202 skimming

[tedmcedd]

I want to put a 202 onto a 998....

Its a standard head, and Ive measured the chambers and they are the standard size of 26.1, I'm guessing I need to get it as close to 25.5 to make it suitable for the 998....?

How much do I need to take off in the skim?

(I know there is some maths way of finding out!! But I don't know it!!)

Thanks!

Ed

[abri]

Once you are sure of the unswept volume you need, subtract the ring land volume, piston dish volume, the piston crown to deck volume, and compressed gasket volume and you end up with the chamber volume you want. (If this is what you did to arrive at 25.5, then fine) Then, using a burette or other accurate instrument, measure out 25.5ml of liquid (I've been advised to use water mixed with food colourant so you can see it clearly) into the chamber (with the plug fitted). Then carefully measure the distance from the deck of the head to the fluid level. That's what you need to skim off to get it to 25.5cc.

Hope that helps.

Knowledgeable people, correct me if I'm wrong.

[Tim]

I've done it a different way. Get some graph paper marked in millimetres, and do a 'rubbing' of the head face (a greasy finger is good enough). Draw a solid line around the outline of the combustion chamber. Next count up all the little squares to work out the cross sectional area and convert to square centimetres.
The difference between 26.1 and 25.5 is 0.6 cm3, so divide 0.6 by the cross sectional area to get the amount in cm to shave off, convert to units your machinist likes and have the head shaved.

The answer will be two tenths of bugger all. Just get the head planed flat and use it as it is, the compression ratio will be barely any difference.

Tim

[Vegard]

If I remember correctly, on an open chamber head 0.010" is approx 1cc. So, if you want to remove 4cc, remove 1mm in new money.

But, you might end up with this:



Which is really a quite easy fix, so don't worry.

[mk1]

ST used to quote 12 thou per cc. On an open chamber head.

M

[Tim]

So about 6 thou. Not really worth worrying about is it?

Tim

[Frogeye61]

From my extensive study of averaging cylinder head dimensions, I estimate 15.7 thousandths per cc.
My data says standard 202 chamber volume 26.1 cm3, and 202 chamber area 25.1 cm2.
The original head if it was a 12A1456 or the like had a chamber volume of 24.5 cm3.

Please correct me if I'm wrong

[Vegard]

Sounds fair.
Although, I'd skim it a bit more. There's no harm in increasing the comp. ratio a tiny bit above 8.0:1

[tedmcedd]

Thanks....

Errm....

I'm useless with all this maths stuff!!

My sizes were from the yellow bible, then I filled the head I had to check if it hadnt already been skimmed, and it hadn't.... I did do the trick of filling it to the '998' size for the higher compression and measured it, I came out with about 1mm...! Interesting!!

I know that high compression is favoured in the small bore engines, or is it over all A series!?

(I'm not really and engine builder type...! Can you tell!!??? )

Ed


EDIT: So If I went for 30 thou..... I might not end up with Vegards style of head!? (or is it fixed with the head gasket!? haha!)

[Spider]

(or is it fixed with the head gasket!? haha!)

No. A head Gasket on it's own won't do it.

You'll also need polish and a buffing pad, but in a pinch, chewing gum will get you home.

[69k1100]

Or take advantage of the lower comp and stick a turbo on it