20 replies to this topic

[Cooperman]

When fitting a 12G295 head, or any replacement head really, it is essential to measure everything, do the compression ratio calculations, then have the head deck machined to give the required compression ratio.

A 12G295 head was designed to work with the original Cooper raised D-top pistons which put the compression ratio up. With currently available pistons, dished or flat top, a fair amount needs to be skimmed to get the comp. ratio correct, but without measuring & calculating it is impossible to say how much.

This is not something which can be 'guessed, it has to be done properly.

[Bubblebobble]

When fitting a 12G295 head, or any replacement head really, it is essential to measure everything, do the compression ratio calculations, then have the head deck machined to give the required compression ratio.

A 12G295 head was designed to work with the original Cooper raised D-top pistons which put the compression ratio up. With currently available pistons, dished or flat top, a fair amount needs to be skimmed to get the comp. ratio correct, but without measuring & calculating it is impossible to say how much.

This is not something which can be 'guessed, it has to be done properly.

 

 

Sorry , i dont think i said im guessing anything....... . As stated earlier the head needs work .

[Captain Mainwaring]

 

 

 

 

Thanks 6joshh6 . I looked on minispares and they sell a cooper 998 rocker but the whole lot would probably be over £100. I think they were 1.3 ratio so not much in it .  A secondhand set it is then , and  some lightening work to them .

 
I wouldn't waste your time lightening them unless you've converted it to desmodromic valve gear.
 

I thought some benefit would come of lightening them , as mentioned in books .  And its free....
 

 
All things that move have a polar moment of inertia . now, this has both velocity and direction - it's a vector, so things like pistons undergo much more loading as a result of acceleration and deceleration thus the need to lighten them.
 
Nahhh take your rocker arm - actually it's balanced anyway, though there will be a little bit of friction on the rocker shaft - but what it is operating ( a vlave spring) takes up much more energy than moving the arm in the first place - though the spring unloads its energy back into the camshaft as the valve shuts.
 
in short a wasted exercise on a standard engine running at normal speeds.

I can only add that those who recommend lightening rockers should back it up with calcs

 

Surley if you lighten them its taking less energy to move them .. .... which is good ?

 

 

No, not really. - they are like a beam on a fulcrum, think about it..

I can guarantee that if you reduced the weight to near zero, that you couldn't measure any change in the energy require to run the valve train - take a look at the whole system - it's hardly designed for low friction and efficiency.

 

Lightening rockers might look great if you are trying to sell a book, but 100% whoever wrote such a thing has (a) never calculated out the inertia involved or (b) tested the valve train on an electric dyno to see what the absorbed power is.

[IainNeon91]

 

I'm running the 1275 pressed steel rockers on my 12g295 (the pressed steel rockers with the oval pads) these have a slightly higher lift than the 998 pressed steel rockers (round pads) and quite a bit more lift than the later A+ sintered rockers. I have the 12g295 on my 998 which is also running a 266 cam, I'm yet to have the engine rolling road'ed but it already seems a lot more pokey! Further down the line I'm think of getting the 1.3 roller rockers to give that final bit of lift, just to maximise the lift of my cam.

Did you have to skim your head down , as mentioned above ?  

 

Yes, I ended up skimming mine by 88thou to make a 10.1:1 CR 

[Bubblebobble]

Thanks .

[Vipernoir]

When fitting a 12G295 head, or any replacement head really, it is essential to measure everything, do the compression ratio calculations, then have the head deck machined to give the required compression ratio.

A 12G295 head was designed to work with the original Cooper raised D-top pistons which put the compression ratio up. With currently available pistons, dished or flat top, a fair amount needs to be skimmed to get the comp. ratio correct, but without measuring & calculating it is impossible to say how much.

This is not something which can be 'guessed, it has to be done properly.

 

I've been wondering about this, and I'm not convinced the 12G295 wasn't designed for something else, with the Cooper application secondary...

 

It's perfect on an 1100 as it is (think MG 1100) and was introduced for that purpose in October 1962.

No-one in their right mind would use raised D-crown pistons unless they were raising compression on an existing head, and sure enough, the 998 Cooper came out in April 1964 - 18 months after the head was first used on the MG 1100.