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Old 09-14-2008, 10:48 AM
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Piston notching for valve relief/clearance

What would be considered the maximum one could notch a forged piston for more piston to valve clearance?

I am having an ongoing discussion with 2 other guys and I just want additional opinions.

Anyone ever used the Isky "Rent a kit" notcher and get good results?

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Old 09-14-2008, 02:57 PM
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Most pistons will be manufactured with a crown thickness equal to 7% to 10% of the piston diameter. 7% on a 4" bore would be 0.280", so there is little chance that you will need to cut valve reliefs so deep that you will compromise the piston integrity. They will also be manufactured to tolerate a "worst case scenario" as far as piston to valve clearance.

Most of the time, you will be ok unless you install larger valves in the heads or change the head/block relationship by angle milling the heads or use some really wild camshaft timing. Although reading Isky will teach you that you need 1/4" clearance, that's just not real in today's world. The most popular generally accepted clearances are 0.080" on the intake valve and 0.100" on the exhaust valve. Clearances will be closest during the overlap period, maybe 30* BTDC and 30* ATDC.

I have not used the Isky notching tool, but have made my own tool by brazing a cutting tool to an old valve. If you do this, make the tool so that it cuts a diameter 0.100" larger than the valve and use a generous radius on the tip of the cutting tool so you don't introduce stress points into the piston crown.
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Old 09-14-2008, 04:02 PM
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Thanks for the info..
.007 X 4.500 = .315

So if a 4.5 inch piston needs a cut approaching .125 there should be more than enough material to allow for it?

This would all be in the dome of a custom forged Hemi piston. Not inside contoured.

(On a 2.25 valve I use a heavily radiused 2.375 cutter.
On a 2.4 valve I use 2.500)

I thought .150 was max all others cautioned at .100, all off a tad.......
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Old 09-14-2008, 04:38 PM
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Before I would start cutting, I'd call up the piston manufacturer to make sure how much material is in that particular crown.
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Old 09-15-2008, 04:48 AM
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how do you get the cut at the correct angle to the piston?
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Old 09-15-2008, 05:46 PM
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Quote:
Originally Posted by BBCMudbogger
how do you get the cut at the correct angle to the piston?
The cutter I'm describing is made from an old valve (make sure it is valid and not bent or the head warped) and a cutting tool (like you would use on a lathe post) sharpened on the edge for clockwise rotation in your drill motor along the edge of the bit from the tip to the center of the valve head. You'll want the bit at a length equal to the valve diameter plus 0.100" (0.050" on the radius). In other words, if you were cutting for a 1.940" valve, the radius of the valve would be 0.970", so you would want the radius of the cutting bit to be 1.020" (center of the valve head to the end of the bit). Leave a radius of maybe 1/16th of an inch on the tip of the bit so that you don't create a sharp corner in your cut that could act as a stress riser in the piston crown, then silver- solder or braze the bit to the valve head. The cutting edge of the tool will be to your left as you look at the valve head with the tool part up and should be relieved as it goes to the other edge of the tool bit (to your right as you look at the valve head). If the valve sizes in the head are fairly close to the same diameter, you may get by with making only one cutter. If they are very dissimilar, like 2.020" and 1.600", you may want to make two cutters, one for intake and one for exhaust. You would have determined if both valve pockets need to be cut when you checked the piston to valve clearance in the first place.

