ausmini :: View topic - 1275 head onto a 998 block

OK, i know this has been brought up a few times before as i have done a search. What i didn't find was a couple of answers to a few questions...

At the moment i have a 12G202 (26.1cc??) head with 1275 valves, extensive port & combustion chamber mods with a final size of 24.3cc. This head needs replacing & i want to maintain the same sort of specs on the new head.

I was just going to use a 998 head but have been told the 12G940 in its standard form is a much better design out of the box.
My block has already been pocketed to an extent, to clear the larger valves and i will get the valves recessed into the new head to help.

Now, as the 940 head is 21.4cc i need to make up 3cc as i don't want any more compression than i already have. I don't think this will be a problem...

To the questions...
1) The bore spacings are different on 1275's, is this a major issue with 940 on a 998
2) Do i need to plug the front water gallery hole on block and re-drill to suit 1275 gasket??
3) Will my rocker assembly bolt straight in?
4) Will my LCB's bolt straight on?

I think that covers everything :wink:

Any help on this appreciated,

Cheers.

Author:	sox61 [ Fri Jul 06, 2007 11:19 pm ]
Post subject:	1275 head onto a 998 block
OK, i know this has been brought up a few times before as i have done a search. What i didn't find was a couple of answers to a few questions... At the moment i have a 12G202 (26.1cc??) head with 1275 valves, extensive port & combustion chamber mods with a final size of 24.3cc. This head needs replacing & i want to maintain the same sort of specs on the new head. I was just going to use a 998 head but have been told the 12G940 in its standard form is a much better design out of the box. My block has already been pocketed to an extent, to clear the larger valves and i will get the valves recessed into the new head to help. Now, as the 940 head is 21.4cc i need to make up 3cc as i don't want any more compression than i already have. I don't think this will be a problem... To the questions... 1) The bore spacings are different on 1275's, is this a major issue with 940 on a 998 2) Do i need to plug the front water gallery hole on block and re-drill to suit 1275 gasket?? 3) Will my rocker assembly bolt straight in? 4) Will my LCB's bolt straight on? I think that covers everything Any help on this appreciated, Cheers.

Author:	drmini in aust [ Fri Jul 06, 2007 11:39 pm ]
Post subject:	Re: 1275 head onto a 998 block
sox61 wrote: OK, i know this has been brought up a few times before as i have done a search. What i didn't find was a couple of answers to a few questions... At the moment i have a 12G202 (26.1cc??) head with 1275 valves, extensive port & combustion chamber mods with a final size of 24.3cc. This head needs replacing & i want to maintain the same sort of specs on the new head. I was just going to use a 998 head but have been told the 12G940 in its standard form is a much better design out of the box. My block has already been pocketed to an extent, to clear the larger valves and i will get the valves recessed into the new head to help. Now, as the 940 head is 21.4cc i need to make up 3cc as i don't want any more compression than i already have. I don't think this will be a problem... To the questions... 1) The bore spacings are different on 1275's, is this a major issue with 940 on a 998 2) Do i need to plug the front water gallery hole on block and re-drill to suit 1275 gasket?? 3) Will my rocker assembly bolt straight in? 4) Will my LCB's bolt straight on? I think that covers everything Any help on this appreciated, Cheers. 1. No. 2. I assume you mean the one near front of #4 cylinder, as detailed by Keith Calver. Good idea, but I and a few others have got away without any waterway mods. Watch the head doesn't hit the water pump at front is all. 3. Yes. Good idea to re-space the exhaust rockers a bit with a flat washer. 4. Yes. I've just finished doing a 1275 head for my 1132cc Moke, the head volume is now 21cc and it's coming down to 20cc. The C/R will finish up at around 10.3:1. This is fine on 98 octane, with an RE13 cam.

Author:	sox61 [ Fri Jul 06, 2007 11:52 pm ]
Post subject:
Beautiful, thanks doc. As far as spacing the exhaust, where does this washer go & how thick? What is that achieving... As for my motor, i'm using a RE13T and am hoping to keep the CR around 8.5:1 so need to find those extra 3cc in the head. Possibly by recessing the valves into the head?!? Not sure, i'm no mechanic, i'll leave the head work to a proffesional.

