12G295 - how much to skim?
Discussion
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
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
The first thing to do is measure what cc you have already, most of these heads will have already been skimmed. Measure it with a burette and a piece of plastic (shown in Vizards book) a cd case is perfect.
You then need to decide what compression ratio you are looking for based on how you will use the engine. Then you can calculate what the total volume you need for the required compression ratio. This chamber volume will be made up of the piston dish+deck volume+gasket volume+ head volume. Once you know what cc you need in the head you then use a burette to fill the current head chamber with the same amount of liquid you need. Assuming you need to skim the head the amount to be removed will be the depth from the face of the head to the surface of the liquid. This has to be measured very very carefully using calipers or something similar.
A quick calculation method is single cylinder volume / compression ratio - 1 = chamber volume
As you mention 12G295 this is probably a 998 engine? if so 10.0:1 is a nice ratio for a fast road type motor with a low/medium spec cam
Eg. (998/4) / (10.0-1) = 249.5/9 = 27.72cc chamber
If you have flat top pistons, there is about about 5cc for the deck and gasket this means you need 22.72 in the head. I recall as standard they are 28 so thats quite a skim? If you have dished pistons you will need even more skimming as these are 6cc also. The cooper that originally had this head had flat top pistons, so its not a great match to a standard 998.
The other option is to fit the 12G940 head from a 1275 and either sink the exhaust valves 40 thou with a mild cam or pocket the block with a wilder cam, but to do this properly you should dismantle the engine really. You can do it in the car but have to be super careful on cleaning up the swarf. The advantage of the 940 head is that it is 21.4cc in the head so the amount to remove to increase or maintain the compression ratio is much smaller.
Hope this helps. I know some of the other guys on here are much more knowledgable than I, if you spot anything wrong please feel free to correct it. You need to consider the engine as a package before you go ahead and machine the head, over skimming the 295 can result in breaking into oilways etc so be careful, measure three times before finalising your numbers.
The other alternative is to contact one of the engine builders such as MED, Bill Richards, VMAX etc etc.
You then need to decide what compression ratio you are looking for based on how you will use the engine. Then you can calculate what the total volume you need for the required compression ratio. This chamber volume will be made up of the piston dish+deck volume+gasket volume+ head volume. Once you know what cc you need in the head you then use a burette to fill the current head chamber with the same amount of liquid you need. Assuming you need to skim the head the amount to be removed will be the depth from the face of the head to the surface of the liquid. This has to be measured very very carefully using calipers or something similar.
A quick calculation method is single cylinder volume / compression ratio - 1 = chamber volume
As you mention 12G295 this is probably a 998 engine? if so 10.0:1 is a nice ratio for a fast road type motor with a low/medium spec cam
Eg. (998/4) / (10.0-1) = 249.5/9 = 27.72cc chamber
If you have flat top pistons, there is about about 5cc for the deck and gasket this means you need 22.72 in the head. I recall as standard they are 28 so thats quite a skim? If you have dished pistons you will need even more skimming as these are 6cc also. The cooper that originally had this head had flat top pistons, so its not a great match to a standard 998.
The other option is to fit the 12G940 head from a 1275 and either sink the exhaust valves 40 thou with a mild cam or pocket the block with a wilder cam, but to do this properly you should dismantle the engine really. You can do it in the car but have to be super careful on cleaning up the swarf. The advantage of the 940 head is that it is 21.4cc in the head so the amount to remove to increase or maintain the compression ratio is much smaller.
Hope this helps. I know some of the other guys on here are much more knowledgable than I, if you spot anything wrong please feel free to correct it. You need to consider the engine as a package before you go ahead and machine the head, over skimming the 295 can result in breaking into oilways etc so be careful, measure three times before finalising your numbers.
The other alternative is to contact one of the engine builders such as MED, Bill Richards, VMAX etc etc.
Edited by dark_helmet on Monday 17th December 20:15
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.
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.
At 29.6 the chamber may have been opened out as part of a previous porting? If you fill your 295 with the cc you want for your chosen CR and measure the distance to the head you might be there? Use a steel rule if you dont have calipers.
