20 replies to this topic

[jinjrich]

Hi all,
Been reading up on 12G295 heads as i'm lucky enogh to have aquired one in very good condition.

I'd like to give it a go on my 998 engine, but i couldnt find specific answers to my questions... can any one help?

1. Can i use a 998 head gasket? or do i need a 1275 one?
2. Will my standard carb be ok? I understand it may need setting up again but will it get enough fuel/air through the bigger valves? i've never been sure what model carb i have, but would hazard a guess its a standard 998 one (Hif 38???)

Many thanks!

[booton]

you need a 1275 head gasket as the 998 one won't seal due to the size of the larger head.

theres a huge range of different ones u can use.

i just fitted a 940 to mine. with a copper 1275 headgasket.


is u have a standard carb prob a 1"1/4 SU if it have a bowl on the side of it.

personally i think you should change to some thing larger like a set of twins 1" 1/4, or hif 38/ 44 depedinig on your other mods.

did u have the head skimmed for your 295 to get the right compression ratio

[jinjrich]

you need a 1275 head gasket as the 998 one won't seal due to the size of the larger head.

theres a huge range of different ones u can use.

i just fitted a 940 to mine. with a copper 1275 headgasket.


is u have a standard carb prob a 1"1/4 SU if it have a bowl on the side of it.

personally i think you should change to some thing larger like a set of twins 1" 1/4, or hif 38/ 44 depedinig on your other mods.

did u have the head skimmed for your 295 to get the right compression ratio

Yeah the carb does have a float bowl. I'll go on the hunt for a bigger carb but do you think it would run ok in the meantime? The head has come straight off an 1098 engine that was running fine, so i would assume it was already skimmed when fitted on that.

[booton]

um not sure some one else will have to conferm that.

but yeh should ruin on it fine but for performance would be worth a change.

[samsfern]

you need to work out your compression ratio, and get the head skimmed accordingly. It wont perform very well if the compression ratios too low.

[jinjrich]

you need to work out your compression ratio, and get the head skimmed accordingly. It wont perform very well if the compression ratios too low.

Sounds like maths is involved.....

[samsfern]

yup, and some pretty precise measurements, easily done though, just research on google, look up "how to calculate compression ratio.

[huw_jenks]

Wait a minute... I thought a 12g295 head was meant for a small bore, so will need a 998 head gasket? If you put a 12g940 on a small bore then you would need a 1275 head gasket.

I'm not 100% sure I'm right, but I'm fairly sure. Hopefully now that this goes back to the top, someone better informed can confirm or deny!

[thebullet]

I put a 998cc gasket on mine and it sealed fine!

[jinjrich]

I put a 998cc gasket on mine and it sealed fine!

This is what i thought, i thought it was the 12g940 was the 1275 one? So does it still need skimming?

[Dan]

Yes this head is a small bore head, and was fitted to the 1098 you pulled it from by the factory so it has chambers the right size for that engine. You will need to calculate the chamber size you need and then how much to skim. Use a small bore head gasket.

[huw_jenks]

Yes this head is a small bore head, and was fitted to the 1098 you pulled it from by the factory so it has chambers the right size for that engine. You will need to calculate the chamber size you need and then how much to skim. Use a small bore head gasket.

I got something right!

[jinjrich]

Yes this head is a small bore head, and was fitted to the 1098 you pulled it from by the factory so it has chambers the right size for that engine. You will need to calculate the chamber size you need and then how much to skim. Use a small bore head gasket.

Great. Its all starting to make sense now! Having trouble finding a guide on how to calculate chamber sizes & compression ratio. Can anyone point me in the right direction?

[ANON]

there are loads of cr calcs on line, think mike had a good one on the ferriday site?
as for chamber volume you've got to measure it yourself.

Edited by ANON, 04 March 2011 - 10:59 PM.

[Turbo Phil]

There's plenty of online calculators, but I wrote a bit of a guide HERE It's aimed at Turbos but it will give you the basics.
As already said the 295 is a small bore head, so you'll need the 998 gasket, NOT a 1275 one.