I was curious. Let's say you were going to shave the deck or head to increase the compression. Is it better to shave the head or the deck, or does it really matter which? On the engine I am currently building, I had a head that was shaved but I had the block decked while it was at the machine shop to get my target compression. So, I have done both on this engine. I am planning on a backup engine (in case I blow this one up) and was just wondering if it made any difference which part was machined, as long as the target compression number is achieved.

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-MattMan
LEANED & MEAN

If you are removing a lot of material (more than 0.100") it's probably smart to split between the two. That way you avoid the risk of thin spots.

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I agree. There's a lot of metal in the deck, but I don't think it wise to just start hacking away. The thickness across the top of the deck and bottom of the head is not uniform; the thickness varies due to production tolerances from one engine to the next. I like the idea of splitting the difference between the two surfaces, but have the machine shop verify the thickness first.

I've only ever had it done once, and that was because of a warped head causing chronic head gasket failure. They took off 0.20" which is not a whole lot.

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If it was easy, everybody would be doing it.

I like to take more off the block deck.

Id surface the head for seal and straightness.......do all your chamber and valve work....then measure to see how much decking is needed for target CR

Reshaping the combustion chamber for larger valves adds volume. Bore notching for flow around larger valves is sketchy. Taking a lot off the head puts the valves closer to the deck.

If all the head work is focussed on flow and flame spread .....it makes some sense to use block decking to achieve the desired compression.

You can easily deck the block .100

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I cc'd the head on my current engine and based the amount to be decked off of that. Basically, I have .060 off the head and .070 off the block. I was just curious about this when I get the spare engine machined.

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-MattMan
LEANED & MEAN

I would machine the block and head to the same specs as your present engine. Then you can mix and match parts in emergencies without changing your CR.

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The backup engine is supposed to be from a 73 Duster. I will know when I look at it tomorrow. Guess I'll measure the deck height and then shave the block to get the came CR with my current head (45cc chambers on it).

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-MattMan
LEANED & MEAN

I always run my calcs, and take a fair amount of the cut off the block first... that way you can assemble the bottom end and check the numbers.

Then you can make your adjustments with the cut on the head (and it also gives the machine shop time to install your OS valves which can change the volume of the chamber depending on how they seat (and if you installed new seats)... so you can measure that then use the .0066=1cc rule to make your final adjustment (as long as the chamber is under 40cc)...

As posted, taking the cut on both surfaces will get you a true surface (well...somewhat true I've had a couple that were .001 with a machinists edge and tape in some spots...

.130 isn't bad, you'll need new pushrods, and you will have to check to see if the headbolts will still work... (on my 10.3:1 motor with a similar set of cuts I was just beyond the point where headbolts would even clamp), a set of ARP Studs will take care of that (I have had good success with them from 10:1-12:1...on the 12:1 motor I had to take over .2 combined between the head and the block...machinists who have never done a slant really love doing that...)

I am always swapping parts around on different engines, so I take most of the cut off the head so I can easily adjust CR and performance by swapping a head and leave the engine in the car. Once you cut the block a bunch, you cannot go down in CR without building a whole new bottom end.

Any combo works, though! 0.100" off each head and block is completely safe, and I would say 0.150" off each is almost certainly safe. I have heard for 0.200" off one or the other. Max I have done is 0.110" off the manifold side and 0.210" off the plug side.

Lou

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You lost me there, Lou.

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-MattMan
LEANED & MEAN

You can "angle mill" the head. This closes up the chamber a bit more on the plug side of the head and leaves more meat on the manifold side. Slightly tilts the head ports up. I do not think this is worth much in terms of performance, but who knows. Meril Bruner told me this trick in 1991 when I visited him in TN and I did it on a couple of heads. Lately I have done more with taller pistons and less head milling. IMHO, net compression ratio and head flow will have a much bigger influence than any tweaking in where you mill, angles, etc... Make sure you put at least the 1.70/1.44 valves in your head and do some bowl porting - biggest gains are there (along with at least 8.5-9:1 of true static comp).

Lou

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I thought you were talking about angle milling the head. I had thought of this but didn't think it would affect much other than the shape of the combustion chamber. That would be more variables than I care to work with for now. I don't want to reinvent the wheel here, just pump it up a little (ok, a lot).

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-MattMan
LEANED & MEAN

Yes, angle milling the head. That is exactly what I was talking about.

Lou

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Home of Slant6-powered fun machines

Wouldn't angle milling the head cause the head bolts to not come in flush contact with the head? You'd need wedge shaped washers, or mill the top of the bolt holes at the same new angle.

I am not sure what others have done, but when I spoke to the engine builder that I am using, they do kiss the top of each head bolt face to straighten it up with the new face angle.

Rick

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