20 replies to this topic

[Minigman]

The 12G295 cylinder head has a standard valve chamber volume of 28.3cc.

Does anyone know what the volume would be after a 0.040 skim please?

Edited by Minigman, 07 November 2017 - 09:03 AM.

[bikerjohn]

Best thing to do Is to get a seringe and measure how much fluid you can inject into it with the head upside doen on a flat surface. use parafin as water won't give you an accurate reading due to surface tension..

[Minigman]

I dont own the head hence I cant do that unfortunately. I know someone who has one for sale. It has no valves either currently. Just trying to ascertain if it will go on my 1132 without further skimming etc.

Edited by Minigman, 07 November 2017 - 09:41 AM.

[Turbo Phil]

0.040" off would reduce the chamber volume roughly 3cc iirc.

Phil.

[Retroman]

A bit depends on the valves too as some are flat, some have a small dish, and also how far recessed the valves/seats are, I've just flooked a 12G295 head that needed a skim, going on a 1217 (1098 to 68mm bore) the compression works out a 10.2:1 perfect.

[Swift_General]

The change in chamber volume depends on the cross sectional area of the chamber and the amount of skim. The CSA of the 998 cylinder is 32.75cc. If you were to say that the chamber CSA is 75% of the cylinder (just my best guess mindful that the 12G295 is of open chamber design) then this would give a CSA of 24.57cc. You could of course be more accurate by overlaying some graph paper on the head and I'm sure someone has a nominal value out there. Assuming the above figure though then with a 40 thou skim this would reduce the chamber volume by 2.5cc.

Edited by Swift_General, 07 November 2017 - 02:35 PM.

[Minigman]

Good info thanks. Not an easy CSA to work out due to the shape. Far easier when its circular 👍

[Cooperman]

It is very unlikely a head which has been skimmed/modified can be simply fitted to an engine without accurate measurements, calculations and skimming.

 

If you don't do this it is a guess as to whether the CR is correct and suitable.

[Retroman]

Got to be said I can't get my head round what Swift_General has said, or is it just me?

I'm not having a go, I just don't understand it...

 

The change in chamber volume does depend on the cross sectional area and as said the skim,  but also how that CSA changes too, as it does when skimming. As the depth gets shallower the CSA gets smaller too so its the change in the area on a head chamber plus the skim. I can't see how this is worked out to 0.01cc using graph paper or am I missing something?

 

Cross sectional area has nothing to do with the volume of the chamber unless its a perfect cylinder or am I wrong? And a CSA should not be measured in CC, as CC is a cubic volume measurement not an area, surely an area is measured in cm squared not cubed? and unless its a perfect cylinder the CC cannot be calculated from an area, or am I wrong?

 

As the chamber in a head is an odd shape CSA has no place in calculating a 3D volume and is uneeded? or am I wrong? I fail to see what use graph paper is or am I missing something? Its a bit confusing.

 

A piece of perspex with a hole, a syringe and some ATF is all I have ever used, perfectly accurate and simple.

 

 

 

[Northernpower]

 

Got to be said I can't get my head round what Swift_General has said, or is it just me?

I'm not having a go, I just don't understand it...

 

The change in chamber volume does depend on the cross sectional area and as said the skim,  but also how that CSA changes too, as it does when skimming. As the depth gets shallower the CSA gets smaller too so its the change in the area on a head chamber plus the skim. I can't see how this is worked out to 0.01cc using graph paper or am I missing something?

 

Cross sectional area has nothing to do with the volume of the chamber unless its a perfect cylinder or am I wrong? And a CSA should not be measured in CC, as CC is a cubic volume measurement not an area, surely an area is measured in cm squared not cubed? and unless its a perfect cylinder the CC cannot be calculated from an area, or am I wrong?

 

As the chamber in a head is an odd shape CSA has no place in calculating a 3D volume and is uneeded? or am I wrong? I fail to see what use graph paper is or am I missing something? Its a bit confusing.

 

A piece of perspex with a hole, a syringe and some ATF is all I have ever used, perfectly accurate and simple.

 

 

 

 

I agree. The CSA changes as the head is skimmed due to the tapered nature of the chamber.

[StuartM]

I did this for my 998 a year or so back and just checked my notes...

 

The 295 head I started with had and average chamber volume of 27.70cc, measured using the 'bit of glass and a big syringe' method (repeated many times - it wasn't easy.)

I did the '5x5mm graph paper and count the squares' method to measure the face area to 2875mm2 to work out the skim depth I needed to get to the CR I wanted. (The chamber doesn't taper at the face where it's being skimmed so don't worry about this being inaccurate.)

I got 40 thou skimmed off (so taking off 2.92cc) to give a new average volume of 24.78cc.

 

My head had new unleaded exhaust valve seats put in before I did all this - I guess that explains the starting volume being different to the expected 28.3cc. I know you're only after a rough figure to decide if it'll work, but It's worth bearing in mind that these heads will likely have been skimmed or otherwise fiddled with before you get hold of them, so if/when you do get the head you obviously need to measure the actual volume before you start fiddling with it yourself.

 

Hope this helps!

[Cooperman]

First of all it is necessary to establish what you want to achieve. The answer will be a set compression ratio.

 

The first thing is to establish what you are starting with. Measure the volume of the combustion chambers, the dish (if any) in the top of the piston and the distance from the piston top to the block deck level at TDC.

 

From this you can establish the existing CR. Assuming this is too low for what is required, the final corrected volume of the combustion chambers can be calculated.

 

All that is then necessary is to put that exact amount of paraffin (or meths, or ATF, or whatever you decide to use) into a combustion chamber when the head is laid so that the face is upwards and absolutely level. Measure how far from the face the surface of the fluid is and that is the amount to be skimmed to get the CR correct.

 

It is not difficult and is necessary if the CR is to be as required. Guessing is not an option.

[StuartM]

...and, remember to include everything in the CR calc. Inculding Head gasket volume, ring land volume, block unswept volume (the unswept bit between the deck and the piston at tdc) all need to be in the unswept side of the calc. These can be either measured or easily googled.

 

I missed out the block unswept volume in my calc and ended with a CR of 9.5:1 rather than my target of 10:1! 

Still goes like stink compared with the standard head though.    

[Minigman]

Thanks for the replies. I think knowing that provided the the valve chambers havent been de-shrouded then a 40 thou skim reduces the cc by roughly 10% irrespective of the actual depth or valve recess shape below which obviously needs to be taken into account to calculate the total cc.

As this head will end up on a 1132cc with flat top pistons its good to know that I can get the CR i need despite the head already being skimmed. Probably wont have to deck the block either.

It will be measured accurately once I receive the head and I chuck some valves in it, then modded from there.

Thanks.

[Minigman]

Incidently, Im building the 1100 for my 60 mk1. Im retaining the std HS2, manifold and pea shooter single box exhaust.

I have an MG1100 2 ring cam on the shelf which Ive thought about installing but am tempted by a SW5 as Ive never used one before (Im a 266 fan!).

Obviously the carb and exhaust is going to restrict performance quite a bit, I know that. But its a very original early 60 so I have my reasons. The existing none original engine is a 69 factory replacement gold seal 848 which really struggles on the hills around here on Exmoor where I live. So the 1100 + 40 looky-likey is going in with a 3.44FD. It should prove a little more useful.

I wonder if its actually worth doing much work on the head as it will be effectively wasted due to my exhaust restriction.

Or will it?

Is it worth my while installing the SW5 and decent head?

Your opinions are welcome.

Edited by Minigman, 09 November 2017 - 07:54 PM.