There are some other mini forums you could search too
www.theminiforum.co.uk
www.turbominis.co.uk (good technical but these chaps look for low CR no use for NA applications)
www.minifinity.com
The vizard book is worth the investment. ISBN1 85960 620 2 Tuning the A Series Engine. I got it on sale at Amazon for 9.99 last year. I just had a quick skim through and there are no specific thou or mm quoted as I guess it is specific to each head dependant on the chamber shape. Page 222 and 223 are what you need to begin with. I dont have a scanner to copy them sorry. One thing he does mention is the oil passage depth from the face of the head should not be less than 0.080 after skimming.
You could use fine graph paper to measure the chamber surface area (lots of counting
)and then work out the cc and determine the depth required for it. I have not done this but it seems sensible. Just need to be careful of the oilways etc.
Good luck!
There are some other mini forums you could search too
www.theminiforum.co.uk
www.turbominis.co.uk (good technical but these chaps look for low CR no use for NA applications)
www.minifinity.com
The vizard book is worth the investment. ISBN1 85960 620 2 Tuning the A Series Engine. I got it on sale at Amazon for 9.99 last year. I just had a quick skim through and there are no specific thou or mm quoted as I guess it is specific to each head dependant on the chamber shape. Page 222 and 223 are what you need to begin with. I dont have a scanner to copy them sorry. One thing he does mention is the oil passage depth from the face of the head should not be less than 0.080 after skimming.
You could use fine graph paper to measure the chamber surface area (lots of counting
)and then work out the cc and determine the depth required for it. I have not done this but it seems sensible. Just need to be careful of the oilways etc.Good luck!
Edited by dark_helmet on Tuesday 18th December 07:35
That sounds like a nice engine.
The A-series engine does like a high-ish comp ratio and about 10.3 or thereabouts with 97 octane petrol plus lead additive (if necessary) seems to give good results.
With an older head you never know what's been done in the past, so it's best to start from scratch.
The first thing you need to know is the volume in the cylinder bore at TDC. If you have flat-top pistons that's easy. In fact with a 295 it it best with flat-tops as the original Cooper 998 had raised piston crowns (the old 'D-Top' type). If there is a dish in the pistons, you need to either measure that or get the piston number and come on here for the actual figure as I have a chart for this. The measure how far down the bore the top of the piston sits at TDC and work out the residual volume. If the engine has been bored out this will effect the CR calculations. If you let me have the dimensions I will be happy to do the calculation for you to give the desired combustion chamber volume. Then it will be up to you to put that amount of liquid in each chamber to find out how much to skim off. With the meniscus caused by surface tension it's always a bit of a guess, but, in fact, a few thou error won't screw up the cr too much, so just get as close as you can with it. I've tried petrol paraffin, water (with and without soap) and it's still always a bit of 'estimation' that's required.
I hope this helps and feel free to email me if you want me to help in any way
Peter
The A-series engine does like a high-ish comp ratio and about 10.3 or thereabouts with 97 octane petrol plus lead additive (if necessary) seems to give good results.
With an older head you never know what's been done in the past, so it's best to start from scratch.
The first thing you need to know is the volume in the cylinder bore at TDC. If you have flat-top pistons that's easy. In fact with a 295 it it best with flat-tops as the original Cooper 998 had raised piston crowns (the old 'D-Top' type). If there is a dish in the pistons, you need to either measure that or get the piston number and come on here for the actual figure as I have a chart for this. The measure how far down the bore the top of the piston sits at TDC and work out the residual volume. If the engine has been bored out this will effect the CR calculations. If you let me have the dimensions I will be happy to do the calculation for you to give the desired combustion chamber volume. Then it will be up to you to put that amount of liquid in each chamber to find out how much to skim off. With the meniscus caused by surface tension it's always a bit of a guess, but, in fact, a few thou error won't screw up the cr too much, so just get as close as you can with it. I've tried petrol paraffin, water (with and without soap) and it's still always a bit of 'estimation' that's required.
I hope this helps and feel free to email me if you want me to help in any way
Peter
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