27 replies to this topic

[Dusky]

Hi there everyone!

 

I'm getting my hands on a 12G295 head wich comes of a original cooper with the D pistons.

The head is probably unskimmed so I'll have to skim it quite a lot I think ( I got flat top pistons in mine).

WIll measure it when it arrives.

 

Howmuch should I skim for what compression ratio? Don't really know how much I'm after with my camshaft ( AEG 643 = BL 544)

 

What will I need to do after skimming? I understand I'll proably need to adjust the rocker posts with shims equal to the amount that I skimmed it. Is that right?

I'll also be fitting 1.3 rockers, anything more toadjust with them?

 

I will run it with a lead additive as I don't think that a 66' cooper had unleaded valve inserts..

 

Cheers!

Edited by Dusky, 25 September 2014 - 08:18 AM.

[gazza82]

You need to work out your compression ratio first ... if you just skim off a random amount you might find it is them too high for the car to run correctly. By putting in flat top pistons you are already increasing the CR as you reduce the space.

 

I've been using this site to work out mine .. http://www.gtsparkpl...pRatioCalc.html

 

by the way if the head is standard the comb chamber should be 28.3cc

Edited by gazza82, 25 September 2014 - 09:04 AM.

[IainNeon91]

If it is standard thickness, then you have good ground to play about. 

 

I went for a CR of 10.1:1 because that worked well with the rest of my engine set up. My 295 was standard thickness and I ended up (after a lot of measuring and calculations) taking off 88thou (~2.25mm) to get my desired CR.

 

But with the head as standard thickness you do have the option of Turbo'ing... 

[Dusky]

But my compression is.lower than the cooper's without skim Turbo would be too much of a hassle I think especially here inbelgium where parts are hard to find. I will calculate current CC's onder the head arrives. And I'll let it be skimmed for a 10.2 compression orso. 88 thou, thats 0.088 inch,right? Is skimming an exact amount something any machine shop can do?

[BritishRacingGreen]

first decide what compression ratio you want, then work out what cc head chamber you need to achieve that with your pistons, bore, deck height etc. Then fill the chambers with that cc of fuel and use a depth gauge to measure from the surface of the head to the surface of the fluid. That will be the amount you need to skim it by.

[Mini Manannán]

Cooperman did a comprehensive guide: http://www.theminifo...o-calculations/

[Dusky]

Found this online, could it be correct? :

 

inch skim___std.__0.010__0.020__0.030__0.040__0.050__0.060
12A1456___24.5__23.93__23.36__22.79__22.21
12G202____26.1__25.46__24.82__24.19__23.55__22.91__22.27
12G295____28.3__27.56__26.82__26.07__25.33__24.59__23.85
12G940____21.4__20.66__19.92__19.17__18.43

 

If so I will need around a 58 thou skim, could that be right?

[Cooperman]

No, measure it correctly first. That is the only way to do the job right. Then do the calculations and then it will be what you need.

It's called 'engineering'.

[Dusky]

No, measure it correctly first. That is the only way to do the job right. Then do the calculations and then it will be what you need.

It's called 'engineering'.

I will measure But just wondering if these standard values are true so I can use them as a reference :)

cheers

[gazza82]

The two values for the standard comp chamber CC for the 12G295 and 12G940 agree with everything I've read or seen but I can't vouch for the other values.

 

What you need to do now is to measure up the rest of the dimensions (bore, stroke, piston dish, deck volume, gasket volume, etc), check your cc value is correct (ie the head hasn't already been skimmed) and then run the calculations to see what your existing CR is ... only then can you make any decisions on how much to skim to bring that to you desired value or whether you need to look at opening up the comp chamber or change pistons to a larger dish to reduce the CR.

Edited by gazza82, 26 September 2014 - 10:48 AM.

[dklawson]

I started to post yesterday and deleted instead.

 

I cannot stress enough that you need to follow Cooperman's advice and measure the head you are working on.  You should not trust online summaries and do not cc just one chamber, measure all four.  Once you have all four values you are likely to find that none of them are the same as each other and that all deviate from the "averaged" published values.  Carefully enlarge all the chambers to have the same volume as the largest chamber you measure (remove a bit of material and re-measure the volume of that chamber... over and over).  With that done you will have the actual chamber volume for you cylinder head and you will know how to proceed accurately.

 

Once the chambers are all the same size, take graph paper with a 1mm grid on it.  Place the paper over the chambers and CAREFULLY use a razor blade to cut the paper along the outline of the chamber entrance.  Once the paper cutout is complete, measure the number of squares, including partial squares.  Convert the number of square millimeters to square centimeters and then you can calculate how much to shave off the head to get the chamber volume you need for the compression ratio you want.

[Dusky]

I will measure it as accurately as possible. My biggest concern is that I do calculate the compression in the right way (thats why I was searching soms values, zo I have a ballpark )
And alzo the things I need to do after skimming , like adjusting/shimming the rocker pedestals etc(thats actually the part I know least about )

cheers!

[Dusky]

SO basically my biggest question is : what do I need to adjust after skimming X amount of a head

Also, does this kit prove any good? : http://www.minispare...c/C-AEG392.aspx

I have normal springs now  

[Cooperman]

There is a tolerance in rocker setting after skimming. Once you have decided how much is to be skimmed fit spacers under the rocker pillars of that thickness. Then it'll be right. Around +/- 0.015" is acceptable as a tolerance.

[carbon]

Dusky - for measurement of combustion chamber cc's and chamber balancing I have been using an alternative method which avoids the need for burette and liquids. It seems to be repeatable to better than 0.1cc, so at least as good as burette.

 

You should be able to get hold of all the kit to do this for less than 20 Euro. If you would like more details let me know.