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Having re-read the IT rules on head modifications, I decided I needed to measure the head thickness (height) again, and go ahead and have the head cc'd. All this to know for sure if the piece is IT legal or not.
Because as it turns out, there are actually 2 criteria that must be met in Improved Touring. The 1st is head thickness, which must be within 0.025 of stock. So, we cannot be less than 3.305, since the shop manual (1971 - US spec) says 3.330 is stock. The 2nd, and over-riding, element is the CR itself, which can only be half a point (0.5) above stock; in this case, we cannot exceed 11.0:1.
My remeasure of the head thickness came out at 3.31+. (I don't know how I came up with 3.28 the first time.) I also came up with .034" as the compressed gasket thickness, NOT the .030" I reported in an earlier post. The memory is definitely going South...
Next, off to a local engine builder that agreed to cc one of the combustion chambers and confirm head thickness for 20 bucks. (Before any of you engine gurus get all upset, you need to understand that by the rules, I can't do a damned thing to correct cylinder-to-cylinder variance. So no real need to do all of the cylinders... we are just trying to get a warm and fuzzy feeling about the general legality of the head.)
The machinist agreed with my thickness measurement pretty closely - he came up with 3.314" to 3.318", depending on which end of the head the measurement was taken. These numbers would back up the claim by the previous owner/builder that the head was a '70 or '71 piece (3.330" stock), and that he had only taken a few thousandths off during the previous build. The machine shop that did the head for me this time had stated they took about 10 thousandths to get it true.
Okay, the head meets legal minimum for thickness, with about .010" to spare. So what about the combustion chamber volume and resultant CR? The builder measured the #1 cylinder at exactly 45cc. Given that number, plus 5.6 cc of head gasket volume (.034" x 3.59"), and 505cc swept volume (.030" overbore), my math says the theoretical CR is 10.98 to 1.
Perfect! Sometimes you just get lucky.
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1971 142E ITB racer, 1973 1800ES, 2002 S60 T5
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