Piston to Valve Clearance
 

'91 HO 302 roller motor

AFR 165cc heads

.499/.510 275/279 cam

Stock forged pistons

Blueprinting/ building the engine and checking the valve to piston clearance. The closest clearance is .027". I've heard the recommended clearance is between .080" and .125" that would mean I need to take off about .060" from 2 eyebrows on each piston. Have 3 questions.

What is the recommended piston to valve clearance in this situation?

How much is safe to take off the piston in the eyebrow area before it becomes too thin?

Is this enough to throw off the balance of the engine?

.100" is the recommended clearance.

You can usually machine the eyebrows on most pistons .125"

Yes, it will change the balance, but likely not enough for you to notice since it is not a major part of the bobweight.

Did you use a set of used headgaskets (already compressed) when you ran the test?

Glad you said something about compressing the head gasket. After I did that the clearance is down to .010".

Machinist said it would compress about .005" but it looks like it was more like .017", looks like I'm going to have a thinner head gasket than planned, make up for the extra room of the larger eyebrows.

I also noted that the valve is hitting in the back radius of the eyebrow, explains why I was getting such a wide variance in reading between cylinders, because it wasn't hitting flat in the bottom of the eyebrow. So I'll make the circumference larger then take another measurement. Some cylinders were up to .060" so maybe I won't need to take that much off the bottom.

Just one other question, are SB Ford valves angled at 20 degrees? I need to make a piston holder jig for the mill so I can cut these eyebrows.

Butterfly cut and go with BF's suggested clearance,

Bounce alone will smash parts at .010.

Iskendarian makes a Piston Notching tool - you can get more info from their website:

http://www.iskycams.com

I have tried to capture some of the data below - You use your cylinder head as the support platform / mandrel for the tool and cut the pistons while they are in the bore.

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Piston Notching Tool
Many racers purchase custom forged pistons made to their engine specifications then find (and usually too late) that they have V/P interference
(valve to piston interference) usually caused by mislocated valve pockets in the piston's domes. Only ISKY markets this tool where your own cylinder
head is used to obtain perfect geometry with the valve that cuts its own proper relief in the piston dome. Available in 1 5/8, 1 3/4, 1 7/8, 2, 2 1/16,
2 1/8, 2 1/4 and 2 3/8 inch diameters for nearly any engine requirement. Proper procedure, is to choose the next larger cutting size than your valve
diameter, for proper valve pocket side clearance. Pilot sizes are 5/16, 11/32 and 3/8 diameters. Perfect for the Racing mechanic or speed shop, the
tool may also be rented at a nominal fee.
Cutter only (specify size) (hand driven with tap handle wrench) . . . . . . . . . . . . . . . . . . .NET $100.00
Pilot (specify stem dia.) (hand driven with tap handle wrench) . . . . . . . . . . . . . . . . . . . . .NET $32.00
(Rental charge) refund due customer when returned (for 1 pilot and cutter) . . . . . . . . . . . . . . . .$40.00
*Above Prices are net, no further discount.
P.N.T. Cutters P.N.T. Pilots
Part No. Diameter Part No. Pilot Diameter
PNT-158 1 5/8” NP-516 5/16”
PNT-134 1 3/4” NP-1132 11/32”
PNT-178 1 7/8” NP-38 3/8”
PNT-200 2”
PNT-216 2 1/16”
PNT-218 2 1/8”
PNT-214 2 1/4”
PNT-238 2 3/8”
NOTE: This Tool is effective for cutting eyebrow only (up to a 50% valve face
area cut). Beyond this point efficiency is significantly reduced.
Isky Invention