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| Skimming the head | Rate Topic |
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| Posted: 05-18-2007 06:19 pm |
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1st Post |
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Jon Plowe Member
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I have had reason to take the head off my engine and although I have had no problems i that area I was pondering whether it would be worthwhile to have the head skimmed before refitting. I have two reasons, both of which may or may not be a good idea and happy to be told so. Firstly to head of any problems of a warped head that may be creeping in, and secondly as it would in theory increase the compression would is it a bonafide way of extracting (a little bit) more performance? Questions - how much can you take off the head before the valves and pistons start colliding? I would think this is in ratio to the amount of standout of the piston above the liners but I cannot find any figures to work from. I have looked all over the head and I cannot see any marks that would indicate it has been skimed before - how could I tell, are there any measurments that can be taken? Any advice gladly received. I really appreciate your site for the specialist knowledge available as over hear in the UK we seem to concetrate on polishing Thanks Jon
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| Posted: 05-20-2007 11:32 am |
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2nd Post |
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Sylva Member
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Evry book and piece of paper I've sen says you can not skim the head, But I can't see a skim does any harm, I skimmed mine about 10 thou, 7 years ago and no problem. the head is interference any way, so valve clearence on correct timing is not a problem
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| Posted: 05-21-2007 03:14 pm |
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3rd Post |
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Ron Earp Member 
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Mine has been skimmed on at least three different occasions that I know of. Who knows what happened before then. I'd say it has had 0.030" to 0.050" off total, and still has good valve clearance and works well. Raises that compression a bit, just calculate to make sure that it is acceptable to your needs. Ron Last edited on 05-21-2007 03:14 pm by Ron Earp |
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| Posted: 05-21-2007 07:23 pm |
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4th Post |
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Greg Fletcher Administrator 
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An important consideration to keep in mind- if you run the JE or CP very high compression pistons, you're lowering the valves to the top of the piston. On those high compression racing pistons, they aren't cutting you any slack. You need to test assemble the engine and be certain you have the room, because it might work out that there isn't enough.
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| Posted: 06-09-2007 08:15 pm |
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5th Post |
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Jon Plowe Member
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Just thought you might like to know I had 20" off the head and nothing has hit things it shouldn't. Any more than this and it looks as if it would have been eating into the valve seats. Of course I don't know what, if anything, has been skimmed off before. I am running 9:5 pistons
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| Posted: 06-10-2007 03:29 am |
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6th Post |
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Judson Manning Member 
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The new Goetz(sp?) head gasket is about .020" thicker than the original steel-asbestos version. It adds a bit of insurance when it comes to piston to valve clearance. With stock cams, running the CP or JE pistons with a decked head is not something to worry about. I wouldn't be concerned with the 107 either as I'm running a custom grind which has slightly more lift than the 107. I've not personally run a cam with as much lift as a 104, but I'm sure there are enough club members with 104 cams and JE pistons who could lend their experiences. Technically the service limit is ~.015", but as Jon points the real limit is the point where the valve seats start to be machined. The fact is you want the head flat so it seals well. The head alloy is quite soft and tends to deform when torqued down on the original steel head gasket, so it's almost a foregone conclusion that every head needs to be skimmed. The "9.5:1" pistons actually spec out in the engines I build somewhere in the 10-10.5:1 range. With this already high c/r, there's no need to do anything but deck the head sufficiently to get a flat surface.
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