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Author Topic:   Piston/Valve clearance
posted January 06, 2004 07:15 PM
I recently purchased a set of domed speed pro 12:1 pistons on 5.7" 4340 H beam rods. They are brand new in the box assembled. The guy I purchased them from needed money badly so I got them for $50. He said they were $600 new but I havent looked, think he may have elevated it a few... My question is will these fit ok with my 64cc double humps or should I put them in the motor with the 76cc 882 heads. My concern is the dome with the already short humpers with the valves and spark plug clearance. can any of you give me some idea of what you can run with what, as far as cc to dome height clearance is concerned. Maybe a shorter spark plug??? thanks......

[This message has been edited by NYhick (edited January 06, 2004).]

posted January 06, 2004 11:51 PM
one way to check look up the part number and see if thats 12:1 with 64 or 76 cc heads that will tell you right there and what ever heads you deside to run when you get the bottom end in put some modeling clay on the piston bolt the heads on and set the valvles on a cylender and turn it over by hand once then messure the thickness of the clay when your done and that will tell you how much clearance you have

posted January 07, 2004 12:10 PM
I have been told bare bare minimums are .030" on piston to head clearance, and .050" piston to valve clearance. It all depends on the lift/duration, etc... on the valve clearance.
And did you just get pistons, or pistons and rods? the pistons new are around 400 or so w/out rings, and the rods are minimum or 350.00. I assume the pistons are forged, I dont think you would want a 12:1 cast piston very much..?

posted January 07, 2004 12:41 PM           
I guess Im just a dummy,and or lucky because I just ask the manufacturer, or my machinest if I needed to check the piston to valve clearance.

Its my understanding that the pistons made today will all clear a 202/16 valve at .570 lift. Ive ran these pistons with no problems yet. Wiseco,Ross, JE,speed pro,keith black, all of which were at least 12-1.

.563 was a common intake lift on a cam I ran in at least 7 diff motors.

I think you need to start checking valve clearance when you angle mill a head, which would change the angle of the valve into the pocket. Or if your degreeing the cam way out of its inteded specs.

But I hear about hobby stock guys with .480 lift checking their piston to valve clearance all the time. Hmmm!!!!!!!

posted January 07, 2004 07:32 PM
goinrcn44h, yes I did get them with the rods and pistons already assembled together. He told me they were over $600 new so I guess maybe he was telling me the truth. Guess I got a smokin deal.... I did find out that the pistons were installed in one of his motors and during warm ups of the first time he ran it he blew a head gasket and found a cracked block so he tore down the motor and pulled the pistons out. Thanks for all the help, guess I will let the engine builder do the math, just didn't want to walk into his shop and look like a fool and ask him to put 10# of stuff in a 5# cylinder.....

[This message has been edited by NYhick (edited January 07, 2004).]

[This message has been edited by True Blue (edited January 08, 2004).]

posted January 08, 2004 05:35 AM
it is not the lift of the camshaft that will get you into trouble, its the duration. and don't go by advertised, the duration at .050 is what should concern you the most.

posted January 09, 2004 12:12 PM
Alot of things affect clearence The rule is .100 without the head gasket.if your cam is advanced or retearded can change which valve is closer and it is at a position other than max valve lift.I put a new cam in mine and ended up with .100 intake and .145 exhaust clearence + head gasket .039 should be adequate.doing it with the light spring method.Put the clay on like 17j said and turn it by hand and check your thickness.I'm running 254/259 duration @ .050 with .584 lift on both

[This message has been edited by dirtracer7 (edited January 09, 2004).]

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