh no said the chief engineer. As long as they ‘look’ within 1 cc of each other it’s fine.

Check in the green book for the measurements - I've got a feeling there isn't a specified minimum overall thickness of the head but perhaps a depth of valve seat to head surface...

...best to check in the book (and rely on the well meaning interweb)