57 replies to this topic

[twrminisport]

The OP states he has swapped for a 1275 TEMP.
He wants to do an engine rebuild on his 998.

Jesus

[Artful Dodger]

Looks fine. Get it skimmed to CR. ChEck first though.
Good clean .


Jesus

yes..?



haha i think you can measure how much the head has been skimmed by measuring through an oil hole on one of the rocker posts, and then measure the thickness of the 'deck'.

Edited by MiniclubmanGT, 16 October 2011 - 07:44 PM.

[Cooperman]

Looks fine. Get it skimmed to CR. ChEck first though.
Good clean .


Jesus

yes..?



haha i think you can measure how much the head has been skimmed by measuring through an oil hole on one of the rocker posts, and then measure the thickness of the 'deck'.

That's aqbsolutely correct. An un-machined head will have around 0.125" difference between the total head thickness and the depth of the oil hole, so a total of 0.080" can safely be removed leaving 0.045" before you break into the oil hole.

[coopdog]

so whats the difference between the 295 and a standard 998 head?

The 295 is what is known as an 'open chamber head'. This means that the edges of the combustion chambers do not shroud the valve edges as they do in a standard 998 head ('closed chamber head').
The 295 is a very well-regarded head for a 998 so long as the compression ratio is checked and the head skimmed as necessary.

could you mod a standard head to be the same??

[charliedurrant]

Jake,

The 1275 is temporary. I want to learn to do full engine rebuild and with a 998.

Charlie

[jaydee]

By looking closer at the last pic, the head seems untouched. I bet if you measure that head is 2,75"

[Cooperman]

so whats the difference between the 295 and a standard 998 head?

The 295 is what is known as an 'open chamber head'. This means that the edges of the combustion chambers do not shroud the valve edges as they do in a standard 998 head ('closed chamber head').
The 295 is a very well-regarded head for a 998 so long as the compression ratio is checked and the head skimmed as necessary.

could you mod a standard head to be the same??

No, the casting is very different. A standard head can be improved, but no matter what you do it still won't be as good as a 295, especially if the 295 has itself been gas-flowed.

[charliedurrant]

By looking closer at the last pic, the head seems untouched. I bet if you measure that head is 2,75"

For the 2.75'' where do measure from and to?

Charlie

[charliedurrant]

Oh and does anyone have any recommendations for anywhere that can gas flow a 12g295 head?

[twrminisport]

A standard head wil NEVER be as good as a 295 thats had the same money spent on it.
My 12g295 is fully gas flowed with big valves and ports, double valve springs etc

You just cant get the same figures out of a standard CAM4180 head due to the closed chambers.
Also the amount of work required to get the standard head even close makes it financially unviable compared to a 12g295.

In summary, start off with something that is right for the job then make it better. Do not use a casting which was never going to produce power.

Jesus

[087dave]

Sorry to hijack, but what about the heads that minisport sell for the 998 are these anything like the 295 head?

[lukepinfield]

I have this head however am planning to make big changes, I am taking the head to stage 3 and have a pair of twin HS2 carbs on a ported inlet manifold, I read it's the best head for a 998 due to the open design of the chambers much like the larger 1275 head and giving more flow. In my opinion as I said although this is a brilliant head for a 998 to get the most spend a bit of money to get the real benefits. On compression I vissited MED who advised me that a 9:1 ratio on a standard block with flat top pistons would be more than enough for a daily drive. Hope that helps

Edited by lukepinfield, 17 October 2011 - 05:00 PM.

[Cooperman]

Sorry to hijack, but what about the heads that minisport sell for the 998 are these anything like the 295 head?

No, see the two posts above. The standard casting can never be made as good as the 295 casting which is an 'open-chamber' design of combustion chamber.
A gas-flowed 295 with the slightly larger inlet valves it's capable of having is a head 'par-excellence' for a 998.

[charliedurrant]

By looking closer at the last pic, the head seems untouched. I bet if you measure that head is 2,75"

Gents, without seeming like an ar*se where do I measure from and to?

Charlie

[Cooperman]

Charlie, you need a digital vernier caliper.
First of all you measure the total thickness on the head to the nearest 2 or 3 thou.
Then you look at the top of the head and where one of the rockers sits you'll see a small-ish hole going down into the head. It's about 1/8" dia and is the hole where the oil feeds to the rocker shaft.
Using the depth measurement capability of the vernier caliper, you measure the depth of this hole. Expect it to be around 0.080" to 0.125" less than the thickness you just measured. That difference is the amount of metal between the face of the head which sits down onto the block and the bottom of the oil hole. This dimension after skimming to get the compression ratio (CR) correct for an engine should not go below 0.040", although some engine builders will go down to 0.030" as an absolute minimum.
After measuring the head in this way, you have to measure to obtain the existing comp ratio, then calculate the combustion chamber volume for your chosen CR., say around 10:1. Put that amount of fluid into the existing combustion chambers of the 295 and see how far that fluid level is below the head face. That's the ideal amount to be machined off, so long as that doesn't take you below the minimum material thickness between head face and oil hole bottom.
Actually it's not as difficult as it sounds and when you get a bit further, we can do the calculations for you if you post the measurements we will ask you to make.
I hope this all helps.