9 replies to this topic

[miniman24]

Hi all, just bought a very nice condition 12g295 head, its had approximately 16 thou skimmed from standard and I have worked out that there is 124 thou of metal between the head surface and the oil way - approximately how much will need to be skimmed to fit this to a standard 998 with dished pistons? I am only looking for an approximate value as I am yet to measure the dish or the compression ratio, both which will be done when I remove the standard head. My question is: can this head be safely skimmed to fit my engine, with 124 thou of metal left to play with?

[minimadles]

Well you need to leave at least 80thou on the deck if you want it to be reliable the thickness of an unskimed head is 2.750 inches but you need to do the calculations and measurements to find out how much needs to come off

[miniman24]

Is that 80 thou above the oil path? I know I need to do the calcs, which will be done when I get round to removing the head, I just need to know if anyone has put a 295 on an engine with dished pistons, ie if it is possible if you see what I mean? Not looking for an accurate figure of how much to remove :)

[minimadles]

Yes it will go on with dished pistons if you have aprox 124 thou from the bottom of the oil hole to the head face you have about 44 thou to play with. Measure the full thickness of the head and as long as the top face hasn't had a skim you can work out how much has been taken off the head. Use 2.750 as the measurement for a standard 295 head

[miniman24]

Ah right I see, thanks - I had come across the 2.75" figure for the thickness on several threads before I started this one and mine measures at 2.734", thats how I arrived at the conclusion that it has had a 16 thou skim at some point previously :) So you reckon around 80 thou needs to come off a 2.75" non skimmed head to run with dished pistons? Sounds about right, thanks for the help mate.

[minimadles]

Ok just been out to the workshop and measured up two standard 295 heads one has .275 thou and the other has .225 thou before the oil way breaks through that said each of them only has .200 thou in the deck so .120 thou is about as far as I could go on these heads hope this helps

[miniman24]

The heads must vary abit if mine only had 140 thou pre-skim before the oil way was broke! One last question, what would be a "safe" amount of metal to leave between the oil way and the surface? Would 1mm be too little?

[ACDodd]

I would avoid a big skim. Thin decks promote weeping go head gaskets. If in doubt run less compression and a thicker deck for longer term reliability.

Ac

[miniman24]

Thanks for the advice AC - what would you call too big a skim? Would 60 thou between the oil path and deck outer surface be too little?

[ACDodd]

The oil way is not the problem, it's the water jacket transfer ports on the front of the head. Mini 0.125". Remember it is always better to under compress that to have thin decks. Aim for the same compression you already have. The engine will run much better on 95 octane because of it.


Ac

Edited by ACDodd, 30 September 2013 - 07:38 AM.