7 replies to this topic

[liirge]

Hey guys,
Well ive got a 12g295 head, thats going onto my 998, and i know im going to have to work out the compression ratio to see if my head will need to be skimmed, and i was wondering a few things:

1. how do i measure the gasket compressed thickness - anyone got a figure for a 998 standard gasket?

2. how do i measure the area in the cylinder head, ive seen glass being put over this area sealed with grease, and water poured into the area, and the amount of water that sits in there is measured and this is converted into cc..is this a good method?

3. after this all i need todo is get the bore and stroke figures from the haynes (its got flat top pistons) and ive got all the info i need?

4. from this how do i work out how much will be needed to be skimmed?


Any help would be much appreciated

Alastair

[Ethel]

Hey guys,
Well ive got a 12g295 head, thats going onto my 998, and i know im going to have to work out the compression ratio to see if my head will need to be skimmed, and i was wondering a few things:

1. how do i measure the gasket compressed thickness - anyone got a figure for a 998 standard gasket?

About 4cc's or trace the outline on squared paper, counts the squares and multiply by gasket thickness - measure the old one with verniers.

2. how do i measure the area in the cylinder head, ive seen glass being put over this area sealed with grease, and water poured into the area, and the amount of water that sits in there is measured and this is converted into cc..is this a good method?

That's the way, use a burette/pipette to fill it. A flat bit of glass or perspex eliminates the surface tension and having the head dead level issues - parraffin also has less surface tension ( or a drop of detergent).

3. after this all i need todo is get the bore and stroke figures from the haynes (its got flat top pistons) and ive got all the info i need?

add on about .75cc for the ring land.

4. from this how do i work out how much will be needed to be skimmed?

CR is swept volume + total chamber volume : total chamber volume


You could get a pretty decent guess by comparing with the spec of a standard engine

E spec A+ is 10:1CR with a 24.5cc in the head and spec the same as yours.


Any help would be much appreciated

Alastair

998cc /4 = 249.5cc's per cylinder

let x be the total combustion chamber volume
for 10:1CR, 249.5+ x = 10x
therefore, 249.5 = 9x so x = 27.72cc's

27.72 - 24.5 = 3.22cc's in the gasket etc.

[liirge]

Thankyou very much for that ethel! brilliant as always.

[Ethel]

If it comes to getting it skimmed, specify the chamber volume you want rather than the amount to mill off. Then it'll be their fault if it doesn't work out

[liirge]

Hee hee, nice tip ethel!
thankyou again!

[liirge]

When i measure the Cylinder Head Part, do i put in a spark plug?

[icklemini]

yes

[liirge]

goodo thought as much!