You do the cutting with the short block mock-assembled (piston at TDC and masking tape around the piston to stabilize it in the bore). Use an old head gasket of the type you will be using on final assembly or you can use the new gasket you're going to use on final assembly because we're not going to torque it down, just snug it. Oil the shaft of your cutter/valve and place it in the valve guide. Slip a drill-stop collar onto the valve stem and snug down the allen screw to hold the cutter in place while you put the head on the block. You can usually find these collars at the local hardware in various shaft diameters. If not, here's a link to a low-buck kit with several sizes. A SBC valve stem is 11/32" (0.34375"), so a 3/8" collar would be used. If you want to get technical, drill the 5/16" collar out to 11/32" for a snug fit.
http://www.jackstoolshed.zoovy.com/p...&utm_campaign=
With the head SNUGGED DOWN to the block with a couple of bolts to maybe 25 ft./lbs. (not torqued to final spec), loosen the collar and let the valve/cutter head drop down onto the piston crown. Move the collar down against the valve guide/boss and snug the set screw. Rotate the cutter counter-clockwise in the guide. If you encounter resistance, loosen the collar, then re-tighten. Keep doing this until you can rotate the cutter 360 degrees and find a little resistance at one point. That will be the high spot on the crown and the point from which you want to start your cut. With the cutter positioned at that point, use a stack of your feeler gauges between the guide/boss and the collar that equals the depth of cut you have determined to make to get sufficient clearance. For instance, if this is an intake valve pocket and you want the minimum 0.080" clearance, but your clearance was only 0.047" when you checked it, you would stack 0.033" of feeler gauge blades between the guide/boss and the collar to limit your cut to 0.033" in depth. Make absolutely certain that the collar is TIGHT. Not to the point of stripping the threads on the set screw, but TIGHT.

Now, you're ready to cut. I will caution you here...GO SLOWLY AND EASILY. If you use just a very slight downward pressure on the drill motor, the tool won't chatter and your stop collar won't slide up on the tool and you'll be ok. This takes a lot of concentration and control, so turn off the radio and tell everyone who might be there to shut up.

Although I've done this with the piston at TDC and it worked out well, actual valve to piston contact will probably occur at some point either before or after TDC during the overlap period. I think the next time I do it, I will cut at TDC, then move the piston to 30* BTDC, re-arrange the collar as above and see if the cutter will engage a little more material from a slightly different angle. Same for 30* ATDC. After you've made the cut, pull the head and intall the valve you will be using, retaining it with the collar and the set screw snugged lightly to prevent scarring the valve stem. With the head back on, set your feeler gauge blades to the theoretical valve lift plus clearance needed. For instance, intake valve theoretical lift 0.480" plus 0.080" clearance = 0.560". Pull the valve up tight against the seat. Stack your feeler gauges for 0.560" and place them between the collar and the guide/boss. Move the collar down and lightly snug it on the valve stem. Slide the valve down into the bore. It there is no space left between the collar and the guide/boss, you're good to go.

For those of you who think this sounds like a lot of work and time, it is. Maybe it will give you a clue as to why racing motors are so expensive. Can you imagine paying someone $100 per hour or more to do this for you, cutting 16 reliefs?

Last edited by techinspector1; 09-16-2008 at 09:12 AM.
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Old 09-15-2008, 07:07 PM
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after measuring the drop and p/v numbers at 10, I just ordered new pistons, it would have required more than I would want to remove. Its only 128 a piston so no big deal...


My intakes are 2.400 and exhausts are 2.0, Isky did not have cuttters that large for the intake valve pocket and made a valid reason to go new.
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Old 09-16-2008, 04:36 AM
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I see you do it with the head on with the valve guide as your arbor or guide...the only way I have ever seen it done was with vertical mill and a flycutter and a fixture that references the piston pin. How deep of a cut is this method good for?
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Old 09-16-2008, 08:38 AM
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Quote:
Originally Posted by BBCMudbogger
I see you do it with the head on with the valve guide as your arbor or guide...the only way I have ever seen it done was with vertical mill and a flycutter and a fixture that references the piston pin. How deep of a cut is this method good for?
Until you break through the crown and begin whittling on the wrist pin. No, just kidding. It would be limited to the amount of valve stem you have available to tighten onto with your drill motor chuck. But even at that, you could start with a longer valve from the aftermarket. If you need more clearance than that would provide, you need to re-arrange your thinking and go with less cam or shorter rocker ratio or less piston dome or sink the valves a little or take a light skim off the valve faces or increase gasket thickness slightly or advance/retard the cam a little. Advancing the cam relative to the crankshaft increases exhaust valve clearance at the cost of decreasing the intake valve clearance. Retarding the cam increases intake valve clearance at the cost of decreasing exhaust valve clearance.

Last edited by techinspector1; 09-16-2008 at 09:30 AM.
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Old 09-16-2008, 10:27 AM
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My little p to V problem got the piston manufacturer pointing to cam manufacturer and vise versa.

Both were faxed the exact same engineering drawings giving valve head distance from the piston pin for specs... PRIOR to making the parts.
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