Author:	cush [ Sat Jul 07, 2007 12:34 am ]
Post subject:
don't forget the block pocketing if you plan any sort of high lift...

Author:	sox61 [ Sat Jul 07, 2007 12:52 am ]
Post subject:	Re: 1275 head onto a 998 block
sox61 wrote: My block has already been pocketed to an extent I know from personal experience what happens when u don't. If only i could find the picture... It's not pretty.

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Author:	drmini in aust [ Sat Jul 07, 2007 7:08 am ]
Post subject:
I agree, if it's going to get a rev, pocket the block. Bent valves and worse are not a good look. Unless ex valves sunk into the head I pocket the block 2.5mm deep, out to the gasket line of a 998 gasket. This keeps it well clear of the top ring. Easy to do on my mill. re the flat washer, on a 1275 the ex valves are spaced out from the inlets by 1mm or so compared to a smallbore. And if using stock 998/1098 rockers the rocker pads are narrower than 1275 ones. S forged rockers have a flat washer next to the exhaust one, for this reason.

Author:	pristic [ Sat Jul 07, 2007 4:19 pm ]
Post subject:
Hey Mate, Im keen to see how your 1275 compares to your worked smallbore head. So it will basically be the same motor but with 1275 (12g940) head on it right? Ive seens Kevs and it looks good... I know Kazjim went to a 1275 head on the 1100 turbo. Ben is running a 12g295 in his beast... hmmm, interesting Cheers, Peter.

Author:	drmini in aust [ Sat Jul 07, 2007 5:02 pm ]
Post subject:
GR says my slightly modded 12G1316 head will easily outflow a 295, but will move the torque peak up the rev range a bit. Basically, HP = torque x RPM... so looking good. Look out, Spiro da Moke...

Author:	Spaceboy [ Sat Jul 07, 2007 5:11 pm ]
Post subject:
why do you need to replace the head? that head sounds good

Author:	sox61 [ Sat Jul 07, 2007 7:56 pm ]
Post subject:
I found my 'trophy' of what can happen when a 998 block isn't pocketed, runs large valves & large cam lift.... (Have a broken rocker somewhere as well!) Also have a shot of how my block has been cut out. These are shown with the 998 gasket on. As u can see doc, they are not right out to the gasket line... Will check clearance when i get the head. Will still try to get exhaust valves recessed. spaceboy wrote: why do you need to replace the head? that head sounds good A lot of money has gone into my 202 but I'm having trouble with the head gasket sealing , this way i can use a metro turbo gasket and it also may flow slightly better as a bonus

Author:	drmini in aust [ Sat Jul 07, 2007 8:00 pm ]
Post subject:
kazjim had continual sealing problems with his 202 head on the 998 turbo at 12psi or more, that's why we built a 12G1316 head when he upsized it to 1152cc. The deck thickness of 1275 heads is way more than any smallbore one. Last I heard it was giving 115HP+ ATW at 15psi, somewhere in Qld....

Author:	sox61 [ Sat Jul 07, 2007 8:03 pm ]
Post subject:
drmini in aust wrote: GR says my slightly modded 12G1316 head will easily outflow a 295, but will move the torque peak up the rev range a bit. Basically, HP = torque x RPM... so looking good. Look out, Spiro da Moke... Would this be the same for me with the head swap??? pristic wrote: Hey Mate, Im keen to see how your 1275 compares to your worked smallbore head. So it will basically be the same motor but with 1275 (12g940) head on it right? Ive seens Kevs and it looks good... I know Kazjim went to a 1275 head on the 1100 turbo. Ben is running a 12g295 in his beast... hmmm, interesting Smile Cheers, Peter. Yep same motor, different head. 12G940 i think- depends what i can get. Hopefully will maintain the previous power output... We'll see.

Author:	Spaceboy [ Sat Jul 07, 2007 8:09 pm ]
Post subject:
ahh okay.. yeah go more power eats bigblocks for breakfast.

Author:	sox61 [ Sat Jul 07, 2007 8:27 pm ]
Post subject:
Thanks for the help! So a 1316 is better to get but no biggie if i use a 940, a bit more work for the guy porting it. Easy!! spaceboy wrote: ahh okay.. yeah go more power eats bigblocks for breakfast. I'm going for reliability, if extra hp is a byproduct, well... i won't say no

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