I am porting a 12g2940 11 stud head from a 1300GT.
I have almost finished the ports and, finally, I am going to do some work on the combustion chambers by de-shrouding the valves and seeking to balance the chamber volumes. I am generally following the Des Hammill guide.
How much can be safely taken off one of these heads? The reason I ask is that I don't want to end up having taken too much out of the chambers so that I cannot then safely plane the head to the desired CR. I don't think this will happen as I am being fairly judicious with the grinder but I would hate the cock up what has turned out quite nicely so far. I'm looking for a static CR of around 10.5.
What is the standard thickness for this head as a matter of interest?
Cheers as always
Jon
Head skimming - how much off?
Re: Head skimming - how much off?
Buy yourself a burette and measure the chambers, theres absolutely no point in guessing.
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Jono
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Re: Head skimming - how much off?
There's no question of me guessing anything
I suppose what I am asking is how close is the water jacket to the face of the head? To know what that means in practice I also need to know the original head thickness as doubtless mine has been skimmed before;)
Jon
I suppose what I am asking is how close is the water jacket to the face of the head? To know what that means in practice I also need to know the original head thickness as doubtless mine has been skimmed before;)
Jon
Re: Head skimming - how much off?
The earlier flat-top 12G940 castings were 2.750" thick (nominally).
I have some 9-stud heads which measure 2.748" (new/unused casting), 2.755" & 2.758"
and two 1805 11-stud heads at 2.742", although I can't be sure that these last two haven't been skimmed at some point
I have some 9-stud heads which measure 2.748" (new/unused casting), 2.755" & 2.758"
and two 1805 11-stud heads at 2.742", although I can't be sure that these last two haven't been skimmed at some point
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carbon
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- Location: Scotland
Re: Head skimming - how much off?
Jon,
The 12G1805 940 heads can be skimmed to give a chamber size of 15 - 16 cc. You shouldn't need to go anthing near this small. What I have seen with heads skimmed this much is the brass plugs seem to have more of a tendency to move and recess into the head, but they can be taken out and slightly oversize plugs made and refitted.
Before skimming the head to get the CR up, would suggest doing dry build and carefully checking the deck height of all the pistons. If you're using 1300GT type pistons these can end up a long way down the bore - up to 35 or 40 thou. Would suggest if this is the case you're better to skim the block first and get a piston to deck clearance of about 7 thou, but make sure it is exactly same for all four pistons, otherwise all your matching of combustion chamber volume will be fairly pointless (one thou piston height difference = about 0.1cc).
Hope this helps.
PS. If anyone else on forum has advice on best piston-deck clearance for fast road 1275 please let me know, information on this is not easy to find - both Hammill and Vizard unusually quiet on this point.
The 12G1805 940 heads can be skimmed to give a chamber size of 15 - 16 cc. You shouldn't need to go anthing near this small. What I have seen with heads skimmed this much is the brass plugs seem to have more of a tendency to move and recess into the head, but they can be taken out and slightly oversize plugs made and refitted.
Before skimming the head to get the CR up, would suggest doing dry build and carefully checking the deck height of all the pistons. If you're using 1300GT type pistons these can end up a long way down the bore - up to 35 or 40 thou. Would suggest if this is the case you're better to skim the block first and get a piston to deck clearance of about 7 thou, but make sure it is exactly same for all four pistons, otherwise all your matching of combustion chamber volume will be fairly pointless (one thou piston height difference = about 0.1cc).
Hope this helps.
PS. If anyone else on forum has advice on best piston-deck clearance for fast road 1275 please let me know, information on this is not easy to find - both Hammill and Vizard unusually quiet on this point.