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Discussion Starter #1
Are the heads with the smaller valves the same as the ones with larger valves (other than the valve siz)? I need some and can get the smaller valves heads and machine to larger size. Thanks,
Gary
 

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Hello Gary, Yes you are right the 1.94 casting 186 head is the same basic casting as the factory 2.02 head, but the factory 2.02 head has differnt machining in the bowls ( below the seat) and also the intake wall around the intake valve is machined with a bigger cutter to unshroud the intake valve. Actually this a good way to go as any good machine shop can easily machine the heads out to 2.02x1.60 valve size, PLUS the valves sit up in the chamber just like a new head would! As far as unshrouding the intake wall i have built lots of motors without doing this and they run just fine. It probably hurts airflow in the chamber slightly, i have never flowed a original head and a converted head to find the difference. Hope this helps. Mike.
 

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I am not sure what "unshrouding" means.Does that mean that the area behind the valve stays the same, only the valve head is larger? The seat must have to change also, correct? Thanks,
Gary
 

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Hello Gary, Unshrouding the intake valve means machining the vertical chamber wall around and above the intake valve in the combustion chamber. If you are looking at the head on the bench with the intake ports toward you and the valves facing straight up the vertical chamber wall is behind the valve. Picture the intake valve as a clock face, the factory unshrouding cutter machines the vertical combustion chamber wall from say 8 o'clock to 12 o'clock posistion. Actually there are not any seatinserts in a original head, the casting simply has the seat machined into the head cast material. You are correct the seats have to be enlarged for the bigger valves. Some machine shops will install stellite valve seats for todays unleaded fuel to stop seat recession. Hope this answers your question. Mike.
 

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Discussion Starter #5
I am looking to make 2.02 valve heads out of 1.94 heads. I understand that there are machinig specs for the milling process to unshroud the intakes in the older GM Performance Parts book. Does anyone have that book or a good close-up picture of a real DZ head? Thanks,
Gary
 

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My stock, big valve, -461 heads cc'ed at 66 vs 62 to 64 generally b/c of the factory chamber relief cut to unshroud the intake valves.. I don't have pic's but it is a pretty straight forward plunge cut with a stone on an arbor. pdq67
 

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The unshrouding cutter should be 2.34" diameter. I've seen flow bench sheets showing that heads with intakes enlarged from 1.94" to 2.02" without unshrouding the intake valve actually flow LESS than they did originally with the 1.94's; there's a good reason the chamber was cut on factory 2.02 heads. It cost over $200,000 (in the late 60's) to add that station in the head machining line, and Chevy didn't spend any money on option tooling without solid justification from Engineering to get it past the bean-counters


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JohnZ
CRG
'69 Z28 Fathom Green
 

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Discussion Starter #9
Thanks!
 

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Discussion Starter #10
I now have the 1.94 heads. I am talking to machine shops about what to do with these heads. I want basically a stock 302. I will open up the valves to the larger size and unshroud the valves. Some pictures I have seen of original heads looks like the exhaust valves were unshrouded along with the intakes. Is this correct? Some shops are telling me to install screw in studs and plates (not stock). Is this something worth
doing to protect the valve train? Did the factory DZ heads have machined bowls (pocket ported)? Thanks for everyone's advice.
Gary

[This message has been edited by Gary L (edited 12-26-2002).]
 
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