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Piston to Valve Clearances

tjurek51

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Bike: 1980 CB750F Supersport.

I'm installing a new camshaft which is much higher lobed than the stock camshafts (Webcam Grind #110, part 55-491). I'm new to this precision measurement stuff, but here's what i have;

Valve lift: .395" (10.03mm) on both Exhaust and Intake.
Starting at 20 degrees BTDC @ 0" lash, using a dial indicator i measure .388" of travel on the exhaust side and at 20 degree ATDC @ 0" lash i measure.326" of travel on the intake side.
Starting at 0 degrees TDC @ 0" lash, i measure .312" of travel on the exhaust side and .245" of travel on the intake side.

here's where i'm stuck. in my head i'm thinking if the valve lift is .395", at all measurements it looks like i could hit the piston. Thus, i know i'm missing something.

Is there some calculation that is used to determine the correct P-V clearance? everything online just tells me the measurement is my clearance and don't actually tell me the calculation (if there is one). if i use the measurements i got, they look very large compared to the recommend .080" on the Exhaust side and .050" on the intake side.

any help is appreciated.
 

amc49

CB750 Guru
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What pistons are you using? You will have to have a load of other parts too, that is way too much cam for a stock engine. If the pistons are stock you quite simply will not clear the valves.

It's not about absolute clearance at all, all parts are moving and it changes all over the place. How close the cams are to each other alone can make them hook valves with each other much less the pistons.

You have to do those checks with parts in place as even the shims will change up how much room you have to clear. So can the valve springs and you MUST have stronger ones for that cam set.

You don't calculate the clearance, you must positively measure for it as you picked cams that put you there, they are for a racebike not running on the street. You can't use the OEM tappets with shim on top, the cams are big enough (over .370" lift) to spit the shim out and instant destruction doing it.
 
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