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Premium Member
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Any cam can be made to have valves hit or miss the pistons depending upon how you time it. Alfar7 has posted setup instructions on several occasions.

I have 12.3 intake and 11.5 exhaust cams and high dome Venolia racing pistons in my Spider. The cams are Richard's and they are timed at 104/104. I have plenty of valve-piston clearance.
 

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Any cam can be made to have valves hit or miss the pistons depending upon how you time it...
... and how much the head has been milled, and how the machinist has set the valve seats into the head, and how thick the head gasket is, and what type of pistons are used, and the phase of the moon, and .....

Honestly, you need the measure the P-V clearance to know whether or not you have a problem. Even with stock cams and pistons, some of the variables mentioned above could conceivably lead to problems (though admittedly, the odds would be low).
 

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Richard Jemison
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7,372 Posts
12mm cams

Hi All, can 12mm Cam Shafts be used with Stock Pistons, (Clearance)? Bob Lesher [email protected]
The answer is dependant on several items.

First the head:
1- the internals of a stock Alfa head will not have clearance INTERNALLY as built for the 12mm lift. To have clearance for 12mm lifts the valve guides, seals and springs will have to be changed to shorter guides, set deeper in the spring bore using short seals (no teflon) and springs with adequate clearance for the additional lift.
2- the cam follower bores have to be notched to clear the additional lift. Or the cams built with small base circles which will increase wear and noise.
In any event the info in "1'' above is required.

Cams:
Cam timing can be altered to provide adequate clearance for both intake and exhaust. Typically at the closest point of possible contact (5-15 degrees after TDC on intake and before TDC on the exh) you should have .040 min clearance on the intake side and .080 on the exhaust. This with adequate springs to manage spring inertia/weight to insure the valves are following the cam lobe and not floating.
If you need more P to V clearance it`s just a matter of retarding the intake cam or advancing the exh cam.
That also means that the correct timimg of the cam and it`s correct valve events (opening and closing) are altered, but sometimes you just have to do what you just have to do!

Bottom line....
12mm cams should not be placed on stock unmodified heads. About 11mm lift cams with BCs of 1.060 (stock is 1.080) will fit but larger BCs will require some notching to clear the cam follower bore in all but the 1991-1994 Motronic motors..
 
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