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Discussion Starter · #1 ·
Mill my Head???

I was thinking of milling the head I'm going to use on my low-comp. '72. what are the pros and cons of doing this???
 

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One of the cons of milling a head is, it throws off the timing. This is where Bob Legere can tell you how to proceed
 

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Re: Mill my Head???

1972ACGT said:
I was thinking of milling the head I'm going to use on my low-comp. '72. what are the pros and cons of doing this???
Some pros:
>increased compression ratio = more torque, more power, better fuel economy, better throttle response

>cheaper than custom pistons to increase compression

>more efficient flame travel with a milled head vs. domed piston

Some cons:
>increased compression may require higher octane (more $$$) fuel

>decreases piston-to-valve clearance. Not an issue with mild milling and stock valves and cam, but may cause contact with hot cam and/or larger valves (at the exhaust valve, due to cam timing being retarded at the same time)

>retards camshaft timing. Unto itself, this reduces low-end power and moves the peak power higher up the rpm scale. Figure about 2 degrees of retard per .025" of milling. I strongly suggest that you correct the cam timing if you mill the head more than .025".

>thinner head casting *may* create the potential for higher water temps (in theory). Realistically, I've milled heads .085" and more (as much as .105") with no heat-related problems. Note: If you mill the head this much (or more), the head bolts may bottom out in the threads of the block. You will not have sufficient preload on the head gasket, and the gasket will blow out. Bolts will need to be shortened at this point.

Bob
 

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Discussion Starter · #4 ·
Thanx Rally Bob! I'm trying to use a Crane cam that has a 434 lift and 264 duration. would I have any problem milling it .025 or .050?? what sort of compression gain would .050 bring??
 

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1972ACGT said:
Thanx Rally Bob! I'm trying to use a Crane cam that has a 434 lift and 264 duration. would I have any problem milling it .025 or .050?? what sort of compression gain would .050 bring??
Milling .050" will increase compression by a little less than 1 full point (about .8 to .9). You'll get about 1.1 point increase on an earlier (flat-top) engine with .050" milled. You shouldn't have any clearance problems, but with .050" milled you'll definitely want to get the cam timing corrected, or you'll lose valuable low-end torque.

Bob
 

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Re: Re: Re: Mill my Head???

Travis said:
Any reason you couldn't use a shim under the bolt head?

-Travis
Sure, that would work as long as it's a hardened washer and it has a chamfered hole to clear the underside of the bolt head. A non-hardened washer would crush and throw off the torque spec.
When I had custom ARP head bolts made many years ago, I used hardened washers under the bolts (About $2 a piece from ARP!).

Bob
 

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Rallybob,
Would there be a problem with sloting the stock cam sprocket and using stock cam bolts to correct the cam timing issue?
 

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You would be shocked at the forces put on a timing chain/gear in both directions. You should utilize an offset bushing to correctly locate and index the timing gear. Imagine the disaster if it slipped.....bent valves, cracked pistons, possibly a broken chain.

Bob
 

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You would be shocked at the forces put on a timing chain/gear in both directions. You should utilize an offset bushing to correctly locate and index the timing gear.
Any reason not to use a Kent Vernier Pulley?
Btw, anybody have a source of these for our Opel CIH application?

Walter
 

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I've used the Kent pulleys for years, and in fact used to sell them. I don't know where to buy them from these days, I used to buy 10 or so at a time from England direct. I understand they won't sell direct to US customers any longer, as they have a US distributor now.

Piper cams in England also sells a vernier pulley for the Opel CIH. I've only seen one of them, it looked a bit nicer than the Kent item to be honest. Still, the cheapest (and equally effective) method is the use the good old fashioned Chevy offset bushings. The bushing degrees don't quite correlate to the Opel, but with a little test fitting and measuring of the degrees of advance/retard, it can be compensated for.

Bob
 

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There must be a trick that I don't know because I found using the chevy bushing to be a real PITA. Reaming the hole in the pulley to accept the bushings was straightforward but the hole in the bushing was smaller than the opel bolt. Using a three jaw vise in a milling machine I had difficulty finding the sweet spot between, too loose(spun and ruined the bushing) and crushing of the bushing...

Any tricks Bob? I considered using the bushing on the cam pin but it was a little too loose for my liking...

-Travis
 

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Huh? The bushings I've used had a 1/4" hole that was bigger than the 6 mm Opel cam drive pin. Maybe I used the Ford bushings? I'll check when I get home.....

Bob
 

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Pin to Bushing Clearance

Travis and Bob

Aren't you both saying the same thing? That the 6mm (.236") dowl pin is smaller than the .25" hole in the Chevy offset bushing kit. If so, it would seem that the question would center around what kind of problems this .014" (or so) clearance would cause if used as is. How has this issue is delt with in the past.

I'm interested in this as well, because the adjustible cam gear is kinda pricey at about $150 USD.

Thanks
Paul
 

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Yes, we are. I was planning to wait until I got home tonight to measure the distance between the pin and the center of the cam. Then we can calculate the amount of 'play'(in degrees).

-Travis
 
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