12G295 - how much to skim?

[UKAuto]

I am working on setting up a 12G295 head for my son's Mini 1000. I have done some volume measurements, and I am trying to calculare the surface area of the combustion chamber to calculate how many thous to skim, but I am not completely confident in the numbers...

Who has been down this road, and (a) what was the surface area of the combustion chamber, and (b) how many thous did you need to skim?


Cheers,

Rob

[UKAuto]

Thanks for the detailed reply.

Yes the car is a 998cc. The car has a stage one kit with a Kent fast road cam (invoiced as an MM785, but can't find reference to that part number).

I measured the cylinder head that was on the engine three times (26.5cc, 26.2cc, 26.3cc; I took 26.3 as more or less average). Then I did the same for the 12g295 (29.6, 29.6, 29.6; clearly the average was 29.6)

My technique was to use a syringe, probably not as accurate as the burette, but same tool used for both heads. I rubbed my finger in washing up soap and rubbed it on the surface of the combustion chamber (to break surface tension on the water making it flatter for a good read). Maybe I would be better off with an alternative liquid?

My next step was trying to figure out the surface area so that I can calculate how many thousandths of an inch need to be shaved to get the same volume (and therefore the same compression ratio - as I see no reason to change it).

so I have a little different approach, but similar. I guess the approach you recommend is use the same volume of fluid, and take a dial gauge to see how far it sits in from the deck height - I think that's a far better approach then mine. I was trying to turn the irregular shape of the combustion chamber in to a series of other shapes with easily calculated surface areas....

Now, when I reference the expected cc for each of the heads; the one that was on the car that I measured as 26.3cc is listed as 25.5cc. And the 12g295 that I measured as 29.6 is listed as 28.3.
(I guess I need to re-measure). Since my readings are higher than the expected it can't be explained away as the heads already being shaved.

I hope to find the actual shave amount a couple of other people have had to do, (or possibly Vizrd's book calls out the typical shave amount - I don't however have access to that book), this way when I do come up with a number if it's way off I know to try again.

Thanks again,

Rob.

[UKAuto]

Thanks again - will try to figure it all out over the holidays... either success, or we will do a valve job on the original head and but it back on. It all started over a blown head gasket.

Cheers

Rob